Crystal Structures of Half-Sandwich Ru(II) Complexes, [( η 6 - p - Cymene)(3-chloro-6-(1 H -pyrazol-1-yl)pyridazine)Ru(X)]BF 4 , (X = Cl, Br, I)

: Herein, we report the synthesis and single-crystal X-ray structures of three ( η 6 - p -cymene)Ru(II) tetrafluoroborate salts, viz., [( η 6 - p -cymene)(3-chloro-6-(1 H -pyrazol-1-yl)pyridazine)Ru(X)]BF 4 , (X = Cl, Br, I), Ru1-3 . They were prepared by the reactions of [( η 6 - p -cymene)Ru( µ -X)(X)] 2 , (X = Cl, Br, I) with two-mole equivalents of 3-chloro-6-(1H-pyrazol-1-yl)pyridazine, under inert conditions at ambient temperatures, and subsequently precipitated by the addition of excess BF 4 − ions. Orange crystalline precipitates were obtained in good yields, from which the respective single crystals for X-ray diffraction analysis were recrystallized by slow evaporation from their methanolic/diethyl ether solutions. The Ru(II) complexes were characterized by various spectroscopic techniques and chemical methods, which included FTIR, 1 H/ 13 C NMR, UV-visible absorption, mass spectrometry, and elemental analysis. The molecular structures were solved by single-crystal X-ray crystal diffraction analysis. The complexes crystallized in the monoclinic crystal system in the P2 1 /c (Ru1-2) and P2 1 /n (Ru3) space groups. Density Functionals Theoretical (DFT) calculations were performed in methanol to gain an understanding of the electronic and structural properties of the complexes. Trends in the data metrics were established, and selected data were compared with the diffraction data. The electrophilicity indices of Ru1-3 follow the order Ru3 > Ru2 > Ru1 , and the trend is in line with their anticipated order of reactivity towards nucleophiles


Introduction
Reactions of suitable precursor complexes of iridium (Ir), rhodium (Rh), ruthenium (Ru), or osmium (Os) with η 6 -arene ligands form chemically stable halogen-bridged intermediates, from which the monomeric half-sandwich-or piano stool-structured complexes, [(η 6 -arene)M (κ 2 -N,N -L)X]Y (M = Ir(I), Rh(1), Ru(II), or Os(II); X = halide; Y = a counter anion, and N,N -L = bidentate with N-donor atoms) are formed by the 1:2-mole ratio reaction between the respective intermediates with the N,N -bidentate ligand [1]. These reactions proceed under mild conditions to form the half-sandwich complexes in good yields. The π-face coordinated η 6 -arene ligand brings extra chemical stability to the complexes. It π-donates its ring electrons to the metal center, forming three strong M-C bonds. It also confers necessary hydrophobicity to the coordination shell, stabilizes the oxidation state of the metal ion, as well as modulates the lability of the co-ligand. Consequently, the reactivity of the complex towards substitution of the labile co-ligand by nucleophiles also depends on the electronic properties of the coordinated η 6 -arene non-leaving ligand. Most

Crystal Structures of Ru1-3
Suitable single crystals of Ru1-3 for X-ray diffraction measurements were afforded by slow evaporations of concentrated methanol/diethyl ether (MeOH/Et2O) solutions, and their molecular structures were solved by crystallographic analysis. The thermal ellipsoids (at 50% probability) depicting the molecular structures of the three complexes are shown in Figure 1. Data on the cell dimensions and other details on the data collection, refinement, and key structural parameters are presented in Table 1. Selected bond lengths and angles are presented in Table 2.

