Red-shifted cyanide stretching frequencies in cyanide-bridged transition metal donor-acceptor complexes. Support for vibronic coupling

Patterns in the cyanide stretching frequencies have been examined in several series of monometal- and CN--bridged transition metal complexes. Metal-to-cyanide back-bonding can be identified as a major factor contributing to red shifts of nu(CN) in monometal complexes. This effect is complicated in cyanide-bridged complexes in two ways: (a) when both metals can back-bond to cyanide, the net interaction is repulsive and results in a blue shift of nu(CN); and (b) when a donor and acceptor are bridged, nu(CN) undergoes a substantial red shift (sometimes more than 60 cm(-1) lower in energy than the parent monometal complex). These effects can be described by simple perturbational models for the electronic interactions. Monometal cyanide complexes and CN--bridged back-bonding metals can be treated in terms of their perturbations of the CN- pi and pi* orbitals by using a simple, Huckel-like, three-center perturbational treatment of electronic interactions. However, bridged donor-acceptor pairs are best described by a vibronic model in which it is assumed that the extent of electronic delocalization is in equilibrium with variations of some nuclear coordinates. Consistent with this approach, it is found that (a) the oscillator strength of the donor-acceptor charge transfer (DACT) absorption is roughly proportional to the red shift of nu(CN) and (b) there are strong symmetry constraints on the coupling. The latter point is demonstrated by a 10-fold larger red shift of the symmetrical than of the antisymmetrical combination of CN- stretching frequencies in the centrosymmetric trans-([14]aneN(4))Cr(CNRU(NH3)(5))(2)(5+) complex ([14]aneN(4) = 1,4,7,11-tetraazacyclotetradecane). The coupling of the metal d pi orbitals to CN- pi and pi* orbitals can be formulated in terms of ligand-to-metal (LMCT) and metal-to-ligand (MCLT) charge transfer perturbations. The associated charge delocalizations provide a basis for the synergistic weakening of the C-N bond and D/A coupling.