Spin Conversion Processes in Solutions

High-spin to low-spin crossover processes in some transition-metal complexes are described in terms of a radiationless nonadiabatic multiphonon process, occurring between two distinct zero-order spin states, which are characterized by different nuclear configurations. The rate constant was expressed in terms of separate electronic and nuclear contributions. An evaluation of the electronic spin-orbit coupling terms is reported, while the nuclear vibrational overlap factors were estimated from spectroscopic and structural data. The calculated rate constants for spin crossover in some Fe(II) and Fe(III) complexes are calculated without introducing any adjustable parameters and are in order-of-magnitude agreement with the available experimental data.