Crystal Structures of Ru1-3
Suitable single crystals of Ru1-3 for X-ray diffraction measurements were afforded by slow evaporations of concentrated methanol/diethyl ether (MeOH/Et 2 O) solutions, and their molecular structures were solved by crystallographic analysis. The thermal ellipsoids (at 50% probability) depicting the molecular structures of the three complexes are shown in Figure 1. Data on the cell dimensions and other details on the data collection, refinement, and key structural parameters are presented in Table 1. Selected bond lengths and angles are presented in Table 2.
The pzn and the pdzn rings of the N,N -bidentate ligand remain coplanar upon their chelation to the Ru(II) ion. The N 1 -N 2 -C 4 -N 3 torsion angles are near zero and equal to −0.1(9) • , −0.8(3) • , and −5.4(3) • for Ru1-3, respectively. The Ru-N bond lengths of the compounds lie between 2.0712(17) and 2.080(2) Å, which are similar to those reported for other η 6 -arene Ru(II) complexes with N,N donor ligands [5,6,14,[17][18][19][20][21]. The N-Ru-N bond angles range from 76.18(9) • to 76.33(9) • , while the N-Ru-X bond angles range from 83.50(5) • to 88.44(6) • . These ranges are similar to those reported for related η 6 -arene Ru(II) complexes with N,N bidentate ligands [20]. The Ru-X bond lengths increase as expected in the order Ru-Cl (2.3920(6) Å) [14,[17][18][19], Ru-Br (2.5228(2) Å [11,22], and Ru-I (2.6996(2) Å) [11,14,23] for Ru1-3 in line with the size of the labile ligand and the increase in the polarizability of the Ru-X bond. The bond lengths are comparable to those reported for related compounds: Ru-Cl [24], Ru-Br [22], and Ru-I [23]. The lengths and angles of all C-C, C-N, and B-F bonds are within the expected ranges for (N,N/O-bidentate)(η 6 -cym)Ru(II) complexes [3]. There are many inter/intramolecular interactions in the solid state of these Ru(II) borate salts (refer to Table S11 for a list of selected stabilizing short contacts or non-conventional hydrogen bonds). Some of these interactions occur between fluorine and hydrogen atoms of p-cym. Examples are the F . . . H-C short contact bonds (e.g., C17-H17···F3B, d = 2.868(4) for Ru1, while for Ru2 and Ru3, similar non-conventional hydrogen bonds occur between the leaving group (X) and the hydrogen atom of the neighboring cation in the lattice, as exemplified by the C-H17 . . . Br1 = 3.024 Å and C16-H16 . . . I1 = 3.67 Å. A noteworthy difference in the relative conformational geometry and orientation of the ligands around the Ru(II) metal center is that the pzn and pdzn rings of Ru3 are coordinated in contrastingly distinct orientations (when referenced to the orientations of substituents on the cymene) to that of the same rings in Ru1-2, as well as the analogue of Ru1 reported in [18]. It is unclear what drives the difference in the orientations of the two non-leaving ligands which incidentally put these complexes in two coordinational or configurational sub-domains. In Ru3, the propyl group of the p-cym is oriented closer to the larger ring of the pdzn ligand, which seems to increase electron-electron repulsion between the groups, leading to a longer Ru-C Cym(centroid) distance for Ru3 relative to that of either Ru1 or Ru2. Contrastingly, the methanolic solutions afforded orange crystalline salts of Ru1-3. This is an indication that the coordinated halide ligand has little or no influence on the strength of the crystal field, and this is supported by the invariance in the magnitude of the HOMO-LUMO energy band gap ( Table 3). The trend in the DFT-calculated data shows that there is no significant difference in the band gap energies of the three complexes. The commonality in the color of the salts of homologous halo-Ru(II) complexes was also reported for a series of [(η 6arene)Ru(II)]N-N )X] + , X = Cl, Br and I complexes [11]. Only when the counter ion in the analogous salts of Ru1 was changed from BF 4 − to PF 6 − was there a distinct color change from orange to yellow [18].

