The Peter Day Series of Magnetic (Super)Conductors †

: Here, we review the different series 3 ] · G, which represents the largest family of paramagnetic metals and superconductors reported to date, with more than fifty reported examples.


Introduction
Among the many legacies of Peter Day's work, besides the Robin and Day classification for mixed-valence compounds [1], one of the best known is the discovery of the superconducting paramagnetic radical salts prepared with bis(ethylenedithio)tetrathiafulvalene BEDT-TTF (ET, Scheme 1) and [M(ox) 3 ] 3− anions (M = Fe, Cr, Ga, etc.) [2,3] The discovery of the first paramagnetic molecular superconductors boosted the research in the field and led to the preparation of around twenty-five molecular superconductors in the so-called Peter Day series. The first member of this series, initially formulated with a water molecule as β"-(BEDT-TTF) 4 [(H 2 O)Fe III (C 2 O 4 ) 3 ]·PhCN [2,3], and later with a H 3 O + cation as β"-(BEDT-TTF) 4 [(H 3 O)Fe III (C 2 O 4 ) 3 ]·PhCN [4], showed a superconducting transition at 8.5(3) K, one of the highest T c observed in any molecular superconductor to date. As Peter Day noticed in his first report, this discovery paved the way to a synthetic strategy for obtaining further magnetic molecular superconductors [3]. Only two months later, P. Day's group published a deeper characterization of this compound and a second phase (pseudo-κ or κ ) obtained with the same donor (BEDT-TTF) and the same anion ([Fe(C 2 O 4 ) 3 ] 3− ), but with different A + cations: κ'-(BEDT-TTF) 4 [AFe III (C 2 O 4 ) 3 ]·PhCN (A = K + and NH 4 + ). This orthorhombic (Pbcn) pseudo-kappa or κ' phase was also paramagnetic, but it was a semiconductor (as we will see below, all the known members of this κ phase are semiconductors) [2]. These two initial reports in 1995 initiated the search for novel molecular conductors and superconductors with interesting magnetic properties. Besides the more than one hundred metals, semiconductors and superconductors that will be presented in this review, this search also led to the synthesis of the first molecular superconducting antiferromagnets [5,6], metallic ferromagnets [7,8] or field-induced magnetic superconductors [9]. The synthesis of these radical salts was performed in H-shaped or U-shaped electrochemical cells containing two compartments separated by a glass frit. The anode compartment is filled with a solution of the TTF-type donor (Scheme 1) in solvents such as CH 2 Cl 2 and CHCl 2 -CH 2 Cl. The cathode compartment is filled with a solution containing the desired oxalate-based magnetic anion dissolved in different solvents or mixtures of solvents. As we will see below, the choice of the solvents is crucial in many cases to determine the structure and properties of the radical salt obtained. Very often, a few drops of water were added to furnish the needed H 3 O + cations. As we will see in Section 4.1, in some cases, a crown ether such as 18-crown-6 was added to the cathode to increase the solubility of the K + or NH 4 + salts of the anions. Application of a very low constant DC current (in the 0.1-1 µA range) through platinum electrodes (usually with a diameter of 1 mm) results in a slow oxidation of the TTF-type donors in the presence of the magnetic oxalate-based anions that yields, after a few days or weeks, high-quality single crystals in most cases. The co-crystallization of neutral donor molecules together with the oxidized ones leads, in most cases, to lattices with a mixed-valence state in the donors and, therefore, to a combination of electrical and magnetic properties [2,4].
As it will be shown here, the continuous efforts of P. Day's group (and others) led to the synthesis and characterization of more than one hundred radical salts prepared with different TTF-type donors (Scheme 1) and metal-oxalato complexes (Figure 1). Although a revision with a part of the work conducted in this area was published by P. Day in 2004 [10], since then, the number of published salts has multiplied by a factor of four, passing from less than thirty to more than one hundred and twenty. Therefore, as a late homage to the legacy of Peter Day and his group in the area of molecular conductors, here, we will revise the work conducted in this field since the first report of a molecular paramagnetic superconductor by P. Day's group in 1995 [3]. In this review, we present all the reported structures of radical salts with TTF-type donors and metal-oxalato complexes, divided into five different sections. In the first section, we present Peter Day's series with more than fifty reported monoclinic conductors and superconductors formulated as β"-(BEDT-TTF) 4 [AM III (C 2 O 4 ) 3 ]·G with A + = H 3 O + , NH 4 + and K + ; M III = Fe, Cr, Ga, Rh and Ru; and G = C 6 H 5 N (py), PhNO 2 , PhCN, 1,2-PhCl 2 , PhI, PhBr, PhCl, PhF, dimethylformamide (dmf), 2-Clpy, 2-Brpy, 3-Clpy, 3-Brpy and CH 2 Cl 2 . In the second section, we show all the reported BEDT-TTF radical salts with the orthorhombic pseudo-κ or κ phase formulated as κ'-(BEDT-TTF) 4  Finally, in the fifth section, we will include the radical salts prepared with metal-oxalate complexes and other TTF-type donors such as tetrathiafulvalene (TTF), tetramethyltetrathiafulvalene (TMTTF), bis(ethylenediseleno)tetrathiafulvalene (BEST), bis(ethylenedithio)tetraselenafulvalene (BETS) and 4,5-bis((2S)-2-hydroxypropylthio)-4 ,5 -(ethylenedithio)tetrathiafulvalene (DMPET) (Scheme 1).
The key role of the solvent on the physical properties was soon noticed by P. Day's group after the synthesis of the py (1, 2) and PhNO 2 (4) derivatives with the same anion and the same structure [14,15]. Thus, the pyridine derivative showed a metal-insulator transition at T M-I = 116 K, whereas the PhNO 2 derivative became a superconductor at T c = 6.2 K, slightly below the PhCN derivative that showed a T c of 8.5 K [3]. Further studies showed that the solvent molecules interact with the ethylene groups of the BEDT-TTF molecules of consecutive layers and play a key role in the order/disorder observed in these ethylene groups. The stronger the solvent-BEDT-TTF interaction, the better, since it facilitates the ordering of the ethylene groups and, therefore, favors the superconducting state. The lack of superconductivity in the salts with pyridine and the study by P. Day's group [16] showing a modulation of T c with the PhCN/py ratio in the series of salts (ET) 4 [17][18][19]. A final proof of this key role of the solvent is provided by the series of salts (ET) 4 Cl, [20]. In this series with halobenzene derivatives, the polarity and size of the halobenzene molecules determine the presence of superconducting transitions at 1.0 and 4.0 K for X = F and Br, respectively, but no superconducting transition is observed for X = Cl above 0.4 K, and a semiconducting behavior is observed for X = I. In this series, the differences were attributed to the combined effect of the size and the electronegativity of X. For X = F, the solvent-BEDT-TTF interaction is quite strong, despite a longer distance, given the high inductive effect of F. For X = Br, the larger size of X reduces the solvent-BEDT-TTF distance, increasing the interaction. For X = Cl, we are in an intermediate situation that does not favor the interaction (too far and not enough electronegativity). Finally, for X = I, the PhI molecule is too large and cannot fit in the hexagonal cavity and appears slightly out of the hexagonal cavity, with a tilted orientation that displaces the BEDT-TTF molecules, resulting in a loss of the metallic behavior. In fact, this compound is the only semiconducting salt reported in P. Day's series with Fe(III) ( Table 1) [20].
