The Coordination Chemistry of Imidomethanedithiolate Di-anions: A Structural Comparison with Their Dithiocarbamate Analogs

: A review of the coordination chemistry along with the structural features of heavy element complexes of dithiocarbimate di-anions in the form of [(R)C=NCS 2 ] 2 − for R = CN, alkyl, and aryl are described. This class of compound is far less studied compared with the well-explored dithiocarbamate mono-anions formulated as [R(R’)NCS 2 ] − for R/R’ = H, alkyl, and aryl. The coordination chemistry of dithiocarbimate di-anions is dominated by a S,S-chelating mode; rare examples of alternative modes of coordination are evident. When comparisons are available, the structural motifs adopted by metal dithiocarbimate complexes match those found for their dithiocarbamate analogs, with only small, non-systematic variations in the M–S bond lengths.


Introduction
The complexation of the most well-known 1,1-dithiolate anion, dithiocarbamate, with a general formula of [R(R')NCS 2 ] − for R/R' = H, alkyl, and aryl, by heavy elements such as transition metals, main group elements and lanthanides and actinides remains a mainstay in coordination chemistry. This assertion is no better vindicated than by a search of the Cambridge Structural Database [1], which reveals over 4300 entries containing dithiocarbamate anions, esters, or ligands. This importance is also borne out by the regular appearance of authoritative bibliographic reviews of the coordination chemistry of dithiocarbamates [2][3][4][5][6][7], reviews of specific aspects of their coordination and supramolecular chemistry [8][9][10][11][12][13], and recently of their mono-and di-seleno analogs [14].
A key feature of the coordination chemistry of dithiocarbamate anions is the significant contribution of resonance structure (III) (Figure 1). Structure (III) contributes 40% to the overall electronic structure of dithiocarbamate anions, while resonance forms (I) and (II) each contributes 30%. This electronic distribution readily explains the ability of dithiocarbamate ligands to complexify heavy element centers and to stabilize higher-than-usually observed oxidation states [2][3][4][5][6][7]. While not the focus of the present survey, canonical form (III) also accounts for the ability of the MS 2 C chelate ring with significant metalloaromatic characteristics [15] to function as an acceptor for structure-directing C-H···π(chelate ring) interactions with significant energy of stabilization in, for example, over a third (37%) of the crystals of homoleptic nickel dithiocarbamate complexes [16][17][18]. By contrast to the well-developed dithiocarbamate chemistry, the coordination chemistry of the closely related dithiocarbimate ligands (i.e., [(R)C=NCS 2 ] 2− ) is significantly less well-explored. The key differences between dithiocarbamate and dithiocarbimate ligands is the presence of just one N-bound substituent in the latter and the consequent increase in the formal sulfur-bound negative charge. This results in electronic structure (IV), shown in Figure 1. The aim of the present survey is to ascertain the structural motifs formed by dithiocarbimate ligands bound to heavy elements and to relate these, where possible, to those formed by dithiocarbamate analogs in terms of structural motif and bonding characteristics.