Spectroscopic Data
The molecular structures of the N,N -bidentate ligand and its complexes Ru1-3 in solution were probed by 1 H/ 13 C NMR spectra and 2D-NMR techniques (correlation spectroscopy (COSY) and the heteronuclear single quantum coherence (HSQC)). NMR data are useful in elucidating the molecular structure behavior of compounds in solution. The 1 H/ 13 C NMR data for the ligand and the complexes are listed in the experimental section and their respective spectra are given in Figures Figure S1) appear upfield relative to those of Ru1-3. The 1 H/ 13 C NMR spectra of Ru1-3 show common chemical shifts in the resonances of the 1 H and 13 C nuclei. This is an indication that the variation in the halide co-ligand has little or no influence on the electronic environment of the spins of the nuclei of the two coordinated non-leaving ligands. The 3J-constants of non-singlet peaks/(hyperfine-split resonances) in the 1 H NMR spectra of Ru1-3 ( Figures S2-S4) were used to confirm correlated protons in distinctive chemical environments of the complexes. The 1 H spectrum of Ru3 features four common resonance (absorption) peaks while Ru1 and Ru2 have three peaks in the range of 5.90-6.30 ppm. The chemical equivalence of four protons due to the C 2 -symmetry of the facially coordinated η 6 -cym is broken by spin coupling with the diastereotopic protons of methyl groups of the isopropyl moiety. Additionally, the pseudo-octahedral (piano stool) arrangement of four different ligands around the Ru(II) ion makes the latter a chiral center. Thus, the complexes will exhibit stereoisomerism that breaks the C 2 -symmetry of the cym and its spin resonance absorption [17]. The resonance peaks of the three protons of pzn and two of pdzn appear as five resonance peaks downfield (δ = 7.16-9.18 ppm), attributed to the 1 H nuclei on the C=N, C-N, C=C groups and those of the η 6 -cym, respectively. As expected, these peaks were slightly deshielded compared to that of the free ligand ( Figure S1), which ranges between 6.69 and 8.80. The aliphatic protons of the cym appear at 2.79 ppm (m, 1 H CH(CH3)2 ), 2.12 ppm (s, 3H methyl ), and 1.15 ppm (dd, 2 × 3 Hmethyl isopropyl ). The total number of carbon atoms and the range of their chemical shifts (ppm) agree with the different possible bonding environments of the C nuclei of the ligand framework for the proposed structures. Curiously, the aromatic resonances for the η 6 -cym protons do not feature as two doublets as in the spectrum of the respective dimer precursors.
The 1D NMR data were corroborated by those of the COSY, HSQC, and other related correlated techniques, and these data are presented only for Ru3 (refer to Figure S9 and Table S1). The absence of the 1 H/ 13 C cross peaks for C4, C7 (N,N-bidentate) and C8, C11 (cym) in the HSQC of Ru3 (see Figure S9a, and for the carbon numbering, see Figure S8) confirms the quaternary nature of these carbons. The correlation assignments of the rest of the 1 H/ 13 C chemical shifts are consistent with the proposed structure of Ru3 and its two analogues. These assignments were further corroborated by the COSY data ( Figure  S9b and Table S1). In the COSY map of Ru3, common 1 H/ 1 H cross-peaks are observed between the absorption resonances of protons on the C1/C2, C2/C3 (pzn), C5/C6 (pdzn), and C9/C10, C12/C13, C15/C16, C15/C17 (cym), showing that their carbons are bonded together ( Figure S9a). Data of the correlated protons and their mutual coupling constant ( 3 J) are given in Table S1.
The chemical purity of Ru1-3 was checked by elemental (CHNS) analysis while their structural elucidation in the solution was accomplished by various spectroscopic techniques. The elemental compositions of Ru1-3 were within 5% of their theoretically calculated data (see the elemental data listed in the experimental section). This confirmed the high purity of the crystalline salts of Ru1-3.
Direct electro-spraying of solutions Ru1-3 (in methanol/0.1% formic/formate buffer) yielded ion peaks at m/z values of 451 amu (100%), 497 amu (100%), and 543 amu (100%) in their low-resolution mass spectra ( Figures S10-S12). The zoomed-out peaks (see insets in Figures S10-S12) exhibit isotopic distribution patterns that are comparable to the theoretical patterns for the respectively calculated formulas. The matched isotopic pattern for each serves as a structural fingerprint for confirming the identity (as observed in the nominal masses of the respective pseudo molecular ions of Ru1-3 when the isotopic distribution of the elements for each complex is considered). These observed mass spectral data (Figures S10-S12) are attributed to their respective pseudo-protonated molecular ions: UV-visible electronic absorption spectra of the complexes Ru1-3 in solution are depicted in Figure S13. All the electronic spectra of the complexes show similar spectral bands to those reported for other pseudo-octahedral complexes [25]. Characteristic absorption peaks include the medium-intensity bands in the region 250-260 nm, which are assigned to π-π* electronic transitions. An intense band also features in the 280-320 nm region and it is assigned to the inter-/intra-ligand π-π*/n-π* transition within the pyrazolyl-pyridazine bidentate ligand [26]. The absorption band (low intensity) in the region 450-470 nm is assigned to the Ru(II)-to-ligand charge transfer (MLCT). However, the intensities of the absorption bands for Ru3 are weaker compared to those of Ru1 or Ru2. In general, these observed spectral data are comparable to those of other [(η 6 -arenes Ru(II) complexes bearing an N,N donor chelate [25]. As already stated, solutions of Ru1-3 precipitate as orange crystalline solids. This shows that the halide ligand has little influence on the crystal field strength of the complexes. The presence of the 3-chloro-6-(1H-pyrazol-1-yl)pyridazine as a common spectator ligand and the LUMO fragment ensures an overriding metal-to-ligand charge transfer (MLCT) limited to a narrow range band gap (3.3-3.1 eV, see data in Table 3) between the three complexes. This leads to homologous halo-Ru(II) complexes with the same color.
Some of the vibrational stretching bands of Ru1-3 (Figures S15-S17) shift to longer wavelengths compared to that of the free ligand, 3-chloro-6-(1H-pyrazol-1-yl)pyridazine (refer to Figure S14). This confirms the coordination of the bidentate ligand on the metal centers. As shown in Figures S15-S17, the FTIR spectra for Ru1-3 feature common vibrational bands due to in-plane stretches of the C-C/N bonds of the p-cym and the pzn-pdzn bidentate ligands. This is an indication that the leaving halide group has minimum influence on the vibrational frequencies of the functional groups on the non-leaving ligands. The observed absorption bands are mainly due to the functional groups of the coordinated pzn-pdzn-N,N bidentate ligand, the p-cym ligands, and the borate counter ion. As an example, in Figure S15 (the spectrum of the Ru1), two absorption bands are observed at 3085 and 2873 cm −1 ; these low-intensity bands are for the ν C−H stretching frequencies for the aromatic and aliphatic stretches, respectively. Another common featuring band is the 1582 cm −1 (medium, ν C=N (pzn/pdzn) and ν N=N (pdzn) stretch). The absorption bands of the cym occur as an intense duet at 1479 and 1404 cm −1 (intense, ν C=C(cym) stretching). The bands that appear within the range 1200 and 1400 cm −1 (sharp, medium) are due to the ν C=C vibration asymmetric stretches of the pzn/pdzn ring carbons. An intense absorption band is observed in the 1020-1051 cm −1 range due to the asymmetric stretching of the ν B-F bond of the BF 4 − counter ion of Ru1-3. The sharp bands below 800 cm −1 (medium intensities) are due to the vibrational stretches of the Ru-C, Ru-N pzn/pdzn , and Ru-Cl bonds. These frequency bands occur in the range reported in the literature [5,10,14,17,19].