A final interesting aspect of this series is the presence of a structural transition from the high-temperature monoclinic C2/c space group to a triclinic P-1 one below ca. 200 K, only observed in the salts with G = PhF (19,20), PhCl (17,18), PhBr (15, 16), 2-Clpy (29, 30) and 2-Brpy (31,32). The first preliminary observation of this transition was reported in compound (ET) 4 (15,16), where a change in the unit cell parameters to a lower symmetry phase was observed at ca. 180-200 K by single-crystal X-ray data and heat capacity measurements [21]. This result was later confirmed and studied in more detail in the series (ET) 4 (15)(16)(17)(18)(19)(20) and (ET) 4 Cl (24,25) and Br (26,27) [18,20], and in compounds (ET) 4 [19]. The symmetry loss that appears due to the change in the space group from the monoclinic C2/c space group to the triclinic P-1 one leads to the appearance of two independent BEDT-TTF layers in the unit cell (layers I and II, Figure 4), both keeping the β" packing motif (Figure 4a,c). The number of independent BEDT-TTF molecules increases from two in the monoclinic C2/c phase (A and B) to four in the triclinic P-1 phase (A-B in layer I and C-D in layer II). As in the monoclinic phase, the chains are also formed following the sequence . . . AABB . . . or . . . CCDD . . . , and the overlap between neighboring molecules follows the same scheme in both layers: A-A is eclipsed but A-B and B-B are shifted in layer I, and C-C is eclipsed but C-D and D-D are shifted in layer II.
Nevertheless, the most important changes in the triclinic P-1 phase are observed in the anionic layer, where the change in symmetry implies a decrease in four of the six sides of the hexagons that forces a change in the location and orientation of the solvent molecules. Thus, in the monoclinic C2/c phase, the hexagons are planar, and the solvent molecule is located with the C-X bond in the hexagon plane pointing towards one Fe atom (Figure 5a). In contrast, in the triclinic P-1 phase, there is a reduction in the sides of the hexagons, the hexagons are not planar and the C-X bond is slightly out of the average plane and does not point to one Fe atom (Figure 5b). The most interesting aspect of the series β"-(ET) 4 [AFe(C 2 O 4 ) 3 ]·G (1-35) is their electrical properties. As mentioned above, compound (ET) 4 (35) is the first molecular paramagnetic superconductor and shows an ordering temperature of 6.5-8.5 K (depending on the quality of the single crystals and on the exact synthetic conditions) [2][3][4]17]. There is a total of seventeen superconductors reported to date in the β"-(ET) 4 [AFe(C 2 O 4 ) 3 ]·G series, with T c ranging from 1.0 to 8.5 K (Table 1). These superconductors include the derivatives with G = PhNO 2 (4) with T c = 6.2 K [14]; G = (PhCN) x (py) 1-x (5, 7-12) with T c = 3.9-7.  (Table 1). All the superconductors in the β"-(ET) 4 [(H 3 O)Fe(C 2 O 4 ) 3 ]·G series show abrupt transitions with a sharp decrease in the resistivity values. Most of the salts reach zero resistance around one kelvin below the onset of the superconducting transition (Figures 6 and 7). The band structure calculations show that the monoclinic β" phase is a 2D metal with stronger inter-chain than intra-chain interactions. These calculations also show the formation of 3 / 4 filled bands with the Fermi level intersecting the two upper bands, leading to the observed metallic behavior [13]. Magnetoresistance measurements show that these salts are type II superconductors with low first critical fields of a few mT (H c1 , beginning of the penetration of the magnetic field) and very high second critical fields of several tesla (H c2 , complete suppression of the superconductivity) (Figure 7a,b). Furthermore, as expected for these quasi-2D superconductors, the effect of the magnetic field and the values of the critical fields are anisotropic and strongly depend on the direction of the applied DC field (Figure 7c,d).
The magnetic properties of these paramagnetic superconductors are those expected for isolated high-spin S = 5/2 [Fe(C 2 O 4 ) 3 ] 3− anions plus a temperature-independent paramagnetism arising from the conducting sublattice (Pauli paramagnetism, Figure 8a). At very low temperatures, there is a sharp decrease in the magnetic moment due to the presence of a zero-field splitting (ZFS) in the S = 5/2 ground spin state. Magnetic measurements in the presence of an alternating magnetic field (AC susceptibility) further confirm the superconducting transitions in these radical salts and allow a precise estimation of T c and of the critical fields. Thus, AC measurements show the presence of a negative in-phase signal (χ m ) below T c , very similar to the ZFC susceptibility. This in-phase signal also reduces its absolute value when a DC field is applied and becomes positive at any temperature above a critical DC field when the paramagnetic contribution of the [Fe(C 2 O 4 ) 3 ] 3− anion becomes dominant (Figure 9a). AC measurements also show and an out-of-phase signal (χ" m ) that becomes non-zero at T c (Figure 9b). The application of a DC field also reduces the χ" m signal, which eventually cancels above the critical field ( Figure 9b).   (47) [24,25] and PhNO 2 (40) [14], which also present superconducting transitions at T c = 6.0 and 5.8 K, respectively. As it can be seen in Table 2, the [Cr(C 2 O 4 ) 3 ] 3− anion has also been combined in BEDT-TTF salts with other solvents such as PhBr, PhCl, CH 2 Cl 2 , dmf, 2-Clpy and 2-Brpy with A + cations such as H 3 O + , K + , NH 4 + and mixtures of them, giving rise to a superconducting salt with T c = 1.7 K for A + /G = H 3 O + /PhBr (53) [21], two metallic salts that remain metallic down to low temperatures for A + /G = (K + /NH 4 + )/dmf (50) and K + /dmf (51) [13] and three metallic salts that show metal-insulator transitions at low temperature for A + /G = (K + /H 3 O + )/2-Clpy (36), (K + /H 3 O + )/2-Brpy (37) and H 3 O + /PhCl (54) [26,27]. In one case, with A + /G = H 3 O + /CH 2 Cl 2 (48), the obtained salt is a semiconductor (although not a classical one), most probably due to the lack of interactions between the BEDT-TTF and solvent molecules [28].