General Aspects of Dithiocarbimate Ligands
Synthetic procedures to form dithiocarbimate ligands mimic, at least to a first approximation, those employed to form dithiocarbamate ligands. Thus, while dithiocarbamate ligands may be isolated from the reaction of an appropriate amine with carbon disulfide in the presence of base, a common dithiocarbimate ligand, namely dipotassium cyanodithioimidocarbonate, can be formed from the reaction of cyanamide (aminomethanenitrile), carbon disulfide, and potassium ethoxide [19]. Other routes are from the reaction of dipotassium cyanamide and carbon disulfide [20] and the reaction of xanthane hydride and a base [21]. The synthesis of dithiocarbimate complexes can also be achieved in a manner similar to the in situ preparation of dithiocarbamate complexes (i.e., without the isolation of the original salt). For example, (Et 3 P) 2 Pt[S 2 C=N(CH 2 Ph)] was prepared from the reaction of (Et 3 P) 2 PtC1 2 with the primary amine, PhCH 2 NH 2 , and carbon disulfide. This product can be converted to the dithiocarbamate species upon the addition of HCl to form {(Et 3 P) 2 Pt[S 2 C=N(H)CH 2 Ph]}Cl [22]. Another general procedure for the formation of dithiocarbimate complexes involves the conversion of a pre-formed dithiocarbamate complex to the dithiocarbimate derivative. As an example of this method, [NBu 4 ] 2 {Pd[S 2 C=N(C 6 H 4 NH 2 -4)] 2 } [23] was prepared from the reaction of Pd[S 2 CN(H)C 6 H 4 NH 2 -4] 2 with excess NaOH, whose function is to abstract the S 2 CN-bound proton from the dithiocarbamate complex, followed by the addition of tetrabutylammonium bromide to yield the dithiocarbimate complex.
The infrared spectra of dithiocarbimate complexes display a characteristic ν(C=NR) stretch in the broad region of 1310-1690 cm −1 (mean = 1543 cm −1 ; standard deviation = 61 cm −1 ). The frequency of the C=N stretch is lowest for the uncomplexed ligand [(R)C=NCS 2 ] 2− , with the free ligand K 2 [S 2 C=N(CN)] displaying a ν(C=N) band at 1315 cm −1 [19]. By contrast, the highest stretching frequencies are found in complexes when the dithiocarbimate ligand is bridging, such as in [CpMo(CO) 2 Mn(CO) 3 ][S 2 C=N(tBu)], where the C=N stretching band is observed at 1684 cm −1 [24]. When these extreme cases are excluded, the C=N bands are found in the narrower window of 1480-1590 cm −1 , with the mean being 1557 cm −1 .
The majority of dithiocarbimate complexes for which 1 H NMR data are available are either palladium(II) or platinum(II) complexes. In the 1 H NMR of these complexes, the protons of the α-carbon atom (HCN=CS 2 ), where present, are found in the 3.10-5.60 ppm region, with a mean of 4.56 ppm, which compares to the 1.80-4.55 ppm region recently reported for related platinum group metal dithiocarbamate complexes [7]. As with the dithiocarbamates, the shifts were generally observed to be greater in the palladium(II) com-Inorganics 2021, 9, 71 3 of 13 plexes than in the platinum(II) complexes. The quaternary carbon atom RNCS 2 resonates over the broad range of 140-230 ppm, with the greatest shifts being found in the cationic complexes. The free ligand K 2 [S 2 C=N(CN)], for example, has a reported shift of 228 ppm. More typically, the shifts are observed in the region of 140-185 ppm (mean = 171 ppm), which is somewhat lower than that observed in dithiocarbamate complexes [7].
A search of the Cambridge Structural Database (CSD, version 5.42) [1] with the aid of ConQuest (version 2.0.4) [25] was conducted, whereby structures bearing a S 2 C=NC moiety, the key characteristic of a dithiocarbimate residue, complexed to a heavy element were identified; no other restrictions were applied. This retrieved 43 distinct "hits". The crystallographic diagrams were generated using the data in the Crystallographic Information File deposited for each structure using the DIAMOND program [26].

Coordination Chemistry of Dithiocarbimate Di-Anions
The following discussion is arranged so that the available crystal structures are described in order of the Group of the Periodic Table they belong to. For completeness, structures of the main group element compounds are also included.

