Solvent dependence of the g-anisotropy in the ESR of cyanide-bridged mixed-valence complexes

Abstract

The low temperature (∼5 K) X-band ESR spectra are reported of the cyanide-bridged mixed-valence complexes [(OC)₅Cr(μ-CN)M(NH₃)₅]X₂ (M = Ru, Os; X = PF₆⁻) in frozen matrices formed from nitromethane, acetonitrile and dimethylformamide with toluene. The anisotropy (g_‖−g_⊥) is greater for the ruthenium than for the osmium complex. It is positive in all cases and is strongly dependent on the hydrogen-bonding interaction between the solvent matrix and the metal–ammine fragment, decreasing in the order nitromethane > acetonitrile > dimethylformamide. The axial ligand field parameter, Δ, is quite insensitive to the ammine metal (M) and is mainly determined by the solvent matrix. Density functional calculations, together with a simplified MO model, show that: (a) The value of Δ is dominated by the interaction between the filled cyanide π-orbitals and the ammine–metal d_xz,yz orbitals, (b) Δ decreases with increasing solvent donicity because the resulting positive shift of the d-orbital energies reduces this interaction, (c) the insensitivity of Δ to the ammine–metal arises because an increase in the energy mismatch between the cyanide π-orbitals and the d-orbitals in osmium compound is offset by an increase in the 5d resonance integrals relative to those in the 4d shell. Semi-quantitative values are obtained for the π and π* resonance integrals. We point out that g_‖ determines that portion of the ammine–metal spin population that interacts with the cyanide bridge, and should therefore be correlated with the degree of metal–metal charge transfer in low-spin d⁶-d⁵ intervalence compounds. X-band ESR spectra of the polycrystalline powders (M = Ru, Os; X = CF₃SO₃⁻) are rhombic with similar axial and rhombic ligand field parameters. The rhombicity is interpreted as resulting from asymmetric cation–anion hydrogen-bonding that is apparent in the crystal structures of these isomorphous compounds.