Parallel  [27]. Yagubskii's group also prepared the first example in this series with a 4d metal: (ET) 4 (49). This compound is a superconductor with T c ranging from 2.8 to 6.3 K, depending on the measured sample [31].

κ -BEDT-TTF Radical Salts with [M(C 2 O 4 ) 3 ] 3− Anions
Besides the monoclinic C2/c β" salts described in the previous section, P. Day's group also obtained an orthorhombic Pbcn phase with the same [Fe(C 2 O 4 ) 3 ] 3− anion and the same PhCN solvent, but with A + = NH 4 + and K + [2]. These orthorhombic salts show an original packing, called pseudo-κ or κ phase, and present the same general formula as the β" phase: κ -(BEDT-TTF) 4 [AM(C 2 O 4 ) 3 ]·G ( Table 3). The structure of the κ phase also consists of alternating cationic layers containing the BEDT-TTF molecules and anionic layers containing the A + cations, the [M(C 2 O 4 ) 3 ] 3− anions and the solvent molecules ( Figure 10). The BEDT-TTF layer is also formed by two independent BEDT-TTF molecules noted as A and B. A-type molecules bear a charge of +1, whereas B-type molecules are neutral. The BEDT-TTF layer presents the so-called pseudo-κ or κ packing motif formed by eclipsed A-A dimers surrounded by six B-type monomers in a distorted hexagonal arrangement ( Figure 11a). As in the monoclinic β" phase, the anionic layer contains the [M(C 2 O 4 ) 3 ] 3− anions and the A + cations forming a hexagonal lattice with the solvent molecules (G) inside the hexagonal cavities. The main difference is that now the anionic layers are equivalent since they contain both enantiomers in a 1:1 ratio arranged in alternating rows ( Figure 11b). In both phases, the anionic layers are related by a C 2 axis. This different disposition of the ⊗ and Λ enantiomers in the anionic layers is at the origin of the different packing motifs of the BEDT-TTF molecules in both phases [4].
As already noted by Peter Day's group [4], a close look at the cation-anion interlayer interactions shows that the relative orientation of the BEDT-TTF molecules depends on   The orthorhombic pseudo-κ phase has been observed in a total of nine salts to date (Table 3). Interestingly, in all these salts, the solvent is always PhCN (in one case with a small fraction of PhCl 2 ) [17]. Despite the low number of reported salts, this series presents a large variability in the anionic layer with three different A + cations (K + , NH 4 + or H 3 O + ) and seven different metal ions (M = Mn, Rh, Cr, Co, Al, Ru and Fe).
Analysis of the bond distances in the BEDT-TTF molecules indicates that the molecules forming the dimers (A-type) are completely ionized, whereas the isolated BEDT-TTF molecules (B-type) are neutral. The presence of totally ionized (BEDT-TTF) 2 2+ dimers surrounded by neutral BEDT-TTF molecules precludes the electron delocalization in the BEDT-TTF layers, and, accordingly, all the reported κ salts are semiconductors (Table 3) with activation energies in the range 140 to 245 meV in all cases ( Figure 13). Band structure calculations indicate that there is a very strong intradimer (A-A) interaction corresponding to the overlap of the BEDT-TTF + molecules of the almost eclipsed face-to-face dimers. The transfer integrals are around ten times larger than those between the neutral isolated molecules and the dimers (A-B). The transfer integrals between the isolated neutral molecules (B-B) are very small except for one case [2]. The presence of (BEDT-TTF) 2 2+ dimers leads to the formation of a full band and an empty band, separated by a band gap, resulting in the observed semiconducting behavior in all these orthorhombic salts [4].

Other Phases with BEDT-TTF and Oxalato Complexes
Besides the β" and κ phases, the intense research in the field has led to the synthesis of several other crystallographic phases with BEDT-TTF, different [M(C 2 O 4 ) 3 ] 3− anions (or the dianions [Ge(C 2 O 4 ) 3 ] 2− and [Cu(C 2 O 4 ) 2 ] 2− ), different solvents (including chiral ones) and even the inclusion of 18-crown-6 molecules. In this section, we will revise all these salts and classify them according to their composition and crystallographic phase.

BEDT-TTF Salts with [M(C 2 O 4 ) 3 ] 3− Anions and 18-Crown-6
The use of the crown ether 18-crown-6 in order to solubilize the precursor NH 4 + salts of different [M(C 2 O 4 ) 3 ] 3− anions has led to the inclusion of the 18-crown-6 molecules into the crystal structure in a total of eight salts ( Table 4). The first of these 18-crown-6-containing salts was also reported by P. Day's group in compound β"-(BEDT-TTF) 4 [34]. The structure shows alternating cationic and anionic layers (Figure 14a). The anionic layers are formed by two layers of [M(C 2 O 4 ) 3 ] 3− anions separated by a layer with the 18-crown-6 molecules and the crystallization water molecules (Figure 14a). Each of the two [M(C 2 O 4 ) 3 ] 3− layers contains only one enantiomer (Figure 14b). The cationic layers are formed by two independent BEDT-TTF molecules with the β" packing mode (Figure 14c) with chains of BEDT-TTF molecules following the sequence . . . AABB . . . with the same overlap mode (Figure 14d) as the superconducting monoclinic phase. This salt presents proton channels formed by the 18-crown-6 molecules containing H 3 O + cations (Figure 14e) and presents a proton conductivity above 10 −3 S/cm at room temperature. There is a Ga/NH 4 + derivative with the same structure: (68), also reported by P. Day's group [35]. Interestingly, both salts are metallic at room temperature but show metalinsulator transitions at 190 and 240 K, respectively. A second phase obtained with 18-crown-6 is the series of salts formulated as β"- ·18-crown-6, with M = Ir (65) [36], Ru (66) [36], Cr (67) [37] and Rh (69) [38] ( Table 3). The structure resembles that of the previously described com- (64) and NH 4 + /Ga (68). It also consists of alternating layers of BEDT-TTF molecules with the β" packing motif and double anionic layers separated by the 18-crown-6 molecules, although, now, the 18-crown-6 molecules are inserted in the hexagonal cavities formed by the [M(C 2 O 4 ) 3 ] 3− anions.
The Cr and Rh salts are superconductors with T c = 4.0-4.9 K and 2.7 K, respectively [37,38]. This phase is the second one with a [M(C 2 O 4 ) 3 ] 3− complex to show superconductivity and is the phase with single-donor packing with the widest gap between conducting layers. The Ir and Ru salts are not superconductors due to the shorter inter-layer M-M distance (worse 2D systems). The M-I transition in these salts may originate from changes in the anionic layer which induce changes in the BEDT-TTF layer, although Berezinski-Kosterlitz-Thouless (BKT) effects are not discarded [36]. The Ir salt is the first radical salt with a 5d tris(oxalato)metalate anion [36].