Cr Group
There are five dithiocarbimate structures in this category, and these adopt four quite distinct structural motifs. This category also includes the description of the sole example of a heterometallic complex. The first structure to be described is a di-nuclear molybdenum complex found in the salt [NEt 4 ] 2 {Mo(=S)(µ 2 -S)[S 2 C=N(Ph)]} 2 , isolated as a DMF hemisolvate (1) [27]. As shown in Figure 2a, the di-anion features two molybdenum(V) centers bridged by two µ 2 -sulfido ligands, and each metal atom is terminally bound by thioxide and chelating phenyl-dithiocarbimate di-anions (Mo-S: 2 × 2.42 and 2.46 Å, respectively). While the di-anion lacks crystallographic symmetry, the Mo=S residues are approximately syn, with the pseudo S=Mo···Mo=S torsion angle being 6.6 • . The penta-coordinated molybdenum atoms defined by the S 5 donor set approximate square pyramidal geometries with the thioxide-sulfur atoms in the apical positions, and the Mo . . . Mo separation is 2.86 Å. The second molybdenum complex is also di-nuclear, but it is organometallic: [CpMo(SMe)] 2 [S 2 C=N(2,6-Me 2 C 6 H 3 )] (2) [28]. The two molybdenum centers are bridged by a pair of µ 2 -methylthiolate ligands, and each is also bound by a Cp ligand (Figure 2b). The 2,6-dimethylphenyldithiocarbimate di-anion is also µ 2 -bridging, as each sulfur atom forms a bond to both molybdenum atoms, with the bridges being symmetric (Mo-S: 2.45 and 2.46 Å for the first Mo atoms and 2 × 2.47 Å for the second).
The next two structures are tungsten species: (Ph 2 P(CH 2 ) 2 P(Ph)C 6 [29]. The molecular structure of the complex in 3 is represented in Figure 2c. Here, the tungsten is tetra-coordinated by the four phosphorus atoms of the meso-1,2-C 6 H 4 (PPhCH 2 CH 2 PPh 2 ) 2 ligand as well as the carbon atom of the isocyanide ligand, and then it is also S,S-chelated by the 4-tolyldithiocarbimate ligand (W-S: 2.55 and 2.56 Å). The resulting seven-coordinate geometry is based on a pentagonal bipyramid, with the axial positions occupied by the isocyanide-C and one of the sulfur atoms. As noted from Figure 2c, the terminal 4-tolyl ligand may be considered anti with respect to the axial-S atom. In the 4-chlorophenyl derivative 4, three independent molecules comprise the asymmetric unit. While for each of these the coordination geometry is essentially the same as that shown in Figure 2c, the terminal 4-chlorophenyl group is described as being syn to the axially bound sulfur atom. The heterometallic complex, comprising molybdenum and manganese atoms, is formulated as [CpMo(CO) 2 Mn(CO) 3 ][S 2 C=N(tBu)] (5) [24] and bears a resemblance to the structure of 2. As is apparent from Figure 2d, the molybdenum atom is coordinated by two carbonyl ligands and a Cp, while the manganese center carries three carbonyls. The t-butyl-dithiocarbimate ligand is µ 2 -bridging, as each sulfur atom is connected to both the molybdenum and manganese atoms (Mo-S: 2.46 and 2.47 Å and Mn-S: 2 × 2.33 Å).  [29]. The molecular structure of the complex in 3 is represented in Figu 2c. Here, the tungsten is tetra-coordinated by the four phosphorus atoms of the meso-1 C6H4(PPhCH2CH2PPh2)2 ligand as well as the carbon atom of the isocyanide ligand, an then it is also S,S-chelated by the 4-tolyl-dithiocarbimate ligand (W-S: 2.55 and 2.56 Å The resulting seven-coordinate geometry is based on a pentagonal bipyramid, with t axial positions occupied by the isocyanide-C and one of the sulfur atoms. As noted fro Figure 2c, the terminal 4-tolyl ligand may be considered anti with respect to the axial atom. In the 4-chlorophenyl derivative 4, three independent molecules comprise t asymmetric unit. While for each of these the coordination geometry is essentially the sam as that shown in Figure 2c, the terminal 4-chlorophenyl group is described as being sy to the axially bound sulfur atom. The heterometallic complex, comprising molybdenum and manganese atoms, is fo mulated as [CpMo(CO)2Mn(CO)3][S2C=N(tBu)] (5) [24] and bears a resemblance to t structure of 2. As is apparent from Figure 2d, the molybdenum atom is coordinated two carbonyl ligands and a Cp, while the manganese center carries three carbonyls. T t-butyl-dithiocarbimate ligand is µ 2-bridging, as each sulfur atom is connected to both t molybdenum and manganese atoms (Mo-S: 2.46 and 2.47 Å and Mn-S: 2 × 2.33 Å ).

Mn Group
Both structures in this category contain rhenium. In the anion (Figure 3a), the rhenium center is chelated by ea of the tetra-sulfide and methyl-dithiocarbimate (Re-S: 2 × 2.34 Å ) ligands, with the fin The color code in this and subsequent diagrams is as follows: primary metal center = orange; manganese = brown; sulfur = yellow; phosphorus = pink; oxygen = red; nitrogen = blue; and carbon = gray. Hydrogen atoms are omitted for clarity. In (c), only the ipso carbon atoms of the phosphorus-bound phenyl groups are shown.