DFT-Calculated Optimized Structures
Geometry-optimized structures for Ru1-3 were computed in methanol as a solvent. The structures (see Table 4, first column) and the energies of the frontier molecular orbitals are presented in Table 4. Some of these DFT-calculated data were compared with the crystal structure (solid-state) and are presented in Tables 3 and 5. Table 4. Overlays of the geometry-optimized structures on their crystal structures, and HOMO and LUMO electron density mappings for Ru1-3.

DFT-Optimized (Blue); Crystal Structure (Red)
HOMO LUMO of the cym occur as an intense duet at 1479 and 1404 cm (intense, bands that appear within the range 1200 and 1400 cm −1 (sharp, m ν C=C vibration asymmetric stretches of the pzn/pdzn ring carbons band is observed in the 1020-1051 cm −1 range due to the asymme bond of the BF 4 − counter ion of Ru1-3. The sharp bands below 80 ties) are due to the vibrational stretches of the Ru-C, Ru-N pzn/pdzn , frequency bands occur in the range reported in the literature [5,10

DFT-Calculated Optimized Structures
Geometry-optimized structures for Ru1-3 were computed in The structures (see Table 4, first column) and the energies of th bitals are presented in Table 4. Some of these DFT-calculated da the crystal structure (solid-state) and are presented in Tables 3 an   Table 4. Overlays of the geometry-optimized structures on their crystal s LUMO electron density mappings for Ru1-3.

Ru1
bands that appear within the range 1200 and 1400 cm −1 (sharp, medium) are d ν C=C vibration asymmetric stretches of the pzn/pdzn ring carbons. An intense ab band is observed in the 1020-1051 cm −1 range due to the asymmetric stretching o bond of the BF 4 − counter ion of Ru1-3. The sharp bands below 800 cm −1 (medium ties) are due to the vibrational stretches of the Ru-C, Ru-N pzn/pdzn , and Ru-Cl bond frequency bands occur in the range reported in the literature [5,10,14,17,19].