BEDT-TTF Salts with [M(C 2 O 4 ) 3 ] 3− Anions and Two Different Donor Layers
The use of larger non-planar solvent molecules and chiral ones has led to some nice examples of double-layered phases. These chiral and large non-planar solvent molecules do not fit in the hexagonal cavities of the anionic layers and are forced to cross the cavities, resulting in anionic layers with two different sides. This asymmetry of the two sides of the anionic layers induces two different packing modes in the BEDT-TTF molecules, resulting in salts with two different cationic layers. There are two different series of double-layered phases: the α,β", and the α,κ .
The α,β" phase has been observed in the series α The main difference with the monoclinic β" phase is that, now, there are two different sides since the large solvent molecules cross the hexagonal cavities with the X groups pointing to one of the two sides to generate a corrugated side, next to the β" layer, and a smoother face, next to the α layer ( Figure 15a). The polar groups of the solvent molecules point to the β" layer (purple circles in Figure 15a). There are two different cationic layers, with β" and α packing modes, alternating along the c axis. The β" layer ( Figure 15d) is formed by two independent BEDT-TTF molecules (A and B) packing in chains with the sequence . . . AABB . . . , as in the monoclinic β" phase. The only difference is that, now, the overlap between the BEDT-TTF molecules in the chain is eclipsed for AA and AB but shifted for BB. This overlap mode generates groups of four eclipsed BEDT-TTF molecules (Figure 15e). In the α layer, the molecules of consecutive stacks are tilted in opposite directions (Figure 15f). There are also two independent molecules (C and D) that form two different stacks containing only C or only D molecules packed with an eclipsed overlap (Figure 15g).
Compounds 72-74, also reported by P. Day's group [40], were the first examples of the α,β" phase (which is the third phase found in the ET 4 series, after the β" and κ ones). All the reported α,β" salts show relatively high room temperature conductivities but are semiconductors, although band structure calculations indicate that both BEDT-TTF layers should be metallic [40].
The other double-layer phase is the α,κ one, reported in two isostructural compounds with the same solvent molecule (1,2-dibromobenzene): α,κ -(ET) 4 Table 5). The structure of these salts also consists of alternating cationic and anionic layers (Figure 16a). There are also two alternating cationic layers with two different packing motifs: κ and α (Figure 16b,c). The κ layers are formed by four independent BEDT-TTF molecules (A-D) forming two different face-to-face dimers (AA and CC) surrounded by isolated BEDT-TTF molecules (B and D) (Figure 16b). The α layers also contain four independent BEDT-TTF molecules (E-H) forming three different stacks containing only E, only H and alternating F/G molecules (Figure 16c). The BEDT-TTF molecules are packed in an eclipsed way in the three stacks. The anionic layers present the same hexagonal disposition observed in the β" and κ phases and contain both enantiomers of the [M(C 2 O 4 ) 3 ] 3− anions (arranged in parallel rows, as observed in the κ phase), together with A + cations (Figure 16d). The 1,2-PhBr 2 solvent molecules are located in the cavities with the two Br atoms pointing towards the α layer (Figure 16a). This asymmetry in both faces of the anionic layers induces the crystallization of two different layers, as observed in the α,β" phase. Both salts are metallic down to low temperatures, although they do not show superconductivity. This metallic behavior is attributed to the α layers that show a homogeneous charge distribution in the BEDT-TTF molecules, in contrast to the κ layers that present the same charge localization observed in the other κ phases: (BEDT-TTF) 2 2+ dimers surrounded by neutral BEDT-TTF molecules.
By using the chiral solvent (R)-(-)-carvone with small solvent molecules such as CH 3 CN and CH 3 NO 2 , P. Day and L. Martin obtained a series of three isostructural chiral salts formulated as (BEDT-TTF) 3 [NaM(C 2 O 4 ) 3 ]·G with M/G = Al/CH 3 NO 2 (79) [43], Cr/CH 3 CN (84) [44] and Cr/CH 3 NO 2 (86) [45] that crystallize in the monoclinic chiral P2 1 space group ( Table 6). The structure of this 3:1 phase also consists of alternating BEDT-TTF and anionic layers (Figure 17b). The BEDT-TTF layers contain three independent BEDT-TTF molecules arranged in rows containing BC dimers alternating with A monomers tilted ca. 45 • with respect to the dimers (Figure 17a). This disposition resembles the κ phase, but now each dimer is surrounded by four monomers and four dimers (compared to six monomers in the κ phase). The anionic layer contains Na + cations and a single enantiomer of the [M(C 2 O 4 ) 3 ] 3− anions, forming a hexagonal honeycomb lattice (Figure 17c). The small solvent molecules (CH 3 CN or CH 3 NO 2 ) are located in the center of the hexagons. It is to be noted that now the hexagons are smaller and, accordingly, the organic layers contain only three BEDT-TTF molecules per hexagonal cavity. The inhomogeneous charge distribution with (BEDT-TTF) 2 2+ dimers and neutral monomers and the lack of short intermolecular contacts explain the semiconductor behavior observed in these salts [43][44][45].  [46]. Although these compounds were also prepared with a chiral solvent (Λ-carvone), there is a disorder between the Li + and M III centers that leads to a non-chiral P2 1 /c space group. The BEDT-TTF and the anionic layers show the same structure as compounds 79, 84 and 86 (Figure 17), although they are not, strictly speaking, isostructural since compounds 81 and 82 are not chiral.
