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

Laidlaw, W. M., Denning, R. G., Murphy, Damien Martin and Green, J. C. 2008. Solvent dependence of the g-anisotropy in the ESR of cyanide-bridged mixed-valence complexes. Dalton Transactions (44) , pp. 6257-6264. 10.1039/b810224d

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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 dxz,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.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Chemistry
Subjects: Q Science > QD Chemistry
Publisher: RSC
ISSN: 1477-9226
Last Modified: 04 Jun 2017 01:55

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