Oxo-bridged dinuclear chromium(III) complexes: correlation between the optical and magnetic properties and the basicity of the oxo bridge.

The synthesis and X-ray structure of a new member of the series of oxo-bridged, dinuclear chromium(III) complexes, the methyl isocyanide complex [(CH3NC)5CrOCr(CNCH3)5](PF6)4·2CH3CN, is reported. This constitutes only the third oxo-bridged, dinuclear chromium(III) complex with a homoleptic auxillay ligand sphere. Experimentally, the system shows unshifted narrow nuclear magnetic resonance (NMR) spectra that are consistent with calculations using broken symmetry density functional theory (DFT), which suggests it to be the strongest coupled, dinuclear chromium(III) complex known. Furthermore, we report the crystal structure and computed magnetic properties for [(bpy)2(SCN)CrOCr(NCS)(bpy)2](ClO4)2·2H2O (bpy = 2,2'-bipyridine), which differs from other reported oxo-bridged species by featuring a bent CrOCr(4+) core. We also interpret the spectacular 10-orders-of-magnitude variation in acid dissociation constant of the bridging hydroxo ligand in mono hydroxo-bridged dinuclear chromium(III) complexes, in terms of a valence bond model parametrized by metal-to-metal charge transfer (MMCT) and ligand-to-metal charge transfer (LMCT) energies.