L. Martin et al., using the same chiral solvent (R)-(-)-carvone, also obtained two isostructural chiral salts formulated as (BEDT-TTF) 3 [43] that crystallize in the orthorhombic P2 1 2 1 2 1 chiral space group. The structure of these two salts also consists of alternating anionic and cationic layers, but now the organic layers present a different packing with double rows of face-to-face BEDT-TTF dimers (A-B) alternating with single rows of isolated BEDT-TTF molecules (C) tilted ca. 90 • with respect to the dimers (Figure 18a). The charge distribution, determined from the bond distances in the BEDT-TTF molecules, indicates an inhomogeneous charge distribution with charges of 0.33, 0.63 and 0.88 for the three independent molecules. The anionic layer is similar to that observed in the P2 1 salts with the same stoichiometry ( Figure 17c) and also contains a single enantiomer of the [Cr(C 2 O 4 ) 3 ] 3− anion. In compound 80, there is an excess of the [Cr(C 2 O 4 ) 3 ] 3− anion that results in a different charge distribution in the BEDT-TTF molecules, although no electrical properties are reported. The isostructural NaCr derivative (85) [45] presents an excess of the ⊗ enantiomer, due to a disorder of the Cr and Na positions, and a slightly different charge distribution in the BEDT-TTF molecules since, now, the anionic layers have a charge of -2 per formula unit.  [47] and (ET) 3 [NaCr(C 2 O 4 ) 3 ]·EtOH (87) [44] that crystallize in the triclinic P1 chiral space group. Salt 83 was the first chiral BEDT-TTF salt with [M(C 2 O 4 ) 3 ] 3− anions and also the first radical salt in this series with Na + ions in the anionic layer [47]. The structure of these two salts also consists of alternating anionic and cationic layers, but now the organic layers present a very slightly different packing to those of the P2 1 salts with similar compositions but different solvent molecules (79, 84 and 86). In the P1 salts (83 and 87), the organic layers are also formed by rows containing alternating BEDT-TTF dimers and monomers, but now the monomers are tilted ca. 80 • with respect to the dimers (compared to ca. 45 • in the P2 1 salts, Figure 17b), and the offset inside the dimers is smaller in the P1 salts. Consecutive stacks are shifted to form a chessboard of monomers and dimers (Figure 18b).
Finally, when using dmf and the chiral solvent R-(-)-carvone, L. Martin et al. also obtained a 3:1 salt formulated as θ-(BEDT-TTF) 3 [NaCr(C 2 O 4 ) 3 ]·dmf (88) [44] that also crystallizes in the triclinic P1 chiral space group (as the CH 2 Cl 2 and EtOH derivatives) but shows a completely different packing in the BEDT-TTF layer. Thus, salt 88 has only two independent BEDT-TTF molecules that pack in segregated parallel stacks with a tilt angle close to 116 • between molecules of different stacks, giving rise to the so-called θ phase (Figure 18c). The two stacks show a similar shift of the BEDT-TTF molecules of half-rings along the stack. The anionic layer contains a single enantiomer of the [Cr(C 2 O 4 ) 3 ] 3− anions and Na + cations with the classical honeycomb structure [44]. The lack of a homogeneous charge distribution on the BEDT-TTF molecules and the absence of short intermolecular contacts in all these chiral salts preclude the existence of an electron delocalization, and, accordingly, all these 3:1 salts obtained with chiral solvents are semiconductors or insulators (Table 6).

Other Phases of BEDT-TTF Salts with [M(C 2 O 4 ) 3 ] 3− Anions
Besides all the above-described phases, the combination of BEDT-TTF donors with [M(C 2 O 4 ) 3 ] 3− anions has also led to other crystal phases with unusual stoichiometries and/or packings in the cationic and anionic layers ( Table 7).
The first of these salts is α -(ET) 9 [47]. These salts present a very original structure with four different layers following the sequence . . . ABCD . . . alternating along the c direction (Figure 19b). A-type layers contain BEDT-TTF molecules with a very unusual α packing (Figure 19a), B and D layers contain Na + cations and [Fe(C 2 O 4 ) 3 ] 3− anions with one single enantiomer in each layer ( Figure 19c) and C layers contain Na + cations and H 2 O molecules in rows parallel to the b axis (Figure 19d) [39]. In the very unusual α packing, the BEDT-TTF molecules form parallel columns as in the α and β" packings, but in one of every three columns, the BEDT-TTF molecules are tilted in the opposite direction (Figure 19a). There are, thus, two columns with one orientation (++) formed by three independent BEDT-TTF molecules (A-C) with the sequence . . . ABC . . . , and one with the opposite orientation (-) formed by two independent BEDT-TTF molecules (D and E) with the sequence . . . DDE . . . In both stacks (+ and -), the BEDT-TTF molecules are eclipsed. The α packing is, therefore, described as parallel columns following the sequence . . . /+/+/-/+/+/-/ . . . The anionic layer is also original since, now, the vertices of the hexagons contain alternating Fe(III) ions and (Na + ) 2 dimers and there is an additional [Fe(C 2 O 4 ) 3 ] 3− anion in the center of the hexagon with its oxalate ligands pointing towards the Na + dimers (Figure 19c). Interestingly, this unusual disposition has recently been found in a salt with the [Fe(NA) 3 ] 3− anion (NA = nitranilato ligand = C 6 O 4 (NO 2 ) 2 ) 2− ) [48], which is topologically identical to the oxalate ligand and also forms tris-chelato anionic complexes [M III (L) 3 ] 3− (L = anilato-type ligand) [49] and even [A + M III (L 3 )] 2− honeycomb layers [50], such as the ones here described for oxalate in previous sections. This original salt, 95, has an inhomogeneous charge distribution in the five independent BEDT-TTF molecules, and, therefore, it is a semiconductor. Salts α-(BEDT-TTF) 12 [Fe(C 2 O 4 ) 3 ] 2 ·15H 2 O (93) and α-(BEDT-TTF) 12 [Fe(C 2 O 4 ) 3 ] 2 ·16H 2 O (94) are two solvates that, despite differing only in one water molecule, show different unit cell parameters and, therefore, are not isostructural [51]. The structure of both salts consists of cationic and anionic layers alternating along the c axis. Both salts show the α packing mode in the BEDT-TTF layer, formed by parallel stacks with an alternating orientation of the BEDT-TTF molecules. The only difference between both salts is that there are twelve independent BEDT-TTF molecules in 93, compared to only three in salt 94. The anionic layers in both compounds are formed by isolated [Fe(C 2 O 4 ) 3 ] 3− anions surrounded by water molecules connected through H bonds in a 2D lattice (Figure 20a). The only difference is the presence of an extra water molecule in 94 and a slight twist in one oxalato ligand. Although the average charge per BEDT-TTF molecule is +0.5, there is an inhomogeneous charge distribution that results in a semiconducting behavior in both salts [51]. When using (NEt 4 ) 3 [Fe(C 2 O 4 ) 3 ] as a precursor salt and CH 2 Cl 2 as a solvent, the lack of small cations in the medium such as H 3 O + , Na + , K + or NH 4 + results in an original salt with no other cations in the structure. This salt, formulated as β"-(BEDT-TTF) 5 [Fe(C 2 O 4 ) 3 ]·2H 2 O·CH 2 Cl 2 (96) [52], shows cationic and anionic layers alternating along the c direction. The cationic layers of this 5:1 salt contain five independent BEDT-TTF molecules with the β" packing mode, as in the superconducting 4:1 phase. The anionic layer is quite original since it contains isolated [Fe(C 2 O 4 ) 3 ] 3− anions packed in parallel rows, surrounded by the H 2 O and CH 2 Cl 2 molecules (Figure 20b). There are two different anionic layers with only one enantiomer each. The analysis of the bond distances shows an inhomogeneous charge distribution with four BEDT-TTF molecules with a charge of +0.5 and one BEDT-TTF with a charge of +1. This inhomogeneous charge distribution explains the semiconducting behavior of this salt [52].