DFT-Calculated Optimized Structures
Geometry-optimized structures for Ru1-3 were computed in methanol as a The structures (see Table 4, first column) and the energies of the frontier mole bitals are presented in Table 4. Some of these DFT-calculated data were compa the crystal structure (solid-state) and are presented in Tables 3 and 5. Table 4. Overlays of the geometry-optimized structures on their crystal structures, and H LUMO electron density mappings for Ru1-3.

Ru1
Ru2 bands that appear within the range 1200 and 1400 cm −1 (sharp, medium) are due to th ν C=C vibration asymmetric stretches of the pzn/pdzn ring carbons. An intense absorptio band is observed in the 1020-1051 cm −1 range due to the asymmetric stretching of the ν B bond of the BF 4 − counter ion of Ru1-3. The sharp bands below 800 cm −1 (medium intens ties) are due to the vibrational stretches of the Ru-C, Ru-N pzn/pdzn , and Ru-Cl bonds. Thes frequency bands occur in the range reported in the literature [5,10,14,17,19].

DFT-Calculated Optimized Structures
Geometry-optimized structures for Ru1-3 were computed in methanol as a solven The structures (see Table 4, first column) and the energies of the frontier molecular or bitals are presented in Table 4. Some of these DFT-calculated data were compared wit the crystal structure (solid-state) and are presented in Tables 3 and 5. Table 4. Overlays of the geometry-optimized structures on their crystal structures, and HOMO an LUMO electron density mappings for Ru1-3.

Ru1
Ru2 of the cym occur as an intense duet at 1479 and 1404 cm −1 (intense, ν bands that appear within the range 1200 and 1400 cm −1 (sharp, m ν C=C vibration asymmetric stretches of the pzn/pdzn ring carbons. band is observed in the 1020-1051 cm −1 range due to the asymmetr bond of the BF 4 − counter ion of Ru1-3. The sharp bands below 800 ties) are due to the vibrational stretches of the Ru-C, Ru-N pzn/pdzn , a frequency bands occur in the range reported in the literature [5,10

DFT-Calculated Optimized Structures
Geometry-optimized structures for Ru1-3 were computed in The structures (see Table 4, first column) and the energies of the bitals are presented in Table 4. Some of these DFT-calculated dat the crystal structure (solid-state) and are presented in Tables 3 and   Table 4. Overlays of the geometry-optimized structures on their crystal st LUMO electron density mappings for Ru1-3.

Ru2
Ru2 C=C(cym) bands that appear within the range 1200 and 1400 cm −1 (sharp, medium) are du ν C=C vibration asymmetric stretches of the pzn/pdzn ring carbons. An intense ab band is observed in the 1020-1051 cm −1 range due to the asymmetric stretching o bond of the BF 4 − counter ion of Ru1-3. The sharp bands below 800 cm −1 (medium ties) are due to the vibrational stretches of the Ru-C, Ru-N pzn/pdzn , and Ru-Cl bond frequency bands occur in the range reported in the literature [5,10,14,17,19].

DFT-Calculated Optimized Structures
Geometry-optimized structures for Ru1-3 were computed in methanol as a The structures (see Table 4, first column) and the energies of the frontier molec bitals are presented in Table 4. Some of these DFT-calculated data were compar the crystal structure (solid-state) and are presented in Tables 3 and 5. Table 4. Overlays of the geometry-optimized structures on their crystal structures, and HO LUMO electron density mappings for Ru1-3.

Ru1
Ru2 C=C(cym) bands that appear within the range 1200 and 1400 cm −1 (sharp, medium) are due to th ν C=C vibration asymmetric stretches of the pzn/pdzn ring carbons. An intense absorptio band is observed in the 1020-1051 cm −1 range due to the asymmetric stretching of the ν bond of the BF 4 − counter ion of Ru1-3. The sharp bands below 800 cm −1 (medium intens ties) are due to the vibrational stretches of the Ru-C, Ru-N pzn/pdzn , and Ru-Cl bonds. The frequency bands occur in the range reported in the literature [5,10,14,17,19].