Mn Group
Both structures in this category contain rhenium. In the anion of [Ph 4 P]{(S 4 )Re(=S)The heterometallic complex, comprising molybdenum and manganese atoms, is formulated as [CpMo(CO) 2 Mn(CO) 3 ][S 2 C=N(tBu)] (5) [24] and bears a resemblance to the structure of 2. As is apparent from Figure 2d, the molybdenum atom is coordinated by two carbonyl ligands and a Cp, while the manganese center carries three carbonyls. The t-butyldithiocarbimate ligand is µ 2 -bridging, as each sulfur atom is connected to both the molybdenum and manganese atoms (Mo-S: 2.46 and 2.47 Å and Mn-S: (Figure 3a), the rhenium center is chelated by each of the tetra-sulfide and methyldithiocarbimate (Re-S: 2 × 2.34 Å) ligands, with the final (apical) site in the square pyramidal geometry occupied by the terminally bound thioxide ligand. Greater congestion is evident in the structure of [(2,6-(iPr) 2 Figure 3b). The rhenium atom in 7 is chelated by the bulky β-diketiminate ligand and terminally coordinated by the N-bound imide ligand, with the N 3 S 2 donor set completed by the chelating (2,4,6-trimethylphenyl)-dithiocarbimate ligand (Re-S: 2.33 and 2.35 Å). The coordination geometry is based on a square pyramid with the imide-nitrogen atom in the apical position.

Fe Group
The only structure in this category is [(CO)2Fe(dppm)Fe(CO)2][S2C=N(4-tolyl)] (8) [32] (Figure 4), where dppm is Ph2PCH2PPh2. The di-nuclear structure resembles those previously described by 2 and 5 ( Figure 2). The iron atoms are bridged by the bidentate bridging dppm ligand, and each also carries two carbonyl ligands. Each sulfur atom of the 4-tolyl-dithiocarbimate ligand connects to both iron atoms (Fe-S: 2.26 and 2.27 Å and 2 × 2.30 Å ). The small disparity in the Fe-S bond lengths is related to the trans effect, with the longer distances having the sulfur atoms approximately trans to the carbonyl ligands.

Co Group
The crystallographic asymmetric unit of the organometallic {Cp*Rh[S2C=N(CN)]}2 compound comprises two independent di-nuclear molecules, each with two-fold symmetry, and two chloroform molecules of solvation (9) [19]. One of the independent dinuclear molecules is shown in Figure 5 and shows that the independent cyano-dithiocarbimate chelates one rhodium atom (Rh-S: 2.36 and 2.40 Å ) and simultaneously bridges the rhodium atom related by two-fold symmetry (Rh-S: 2.42 Å ); the longer Rh-S bond lengths involve the sulfur-bridging atom. A consequence of the symmetry is that the two cyano-dithiocarbimate ligands lie on the same side of the central Rh2S2 core.
The coordination geometry is based on a square pyramid with the imide-nitrogen atom in the apical position.

Fe Group
The only structure in this category is [(CO)2Fe(dppm)Fe(CO)2][S2C=N(4-tolyl)] (8) [32] (Figure 4), where dppm is Ph2PCH2PPh2. The di-nuclear structure resembles those previously described by 2 and 5 ( Figure 2). The iron atoms are bridged by the bidentate bridging dppm ligand, and each also carries two carbonyl ligands. Each sulfur atom of the 4-tolyl-dithiocarbimate ligand connects to both iron atoms (Fe-S: 2.26 and 2.27 Å and 2 × 2.30 Å ). The small disparity in the Fe-S bond lengths is related to the trans effect, with the longer distances having the sulfur atoms approximately trans to the carbonyl ligands.

Co Group
The crystallographic asymmetric unit of the organometallic {Cp*Rh[S2C=N(CN)]}2 compound comprises two independent di-nuclear molecules, each with two-fold symmetry, and two chloroform molecules of solvation (9) [19]. One of the independent dinuclear molecules is shown in Figure 5 and shows that the independent cyano-dithiocarbimate chelates one rhodium atom (Rh-S: 2.36 and 2.40 Å ) and simultaneously bridges the rhodium atom related by two-fold symmetry (Rh-S: 2.42 Å ); the longer Rh-S bond lengths involve the sulfur-bridging atom. A consequence of the symmetry is that the two cyano-dithiocarbimate ligands lie on the same side of the central Rh2S2 core.