Another original salt is α-(BEDT-TTF) 6 [Fe(C 2 O 4 ) 3 ] (92) [17], which shows a 6:1 stoichiometry with an α packing type in the BEDT-TTF layer. Unfortunately, the crystals are not stable, precluding the determination of the anionic layer. The use of a large solvent such as 1,2,4-trichlorobenzene may be at the origin of the unusual structure of this salt [17].
The use of Li 3 [Fe(C 2 O 4 ) 3 ] as a precursor salt, besides the above-mentioned 3:1 phases (ET) 3 [46]. The structure of this salt consists of cationic and anionic layers alternating along the b axis. In this salt, the BEDT-TTF layer shows an original η packing mode (Figure 21a) formed by parallel stacks of eclipsed BEDT-TTF molecules (Figure 21b) tilted with respect to the stack direction (as in the β" phase), but now the orientation of the molecules changes every two stacks, following the sequence: . . . /+/+/-/-/ . . . (Figure 21a). This original packing can be considered as a mixture of the β" phase, where all the stacks have the same orientation: (+/+/+/+), and the α phase, where the stacks show alternating orientations: (+/-/+/-). The two stacks with a given orientation are equivalent and are formed by two independent BEDT-TTF molecules following the sequence . . . AB . . . (or . . . CD . . . in the stacks with the opposite orientation, Figure 21a). The BEDT-TTF molecules are eclipsed in both types of stacks (Figure 21b). The anionic layer is also very original: it shows a distorted hexagonal packing of the [Fe(C 2 O 4 ) 3 ] 3− anions and the Li + cations. This distortion is due to the presence of a water molecule in the layer connected to the Li + ions, resulting in elongated Li(H 2 O) + entities that occupy alternating vertices of the hexagons (Figure 21c). The elongated hexagonal cavities are occupied by disordered CH 2 Cl 2 solvent molecules. The BEDT-TTF molecules show an inhomogeneous charge distribution, and, accordingly, this original salt is a semiconductor ( Table 7) [46].

BEDT-TTF Salts with [Ge(C 2 O 4 ) 3 ] 2− and [Cu(C 2 O 4 ) 2 ] 2− Dianions
Interestingly, besides all the above-mentioned M(III)-based [M(C 2 O 4 ) 3 ] 3− anions, there are also two oxalate-based dianions that have been combined with BEDT-TTF (Table 8) anion, also with a −2 charge, but with a 2:1 stoichiometry and a square planar geometry (Figure 1b). As we will show here, the change in the charge in the [Ge(C 2 O 4 ) 3 ] 2− anion leads to very important changes in the structure and properties of these radical salts.
The only known salt with BEDT-TTF and a dimeric anion is: (BEDT-TTF) 4 [Fe 2 (C 2 O 4 ) 5 ] (105), also reported by P. Day's group [58]. In this radical salt, the dimeric anion [  (Figure 1c). There are two independent BEDT-TTF molecules, both with a +1 charge, in agreement with the 4:1 stoichiometry and the -4 charge of the anion, resulting in a semiconducting salt with a high activation energy and a low room temperature conductivity (Table 9) [58]. The oxalate-bridged Fe(III) dimer in this salt shows, as expected, a weak antiferromagnetic interaction with J = −3.44 cm −1 , similar to that found in the TTF and TM-TTF salts with the same dimer that will be described in the next section [59,60].  Salt (BEDT-TTF) 3 [MnCr(C 2 O 4 ) 3 ] (106) was the first molecular compound showing metallic conductivity and a ferromagnetic long-range ordering, with an ordering temperature T curie of ca. 5.5 K (Figure 30a) [7]. This salt presents magnetoresistance below ca. 10 K (Figure 30b), indicating that both sublattices are quasi-independent from the electronic point of view. Attempts to change the metal ions and the donor molecules and the use of other solvents led to different radical salts [61], including isostructural compounds (ET) 2.53 [MnCr(C 2 O 4 ) 3 ]·CH 2 Cl 2 (103) and (ET) 2.53 [MnRh(C 2 O 4 ) 3 ]·CH 2 Cl 2 (104) that are also metallic and show long-range magnetic ordering, although in compound 104, there is a broad metal to semiconducting transition at around 100 K [8]. The use of this honeycomb layer with other TTF-type donors will be revised in the next section.  (Figure 31a). The anionic layer is also a honeycomb hexagonal lattice formed by Cu(II) ions in the vertices of the hexagons and oxalate bridges as the sides of the hexagons. The crystallization solvent molecules are located in the hexagonal cavities. As a result of the expected Jahn-Teller distortions in the Cu(II) ions, the hexagons are quite distorted (Figure 31c). The properties of the CH 3 OH derivative (the only one reported to date) show that this radical salt is a semiconductor with an activation energy of 50 meV and presents a moderate antiferromagnetic Cu-Cu interaction through the oxalate bridge [62]. The tuneability of these metallic magnets was evidenced by the synthesis of different derivatives of the metallic ferromagnet (BEDT-TTF) 3 [MnCr(C 2 O 4 ) 3 ] (106). One of these derivatives is the salt (BEDT-TTF) 3 [CoCr(C 2 O 4 ) 3 ]·CH 2 Cl 2 , which showed an ordering temperature of 9.2 K and a high electrical conductivity of 1 S/cm at room temperature, although no metallic behavior was observed, maybe because the measurements were performed on a pressed pellet as no big single crystals could be obtained [63]. The other derivatives, prepared with different TTF-type donors, will be revised in the next section.