DFT-Calculated Optimized Structures
Geometry-optimized structures for Ru1-3 were computed in methanol as a solven The structures (see Table 4, first column) and the energies of the frontier molecular o bitals are presented in Table 4. Some of these DFT-calculated data were compared wi the crystal structure (solid-state) and are presented in Tables 3 and 5. The HOMO mappings of Ru1-3 (Table 4) have a common distribution. Th significant electron density which is delocalized over the η 6 -cym ring, the Ru(I Cl, Br, I) bond, and the pzn donor arm. However, for Ru3, the density over the p stricted only to the N donor atoms and is much smaller than that on the iodo lig  The HOMO mappings of Ru1-3 (Table 4) have a common distribution. They show significant electron density which is delocalized over the η 6 -cym ring, the Ru(II)-X (X Cl, Br, I) bond, and the pzn donor arm. However, for Ru3, the density over the pzn is re stricted only to the N donor atoms and is much smaller than that on the iodo ligand. Fo Table 5. Comparison of selected DFT data metrics with that from the crystal structure for complexes Ru1-3.

Bond Lengths
Theoret The HOMO mappings of Ru1-3 (Table 4) have a common distribution. They show significant electron density which is delocalized over the η 6 -cym ring, the Ru(II)-X (X = Cl, Br, I) bond, and the pzn donor arm. However, for Ru3, the density over the pzn is restricted only to the N donor atoms and is much smaller than that on the iodo ligand. For all, there is hardly any contribution to the HOMO by the pdzn ring for Ru1-2. This is as expected, since there is πand σ-donation of electron density from their η 6 -cym and pzn rings, respectively. These donations reduce the effective positive charge on the Ru(II) ion, making it less electrophilic towards substituting nucleophiles. The LUMOs of Ru1-3 are more localized on the pdzn rings. Conversely, there is no significant contribution to the LUMOs from the pzn rings. Thus, the electron density on the metal can easily be back-donated into π* orbitals of the pdzn ring than into the pzn donor arm. This means that the pdzn arm of the N,N -bidentate is a π-acceptor comparable to pyridine or pyrazine. Overall, the N,N bidentate of these complexes can be an effective electron density modulator through a pulland-push mechanism towards the Ru(II) metal center since pdzn is a better π-acceptor than pzn. This is important in producing complexes that have moderate reactivity and are therefore less prone to deactivation on transit to the cellular targets. Furthermore, the energy for Ru3 s LUMO is the highest, and hence this complex is the poorest π-acceptor, and least electrophilic. However, it is likely to be the most reactive towards incoming nucleophiles because its iodo ligand is the best leaving group due to the high polarizability of its metal bond.
To probe and predict the substitutional reactivity of Ru1-3, their HOMO-LUMO band gaps and global electrochemical descriptors (chemical hardness (η) [27], chemical potential (µ), electrophilicity (ω) [28], and charge density) were compared. As the halide, X is changed from Cl (Ru1) to I (Ru3), and the band gap, electronegativity, and η decrease; thus, Ru1 is predicted to be the most stable and least reactive. The chemical potential, softness, and electrophilicity also increase from Ru1 to Ru3, which is a clear indication of the greater ability of Ru3 to accept electron density from the incoming nucleophile, thus enhancing the π-back bonding of electrons from the Ru into the π* orbital and stabilizing the transition state.
Looking at the calculated NBO charges in Table 4, the effective charge on the Ru(II) center decreases in the order Ru1 > Ru2 > Ru3. Thus, the charges on the complexes are affected by the nature of the leaving group. The Ru(II) ion in Ru1 has the highest positive charge, indicating that it is the most electrophilic. Ru3 is soft and has a high ability for accepting electrons from the soft Ru(II) ion, and hence is expected to be slightly more reactive than Ru1 or Ru2. The large size of the atomic orbitals of the iodo ligand and hence its ease of polarizability results in an instantaneously large dipole that decreases the positive charge on the Ru(II). On one hand, the chloro ligand withdraws electron density from the metal center by resonance, which increases the positive charge on Ru(II). As a result, it is not the charge on the metal that determines reactivity, but rather the general increase in the negative charge on the ligand moiety, as indicated by electrophilicity indices [28].
Overlays of the DFT optimized structures (in methanol) of Ru1-3 and their respective X-ray crystal structures show a good fit (root mean square deviation (rmsd) of < 2%) of the DFT-optimized structures. The superimposed structures are shown in the first column of Table 4. The computed length, distances, and angles are all within a 10% difference from the solid-state data (Tables S5-S10) from X-ray crystallographic analysis, thereby validating the accuracy of the theoretically calculated metric data of the complexes.