Co Group
The crystallographic asymmetric unit of the organometallic {Cp*Rh[S 2 C=N(CN)]} 2 compound comprises two independent di-nuclear molecules, each with two-fold symmetry, and two chloroform molecules of solvation (9) [19]. One of the independent di-nuclear molecules is shown in Figure 5 and shows that the independent cyano-dithiocarbimate chelates one rhodium atom (Rh-S: 2.36 and 2.40 Å) and simultaneously bridges the rhodium atom related by two-fold symmetry (Rh-S: 2.42 Å); the longer Rh-S bond lengths involve the sulfur-bridging atom. A consequence of the symmetry is that the two cyanodithiocarbimate ligands lie on the same side of the central Rh 2 S 2 core.

Ni Group
By far, the most represented group of elements complexed by dithiocarbimate ligands is that of the nickel-triad elements. A list of structures and key geometric parameters for these are given in Table 1. The least-represented structures are those containing nickel, but the two observed structural motifs are representative of all structures in this category, By far, the most represented group of elements complexed by dithiocarbimate ligands is that of the nickel-triad elements. A list of structures and key geometric parameters for these are given in Table 1. The least-represented structures are those containing nickel, but the two observed structural motifs are representative of all structures in this category, consistent with a high degree of homogeneity. There are two bis-chelated nickel(II) dianions, namely {Ni[S 2 C=N(CN)] 2 } 2− (10) [21] and {Ni[S 2 C=N(C 6 H 4 CN-4)] 2 } 2− (11) [33], and these comprise the first structural motif. The di-anion in 10 is illustrated in Figure 6a. The nickel(II) center in 10 is located on an inversion center, as is the case for the di-anion in 11. The four sulfur atoms define a square planar geometry, and the Ni-S bond lengths are chemically equivalent ( Table 1). The second structural motif is observed in neutral (dppe)Ni[S 2 C=N(CN)] (12) [34], where dppe is Ph 2 PCH 2 CH 2 PPh 2 . Two independent molecules comprise the asymmetric unit, but they differ only trivially. A representative molecule is shown in Figure 6b. The nickel atom in 12 is chelated by both the diphosphane and cyano-dithiocarbimate ligand, where the latter forms symmetric Ni-S bond lengths, ranging from 2.20 to 2.22 Å (Table 1). Two di-anionic palladium(II) complex molecules adopt the square planar motif illustrated in Figure 6a: {Pd[S 2 C=N(CN)] 2 } 2− in 13 [19] and {Pd[S 2 C=N(C 6 H 4 NH 2 -4)]} 2− in 14 [23]. The palladium atoms in the two independent di-anions in the asymmetric unit of 13 and in the di-anion in 14 are each located on a center of inversion. Despite having monodentate phosphane ligands, the molecules in each of (Me 3 P) 2 Figure 6b,  a Two independent molecules. b R-stereoisomer. c S-stereoisomer.  Figure 7b. The counter-cations are bis(3,4;3',4'-ethylenedithio)-2,2',5,5'-tetrathiafulvalene for 29 [41], the bithieno(3,4-d)-1,3-dithiol radical for 30 [42], and bithieno(3,4-d)-1,3-dithiol for 31 [42], with 31 having co-crystallized with a neutral molecule: bithieno(3,4-d)-1,3-dithiol. The cyano-dithiocarbimate ligands coordinate in the symmetric mode in each case with the range of Au-S bond lengths in the three anions ranging from 2.32 to 2.34 Å . The fifth structure features a cyclometallated phosphane ligand as illustrated in Figure 7c. In the crystal of (iPr)2P(C10H6-P,C)Au[S2C=N(Ph)] (32) [43], the gold(III) atom is P,C-chelated by the phosphane anion and chelated in a slightly asymmetric fashion by the cyano-dithiocarbimate ligand. The shorter Au-S bond length of 2.34  Figure 7b. The counter-cations are bis(3,4;3',4'ethylenedithio)-2,2',5,5'-tetrathiafulvalene for 29 [41], the bithieno(3,4-d)-1,3-dithiol radical for 30 [42], and bithieno(3,4-d)-1,3-dithiol for 31 [42], with 31 having co-crystallized with a neutral molecule: bithieno(3,4-d)-1,3-dithiol. The cyano-dithiocarbimate ligands coordinate in the symmetric mode in each case with the range of Au-S bond lengths in the three anions ranging from 2.32 to 2.34 Å. The fifth structure features a cyclometallated phosphane ligand as illustrated in Figure 7c. In the crystal of (iPr) 2 P(C 10 H 6 -P,C)Au[S 2 C=N(Ph)] (32) [43], the gold(III) atom is P,C-chelated by the phosphane anion and chelated in a slightly asymmetric fashion by the cyano-dithiocarbimate ligand. The shorter Au-S bond length of 2.34 Å has the sulfur atom trans to the phosphorus atom and the longer bond (2.37 Å) trans to the carbon donor.

