Solvent and metal dependent (1)H NMR hyperfine shifts in paramagnetic pentaamminemetal cyanide-bridged mixed-valence complexes.

(1)H NMR resonances, in several aprotic solvents, are reported for axial and equatorial ammonias coordinated to a single spin paramagnetic centre in the Robin-Day Class II cyanide-bridged mixed-valence cations [(OC)(5)Cr(μ-CN)M(NH(3))(5)](2+) (M = Ru, Os) as well as in the complex [(OC)(5)Re(μ-CN)Ru(NH(3))(5)](3+), whose synthesis and properties are reported herein. Using the appropriate isotropic hexaammine complex as a reference, the chemical shift difference between the ammonia protons, δ(ax) - δ(eq), is found to be very sensitive to the paramagnetic metal (M), the remote diamagnetic metal (Cr or Re) and also to the donor properties of the solvent (as well as the counter-ion) as a result of hydrogen bonding interactions. The difference varies linearly with the MMCT energy, and in [(OC)(5)Re(μ-CN)Ru(NH(3))(5)](3+) can be tuned from positive (δ(ax) > δ(eq)) to negative (δ(ax) < δ(eq)) through zero (δ(ax) = δ(eq)) by the choice of solvent.

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

The low temperature (approximately 5 K) X-band ESR spectra are reported of the cyanide-bridged mixed-valence complexes [(OC)5Cr(mu-CN)M(NH3)5]X2 (M = Ru, Os; X = PF6(-)) in frozen matrices formed from nitromethane, acetonitrile and dimethylformamide with toluene. The anisotropy (g paralell-g perpendicular) 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.