Reagents
The reagents and solvents used for the synthesis of the η 6 -p-cymene Ru(II) complexes were procured from commercial suppliers (Sigma Aldrich and Merck, Johannesburg, South Africa) and used without any further purification. The ligand 3-chloro-6-(1H-pyrazol-1yl)pyridazine (pdzn-pzn) was prepared by following a literature procedure [29].

Synthesis of [(η 6 -p-cymene)(3-chloro-6-(1H-pyrazol-1-yl)pyridazine)Ru(X)]BF 4 , Ru1
(X = Cl); Ru2 (X = Br); Ru3 (X = I) The complexes (Ru1-3) were all synthesized by the method of [20] with slight modifications in the purification stages. An acetonitrile solution (10 mL) of each of the precursor dimer complexes, [(η 6 -p-cymene)Ru(µ-X)(X)] 2 , (X = Cl, Br, I), was prepared by dissolving 674, 869, and 1076 mg (1.1 mmol), respectively. The solution of each was separately added to 2.2 mmol (in acetonitrile) of 3-chloro-6-(1H-pyrazol-1-yl)pyridazine [29]. The respective mixture was stirred at room temperature for 4 h under an inert atmosphere of N 2 provided by the Schlenk line and accessories. This prevented the reactants from reacting with air. Thereafter, the mixture was concentrated under vacuo. About 5 mL of an ethanolic solution of NH 4 BF 4 (2.5 mmol) was slowly added to precipitate the complexes. The mixtures were left under an ice bath for one hour, leading to the formation of orange crystalline precipitates. These were collected by filtration and washed with cold diethyl ether and dried under vacuo. The formation of the tetrafluoroborate salts is depicted in synthetic Scheme 1. Single crystals for X-ray diffraction analysis were grown by slow (1-2 weeks) diffusion of their vapors out of the methanol/diethyl ether solutions of Ru1-Ru3.

Characterization of Complexes
1 H and 13 C NMR spectra were recorded on a Bruker Avance DPX 500 MHZ spectrometer (Bruker BioSpin GmbH, Rheinstetten, Germany) fitted with a 5 mm sample probe at 30 °C to confirm the purity and identity of the compounds. All chemical shifts of protons or carbons of 3-chloro-6-(1H-pyrazol-1-yl)pyridazine are reported relative to those of tetramethyl silane (SiMe 4 ). The mass spectral data of the ligands and complexes were acquired on an LC Premier micro-mass spectrometer. FTIR spectra of complexes were recorded (in KBr) on the Agilent Technologies Cary 630 spectrometer (Vienna, Austria) in the 3800-600 cm −1 range. The UV-visible absorption spectra were acquired on a carry 100 Bio UV-visible spectrophotometer with a temperature controller of ± 0.05 °C. Elemental compositions (CHN) of Ru1-3 were determined on a Thermo Scientific Flash 2000 Spectrometer. The characterization data are given in the Supporting Information (SI). Single-crystal X-ray crystallographic data of Ru1-3 were collected on a Bruker APEX Duo CCD area detector diffractometer [30] with an Incoatec microsource operating at 30 W of power. The crystal was kept at a constant temperature within the range of 99.9-104 K during data collection using an Oxford Instruments Cryojet accessory (Oxford, UK). Diffraction was by graphite-monochromated Cu Kα radiation (λ = 1.54178 Å) at a crystal-to-detector distance of 50 mm. Data collection was conducted at the following set conditions: ω-/φ-scans with exposures, taken at 30 W X-ray power and 0.50 frame widths, 2θ range in the range 6.98-142.948° using SAINTS′ APEX2 [31]. The crystal structures were solved by the OLEX2 program [32], using the SHELXT [33] structure solution programs and Intrinsic Phasing and refined by SHELXL [31] refinement programs. The non-hydrogen atoms were refined anisotropically by full-matrix least-squares minimization/refinement of F 2 . Hydrogen atoms were included but not refined. Visualization of the crystallographic data was performed in Mercury v.4.3 (Cambridge Crystallographic Data Centre, Cambridge, UK), and the crystal data are given in Tables S2-S10.