Actinides
There are three thorium structures coordinated by dithiocarbimate ligands. As shown in Figure 8a, in (1,2,4-tBu3C5H2)2Th[S2C=N(Ph)] (33) [44], the central atom is connected to two 1,2,4-tri-t-butyl-cyclopentadienyl ligands as well as a chelating phenyl-dithiocarbimate ligand (Th-S: 2 × 2.73 Å ); 33 crystallized as an unknown solvate. Additional coordination is noted in (Cp*)2Th(NC5H4NMe2-4)[S2C=N(mesityl)] (34) [45], for which two independent but similar molecules comprise the asymmetric unit. Thus, in addition to two Cp* anions and a mesityl-dithiocarbimate ligand, the central atom is coordinated by the pyridine-nitrogen atom of the 4-dimethylamine-pyridine ligand (Figure 8b). The increase in the coordination number in 34 leads to an elongation of the Th-S bond lengths (i.e., 2.81 and 2.82 Å ) for the chelating ligand compared with the equivalent bonds in 33. The exceptional thorium structure is that of di-nuclear {(Cp*)2Th[S2C=N(Ph)]}2, isolated as a benzene mono-solvate (35) [46]. This is readily related to that of 33 in that the The exceptional thorium structure is that of di-nuclear {(Cp*) 2 Th[S 2 C=N(Ph)]} 2 , isolated as a benzene mono-solvate (35) [46]. This is readily related to that of 33 in that the bulky t-butyl substituents are substituted by methyl groups. It is likely that the reduction in steric congestion in 35 allows for dimerization about a center of inversion to generate the di-nuclear species illustrated in Figure 8c. The independent phenyl-dithiocarbimate ligand chelates one thorium atom (Th-S: 2.86 and 2.99 Å) and connects to the second thorium atom via a Th-N(imine) bond (Th-N: 2.76 Å). Hence, the phenyl-dithiocarbimate ligand may be considered a rare example of a κ 3 -ligand employing all potential donors in coordination.
Of the three available uranium structures, (1,2,4-tBu 3 C 5 H 2 ) 2 U[S 2 C=N(Ph)] (36) [47] matches that found for the thorium analog shown in Figure 8a (38) [49] are similar, with the latter illustrated in Figure 8d, and 37 was crystallized as a benzene di-solvate. In each structure, the uranium center is additionally coordinated by a phosphane-oxide atom. Small asymmetry is noted in the U-S bond lengths (U-S: 2.76 and 2.78 Å for 37 and 2.75 and 2.78 Å for 38).
Two structural motifs were noted for the dithiocarbimate complexes of the nickel-triad elements (Section 3.1.5), namely a di-anionic square planar complex within a S 4 donor set formulated as [M(dithiocarbimate) 2 ] 2− and neutral square planar (P 2 S 2 ) complexes formulated as (P) 2 M(dithiocarbimate), where (P) 2 equates to two monodentate phosphane ligands or a bidentate phosphane ligand. There are many examples of precedence in the literature of related dithiocarbamate structures found in the CSD [1]. The crucial difference relates to the overall charges in the series of complexes. Thus, M(dithiocarbamate) 2 species are neutral, while [(P) 2 M(dithiocarbamate)] + species are mono-cationic. For the former series, with centrosymmetric Ni(S 2 CNEt 2 ) 2 [56] as the prototype structure for comparison, the Ni-S bond lengths (i.e., 2 × 2.20 Å) are indistinguishable from those in 10 and 11 ( Table 1). The same observations pertain to Pd(S 2 CNEt 2 ) 2 [57] (i.e., 2 × 2.32 Å) and the equivalent Pd-S bond lengths in 13 and 14 (Table 1). A comparison of the Ni-S bond lengths (i.e., 3 × 2.20 and 2.21 Å) for the two independent cations of [(dppe)Ni(S 2 CNEt 2 ) 2 ] + in its [HgBr 4 ] 2− salt [58], cf. 12 (Table 1), again reveal