Radical Salts of Metal-Oxalate Anions with Other TTF-Type Donor Molecules
Although BEDT-TTF is, by far, the most used donor molecule with metal-oxalato complexes, with more than one hundred reported salts (see Tables 1-9), other TTF-type donors have been combined with metal-oxalato complexes and lattices (Scheme 1). Here, we will revise all these salts, prepared with donors such as tetrathiafulvalene (TTF), tetramethyl-tetrathiafulvalene (TM-TTF), bis(ethylenediseleno)tetrathiafulvalene (BEDS-TTF = BEST), bis(ethylenedithio)tetraselenafulvalene (BEDT-TSF = BETS) and 4,5-bis((2S)-2-hydroxypropylthio)-4 ,5 -(ethylenedithio)tetrathiafulvalene (DMPET).   [64]. Its structure consists of alternating cationic and anionic layers parallel to the ab plane (Figure 33a). There are two different anionic layers each containing a single enantiomer. The anionic layers contain the [M(C 2 O 4 ) 3 ] 3− anions and the solvent molecules with a hexagonal arrangement (Figure 33b), although different to the one observed in the BEDT-TTF salts of previous sections since, now, there is no extra A + cation orienting the terminal O atoms of the oxalate ligands. The TTF layers are also original: they contain three independent TTF molecules (A-C) arranged in AA dimers and chains following the sequence . . . BBCC . . . (Figure 33c). The TTF molecules of consecutive layers run in opposite directions (Figure 33d). This salt shows a paramagnetic behavior with contributions from the [Ru(C 2 O 4 ) 3 ] 3− S = 1 2 anions and from the TTF molecules that show a charge localization, in agreement with the semiconducting behavior observed in this salt [64]. Salt 115 shows alternating cationic and anionic layers parallel to the ab plane (Figure 34b). The cationic layers contain two independent TTF molecules (A and B). A-type TTF molecules form chains running parallel to the b axis, whereas B-type molecules are monomers located between the chains. The packing of the TTF molecules is unique because not only the planes of the A-and B-type molecules are orthogonal but also their long molecular axis (Figure 34a,b). The [Fe 2 (C 2 O 4 ) 5 ] 4− dimer is formed by the fusion of one ⊗ and one Λ monomer through a bis-bidentate oxalate ligand (Figure 1c). These dimers are packed in rows along the b axis with PhCH 3 molecules located between the rows (Figure 34c). The electrical properties show that this salt is a semiconductor (Table 10), in agreement with the presence of neutral isolated TTF molecules. The magnetic properties show that the [Fe 2 (C 2 O 4 ) 5 ] 4− dimer presents a weak antiferromagnetic Fe-Fe coupling with J = −3.57 cm −1 , through the oxalate bridge, as observed in the other salts prepared with this dimer [58,60].  [65,66]. This salt was the first one prepared with TTF and any oxalate complex and contains the then-unknown {Mn(H 2 O) 2 [Cr(C 2 O 4 ) 3 ] 2 } 4− trimer. The structure shows alternating cationic and anionic layers parallel to the ab plane ( Figure 35b). There are three independent TTF molecules forming orthogonal AA and BC dimers (similar to the κ-phase in BEDT-TTF) [67], although now the dimers form stacks separated by water molecules (Figure 35a (Figure 1d). These trimers are packed in a rhombic disposition with strong H bonds between the two coordinated water molecules and the terminal O atoms of the oxalato ligands of neighboring trimers (Figure 35c).
The magnetic properties show the presence of a weak ferromagnetic Cr-Mn coupling with J = 1.08 cm −1 , although the salt is a semiconductor since the TTF molecules are completely oxidized and there are no short intermolecular contacts (Table 10) [65,66]. An additional interest of this salt is the possibility to change the anion by simply changing Cr(III) to Fe(III) and Mn(II) to other metal ions such as Fe(II), Co(II), Ni(II), Cu(II) and Zn(II). In this way, it was possible to prepare the series of salts (TTF) 4 [66]. The use of different metal ions in these series allowed a modulation of the magnetic coupling that is weak ferromagnetic for the Cr(III) derivatives and weak antiferromagnetic for the Fe(III) ones [66]. The then-unknown [Fe 2 (C 2 O 4 ) 5 ] 4− dimer was also obtained with the donor tetramethyltetrathiafulvalene (TM-TTF) and published with the TTF salt described above [60]. This salt, formulated as (TM-TTF) 4 (Figure 36b). The TM-TTF stacks are formed by two independent TM-TTF molecules following the sequence . . . AABB . . . Although the molecular planes of the donor molecules are parallel, the long axis of the molecules is slightly tilted (Figure 36c). As the TM-TTF molecules are completely oxidized, the salt is a semiconductor with a high activation energy (Table 10). As observed in the TTF and BEDT-TTF salts of the same [Fe 2 (C 2 O 4 ) 5 ] 4− anion, in compound 116, this anion shows a weak antiferromagnetic Fe-Fe coupling with J = −3.69 cm −1 , through the oxalate bridge, similar to those of the other salts with this dimer [58,60].
The structure of salts (BEST) 4 [M(C 2 O 4 ) 3 ]·PhCOOH·H 2 O, with M = Cr (109) and Fe (110) consists of alternating cationic and anionic layers parallel to the ab plane (Figure 37b). The cationic layer is formed by four independent BEST molecules (A-D) packed with the β packing mode (Figure 37a) in parallel stacks, with a step every four BEST molecules (Figure 37b), following the sequence . . . ABCDDCBA . . . (Figure 37a). The anionic layers show an original arrangement with pairs of anions with different chirality separated by PhCOOH molecules, also arranged in pairs, and water molecules forming moderate H bonds (Figure 37c). A-and C-type BEST molecules are completely ionized, whereas B and D molecules bear a charge of +1/2. The presence of totally ionized BEST molecules is responsible of the semiconducting behavior of these two salts (Table 10) [68]. The magnetic properties show that there is no noticeable contribution from the organic layers, and the salts behave as isolated S = 3/2 or S = 5/2 ions for 109 and 110, respectively.  [68]. The structure of salts 111 and 113 also consists of alternating cationic and anionic layers parallel to the ab plane (Figure 38b). The cationic layer contains two independent BEST molecules (A and B) packed in parallel stacks following the sequence . . . ABAB . . . , also with the β packing mode (Figure 38a). The anions appear with a disorder since they are located close to an inversion center. When only one of the two possible locations is considered, the anions show zigzag chains with alternating chirality along the c axis (Figure 38c). The A-type BEST molecules are completely ionized, whereas B molecules bear a charge of +1/2, in agreement with the stoichiometry and the anionic charge. The presence of alternating totally ionized BEST molecules results in a semiconducting behavior for these two salts ( Table 10). The magnetic properties are, as expected, similar to those of salts 109 and 110 since the Cr(III) S = 3/2 and Fe(III) S = 5/2 ions are isolated from the magnetic point of view and there is no noticeable contribution from the organic layers [68]. The last salt containing the donor BEST is a very original 9:2 phase, formulated as (BEST) 9 [Fe(C 2 O 4 ) 3 ] 2 ·7H 2 O (112) [68]. This salt also presents alternating cationic and anionic layers parallel to the ab plane (Figure 39b), and the cationic layer also shows the β packing mode (Figure 39a). The main difference with the other BEST layers is the presence of five independent BEST molecules (A-E) that form two different chains (I and II) with steps every three BEST molecules (Figure 39b). Chains of type I are formed by D and E molecules packed following the sequence . . . DED . . . , whereas chains of type II contain three independent molecules (A-C) packed following the sequence . . . ABC . . . There are two chains of type II and one of type I alternating in the direction perpendicular to the stacks following the sequence . . . /I/II/II/ . . . (Figure 39a). The [Fe(C 2 O 4 ) 3 ] 3− anions are located to form dimers with the opposite chirality, and the crystallization water molecules are located in between the anions forming several H bonds with the anions and with the water molecules (Figure 39c). The inhomogeneous charge distribution of the five BEST-TTF molecules is at the origin of the semiconducting behavior observed in this salt (Table 10). As in the previous BEST salts, the magnetic properties correspond to isolated [Fe(C 2 O 4 ) 3 ] 3− anions with no noticeable contribution from the organic layers [68].  [70].