Characterization of Complexes
1 H and 13 C NMR spectra were recorded on a Bruker Avance DPX 500 MHZ spectrometer (Bruker BioSpin GmbH, Rheinstetten, Germany) fitted with a 5 mm sample probe at 30 • C to confirm the purity and identity of the compounds. All chemical shifts of protons or carbons of 3-chloro-6-(1H-pyrazol-1-yl)pyridazine are reported relative to those of tetramethyl silane (SiMe 4 ). The mass spectral data of the ligands and complexes were acquired on an LC Premier micro-mass spectrometer. FTIR spectra of complexes were recorded (in KBr) on the Agilent Technologies Cary 630 spectrometer (Vienna, Austria) in the 3800-600 cm −1 range. The UV-visible absorption spectra were acquired on a carry 100 Bio UV-visible spectrophotometer with a temperature controller of ± 0.05 • C. Elemental compositions (CHN) of Ru1-3 were determined on a Thermo Scientific Flash 2000 Spectrometer. The characterization data are given in the Supporting Information (SI). Single-crystal X-ray crystallographic data of Ru1-3 were collected on a Bruker APEX Duo CCD area detector diffractometer [30] with an Incoatec microsource operating at 30 W of power. The crystal was kept at a constant temperature within the range of 99.9-104 K during data collection using an Oxford Instruments Cryojet accessory (Oxford, UK). Diffraction was by graphitemonochromated Cu K α radiation (λ = 1.54178 Å) at a crystal-to-detector distance of 50 mm. Data collection was conducted at the following set conditions: ω-/ϕ-scans with exposures, taken at 30 W X-ray power and 0.50 frame widths, 2θ range in the range 6.98-142.948 • using SAINTS APEX2 [31]. The crystal structures were solved by the OLEX2 program [32], using the SHELXT [33] structure solution programs and Intrinsic Phasing and refined by SHELXL [31] refinement programs. The non-hydrogen atoms were refined anisotropically by full-matrix least-squares minimization/refinement of F 2 . Hydrogen atoms were included but not refined. Visualization of the crystallographic data was performed in Mercury v.4.3 (Cambridge Crystallographic Data Centre, Cambridge, UK), and the crystal data are given in Tables S2-S10.

Conclusions
Three borate salts of half-sandwich Ru(II) complexes, [(η6-p-cymene)(3-chloro-6-(1Hpyrazol-1-yl)pyridazine)Ru(X)]BF 4 (X = Cl (Ru1), Br (Ru2), or I (Ru3)), were synthesized from the reactions of their respective dimer precursors, [(η 6 -p-cymene)Ru(µ-X)(X)] 2 (X = Cl, Br, I), with two-mole equivalents of 3-chloro-6-(1H-pyrazol-1-yl)pyridazine under inert conditions and subsequent precipitation by the addition of excess BF 4 − ions. Orange crystalline precipitates were obtained in good yields, from which respective single crystals for X-ray diffraction analysis were crystallized by slow evaporation from their methanolic/diethyl ether solutions. Complete characterization of all complexes was achieved using 1 H and 13 C NMR, FTIR and UV-visible absorption spectroscopies, elemental analysis, and singlecrystal X-ray crystallography. The η 6 -cym group is facially coordinated to the Ru(II) ion and occupies the apex position of the pseudo-octahedral geometry. The pyrazole and pyridazine donor arms of the ancillary N,N -bidentate (3-chloro-6-(1H-pyrazol-1-yl)pyridazine) and a halide occupy the other sites of the piano stool geometry. DFT computed data show that the nature of the leaving group affects the effective charge electrophilicity, and hence the reactivity of the complex. Ru3 has the greatest tendency to accept the electron density of incoming nucleophiles during a ligand substitution process.
Author Contributions: A.K.K.: Investigation, conceptualization, synthesis, drafting; P.O.: conceptualization, partial resourcing supervision; J.G.: partial resourcing on synthesis, reviewing; R.O.O.: crystallization, spectral characterization and reviewing, L.A.H.: DFT computations, X-ray diffraction data collection, visualization, handling, and analysis; A.M.: DFT and X-ray diffraction data analyses and partial resourcing on the characterization of compounds, and drafting and reviewing. All authors have read and agreed to the published version of the manuscript.