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Reduction of (4,0)-Ru2(chp)4Cl (1) (chp = 6-chloro-2-oxypyridinate) with Zn or FeCl2 yields a series of axial ligand adducts of the Ru2(II,II) species Ru2(chp)4(L), with L = tetrahydrofuran (2), dimethyl sulfoxide (DMSO; 3), PPh3 (4), pyridine (5), or MeCN (6). Zn reduction in noncoordinating solvents such as toluene or CH2Cl2 leads to the dimeric species [Ru2(chp)4]2 (7) or [Ru2(chp)4]2(ZnCl2) (8), whereas addition of strongly σ-donating ligands such as CO causes cleavage of the Ru-Ru bond. Density functional theory (DFT) models of these complexes, the axially free species, and the axial adducts of several other potential ligands (H2O, NH3, CH2Cl2, S-bound DMSO, N2, and CO) indicate that these compounds can be divided into three distinct categories, based on their Ru-Ru bond length and electronic structure. Compounds 2, 3, 5, 6, 7, and 8, the hypothetical axially free species, and adducts of H2O and NH3 fit in Category 1 with a (δ*)²(π*)² ground state, as indicated by their electronic spectra, magnetic properties, and Ru-Ru bond distances. However, compound 4 and the CH2Cl2 adduct (Category 2) show a pseudo-Jahn-Teller distortion and spectroscopic signs of δ*/π* orbital mixing suggestive of a new electronic ground state intermediate between the (δ*)²(π*)² and (δ*)¹(π*)³ configurations. Category 3 consists of the hypothetical adducts of N2, S-bound DMSO, and CO, all of which are predicted to have a (δ*)¹(π*)³ configuration. Electronic spectra were recorded and assigned using time-dependent DFT, allowing assignment of a band in the 10 000-13 000 cm-1 range as the δ → π* transition. The axial ligand's π-acid character heavily influences the δ*−π* gap, and thereby the ground-state electronic configuration, but not the axial ligand binding strength, which is dictated more by the σ-donor character of the ligands. Thus, this work greatly expands the number of axial ligand adducts known for Ru2(II,II) complexes supported by N,O-donor ligands and provides a predictive theoretical framework for their stability and electronic structures. [ABSTRACT FROM AUTHOR]

We report here the syntheses, X-ray crystal structures, electro-chemistry, and density functional theory (DFT) single-point calculations of three new complexes: tetrakis(monothiosuccinimidato)dimolybdenum(II) [Mo2(SNO5)4, la], tetrakis(6-thioxo-2-piperidinonato)dimolybdenum(II) [Mo2 ( SNO6 )4, lb], and chlorotetrakis(monothiosuccinimidato)- pyridinelithiumdimolybdenum(II) [pyLiMo2(SNOS)4Cl, 2-py]. X-ray crystallography shows unusually short axial Mo2--Cl bond lengths in 2-py, 2.6533(6) Å, and dimeric 2-dim, 2.644(1) Å, which we propose result from an increased Lewis acidity of the Mo2 unit in the presence of the proximal Li+ ion. When 2-py is dissolved in MeCN, the lithium reversibly dissociates, forming an equilibrium mixture of (MeCNLiMo2(SN05)4Cl) (2-MeCN) and [Li(MeCN)4]+[Mo2(SN05)4Cl]- (3). Cyclic voltammetry was used to determine the equilibrium lithium binding constant (room temperature, Keq = 95 ± 1). From analysis of the temperature dependence of the equilibrium constant, thermodynamic parameters for the formation of 2-MeCN from 3 (ΔH° = --6.96 ± 0.93 kj mol-1 and ΔS° = 13.9 ± 3.5 J mol-1 K-1) were extracted. DFT calculations indicate that Li+ affects the Mo--CI bond length through polarization of metal--metal bonding/antibonding molecular orbitals when lithium and chloride are added to the dimolybdenum core. [ABSTRACT FROM AUTHOR]