a high degree of concordance in the Ni-S bond lengths between the two classes of structures. In contrast to the preceding, some evidence for the elongation of the Pd-S and Pt-S bond lengths in relevant analogs is apparent, as illustrated for a pair of di-nuclear platinum(II) structures. Thus, in each of the centrosymmetric di-cations [(Et 3 P) 2 Pt(S 2 CN(CH 2 CH 2 ) 2 NCS 2 )Pt(PEt 3 ) 2 ] 2+ and [(Ph 3 P) 2 Pt(S 2 CN(CH 2 CH 2 ) 2 NCS 2 )Pt(PPh 3 ) 2 ] 2+ [59], the Pt-S bond lengths of 2.35 and 2.38 Å and 2.35 and 2.36 Å, respectively, are longer than their counterparts in 20 and 23, respectively, Table 1.
Precedence in the literature also exists for several of the gold dithiocarbimate complexes included in the present survey. Thus, neutral di-nuclear [Au(S 2 CNEt 2 )] 2 [60] adopts a similar molecular structure but lacks the symmetry of 28. The Au-S bond lengths tend to be longer (e.g., 2 × 2.29 and 2 × 2.30 Å cf. 2 × 2.28 Å in 28), but the intramolecular aurophilic interaction of 2.78 Å is shorter than 2.81 Å in 28. Such potential for the finetuning of aurophilic interactions is well-known to have implications for the luminescence characteristics of gold complexes [61,62].
The centrosymmetric square planar (S 4 ) molecular structures of 29-31 have much precedence in the literature, as exemplified by the salt [Au(S 2 CNEt 2 ) 2 ]Cl [63]. The Au-S bond lengths in the latter (i.e., 2 × 2.33 Å) lie in the range of 2.32-2.34 Å noted for the Au-S bonds in 29-31.
From the foregoing, several conclusions may be surmised despite the relatively small number of structures available for comparison. First, the presence of two formal changes on the dithiocarbimate ligand allows access to structures not always accessible to analogous dithiocarbamate ligands. When analogs do exist, the overall molecular structures are invariably closely related. Relatively small differences were noted in the magnitudes of the M-S bond lengths occurring in analogous dithiocarbimate and dithiocarbamate molecules, and no definitive trends were discerned. Finally, but crucially, in the latter circumstances where molecules differed only in terms of the presence of dithiocarbimate and dithiocarbamate, the interchange between one or the other allowed control over the charge of the complex molecule, an attribute important, for example, for biological activity [64].

Conclusions
The synthetic and spectroscopic characteristics along with structural studies of coordination complexes of heavy metal dithiocarbimate complexes were reviewed. Perhaps surprisingly, this is an under-studied class of compound, especially when compared with the ongoing research efforts in the analogous chemistry of dithiocarbamate ligands. Coordination normally involves a S,S-chelating mode, with rare examples of tetradentate modes of coordination whereby both sulfur atoms connect to a pair of metal atoms. Even rarer examples of coordination exist in di-nuclear molecules, where the sulfur atoms chelate a metal center and one of the sulfur atoms also links a second metal center. Here, a bridge is formed between the two metals, as each sulfur atom forms a bond with one metal center, one metal is S,S-chelated, and the imine-N atom connects to a second metal center. In terms of coordination modes and the magnitudes of the M-S bond lengths, the structural chemistry of dithiocarbimate and dithiocarbamate are very similar in all respects except, most importantly, in the overall charge on the complex.