Compound (BETS) 3 [Cu 2 (C 2 O 4 ) 3 ]·2CH 3 OH (117) contains alternating cationic and anionic layers parallel to the ab plane (Figure 40b) [69]. The organic layer presents the θ 21 phase with two stacks tilted in one direction and one stack in the opposite direction (Figure 40a), also observed in the BEDT-TTF derivatives with the same [Cu 2 (C 2 O 4 ) 3 ] 2− layer (compound 107, see above). The anionic sublattice in 117 is a hexagonal honeycomb layer that shows important distortion due to the Jahn-Teller effect on the Cu(II) ions (Figure 40c). The electrical properties show that the BETS salt is a much better electrical conductor than the BEDT-TTF one with a higher room temperature conductivity and a metallic behavior down to 180 K (the BEDT-TTF salt is semiconducting). The improved electrical properties are attributed to the enhanced intermolecular interactions when S is substituted by Se. The Cu···Cu interaction is also antiferromagnetic, as in the BEDT-TTF derivative [69]. The other salt prepared with BETS is (BETS) 3 [MnCr(C 2 O 4 ) 3 ]·CH 2 Cl 2 (119) [70]. This salt is the first one prepared with the BETS donor and an oxalate complex and also shows alternating cationic and anionic layers parallel to the ab plane. The cationic layer shows the α packing mode, where consecutive stacks are tilted in opposite directions (Figure 41a). The anionic layer is identical to the one observed in the BEDT-TTF derivative: it shows the classical honeycomb structure with alternating Mn(II) and Cr(III) centers connected through oxalato bridges. This salt is metallic down to 150 K ( Figure 41b) and shows the expected ferromagnetic long-range order below 5.3 K (Figure 41c). In this case, the change of S to Se in the donor molecule did not improve the electrical properties [70]. Finally, although its structure could not be determined, the [MnCr(C 2 O 4 ) 3 ] − lattice has also been combined with BEST to obtain a radical salt with a formula of (BEST) 3 [MnCr(C 2 O 4 ) 3 ] that shows a low room temperature conductivity of 10 −6 S/cm and a ferromagnetic order at T c = 5.6 K [61]. The change of Mn(II) to Co(II) in this honeycomb lattice leads to the isostructural lattice [CoCr(C 2 O 4 ) 3 ] − with a higher ordering temperature that has also been combined with the donors BEST and BETS to prepare salts (BEST) 3 [CoCr(C 2 O 4 ) 3 ] and (BETS) 3 [CoCr(C 2 O 4 ) 3 ]. These two salts show ferromagnetic ordering temperatures of 10.8 and 9.2 K, respectively, and show room temperature conductivities of 10 −6 S/cm for the BEST salt and 2.3 S/cm for the BETS one [61].  Finally, although there is no structural report, the honeycomb lattice [MnCr(C 2 O 4 ) 3 ] − has also been combined with other donors such as bis(ethylenethio)tetrathiafulvalene (BET), bis(methylenedithio)tetrathiafulvalene (BMDT-TTF), bis(ethylenedioxo)tetrathiafulvalene (BEDO-TTF) and bis(ethylenedithio)trithiaselenafulvalene (ET-1Se), whereas the isostructural [CoCr(C 2 O 4 ) 3 ] − lattice was combined with the donor BET. The ferromagnetic ordering temperatures are in the range 5.0-5.6 K for the MnCr lattices and 13.0 K for the CoCr one. The room temperature conductivities, measured on pressed pellets, are quite high, in the range 0.1-21 S/cm [61].

Conclusions
The seminal work of Peter Day's group in 1995 [2,3] with the synthesis of the first paramagnetic superconductors in the family of salts β"-(BEDT-TTF) 4 [AM(C 2 O 4 ) 3 ]·G (A = monocation, M = trivalent metal ion and G = solvent) is at the origin of the largest series of molecular metals and superconductors prepared to date. This series constitutes a paradigmatic example of the tuneability of molecular materials, as shown by the large number of related compounds prepared by changing: (i) the A + cation with other monocations (H 3 O + , NH 4 + , K + , Na + and Li + ) or even with dications (Mn 2+ , Co 2+ and Cu 2+ ); (ii) the TTF-type donors (TTF, TM-TTF, BEST, BETS, BET, BEDO, BMDT-TTF, etc.); (iii) the M(III) ion (Fe, Cr, Ga, Mn, Rh, Ru, Al, Co and Ir), or even M(IV) as Ge(IV) and M(II) as Cu(II); and (iv) the solvent molecule (PhCN, PhNO2, PhF, PhCl, PhBr, PhI, PhCOOH, H 2 O, CH 2 Cl 2 , CH 3 OH, py, dmf, Cl-Py, Br-py, etc.). These relatively easy-to-perform modifications have led to the synthesis of more than one hundred and twenty radical salts with oxalate complexes combining electrical properties (semiconductors, metals and superconductors) with magnetic properties (paramagnetism, ferro-, ferri-and antiferromagnetic couplings and even long-range magnetic ordering). These series constitute, by far, the largest family of multifunctional molecular materials prepared to date.
No doubt, the research on this type of salt was boosted by the preparation by P. Day's group of the first molecular paramagnetic superconductor in 1995 [2,3]. Since then, several groups have prepared and characterized many different salts to try to understand the key aspects of these magnetic superconductors and to improve the magnetic and/or the electrical properties. Furthermore, this search has allowed the synthesis of magnetic conductors with other properties such as chirality or proton conductivity.
As a homage to the legacy of the late Peter Day, we have shown, here, the so-called Day series of radical salts formulated as β"-(BEDT-TTF) 4 [AM(C 2 O 4 ) 3 ]·G, with more than fifty reported structures to date, and of the closely related series prepared with other oxalate complexes and different donors. Many of these related series were also initiated by Peter Day's group, and still, twenty-five years later, most of the most active researchers in this field were part of his group in the past.