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 CrOCr4+ 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.