Metal spin-equilibrium complexes

A change in spin state among transition metal complexes in spin equilibrium invariably involves a change in the electron population of the a antibonding eg orbitals. This produces a substantial change in the properties of the metal-ligand bonds. This variability in the population of a antibonding orbitals is a conspicuous feature of the complexes of the 3d transition metals and accounts for many of their unique properties. 

The conclusions described in the previous section are inferred from a relatively small number of observations of spin-equilibrium dynamics. Nevertheless, they are internally self-consistent and also compatible with a much wider set of observations derived from studies of electron transfer reactions of metal complexes. For these reasons there is hope that they possess some generality and can be applied to other systems. 

The results obtained from thermal spin equilibria indicate that AS = 1 transitions are adiabatic. The rates, therefore, depend on the coordination sphere reorganization energy, or the Franck-Condon factors. Radiationless deactivation processes are exothermic. Consequently, they can proceed more rapidly than thermally activated spin-equilibria reactions, that is, in less than nanoseconds in solution at room temperature. Evidence for this includes the observation that few transition metal complexes luminesce under these conditions. Other evidence is the very success of the photoperturbation method for studying thermal spin equilibria intersystem crossing to the ground state of the other spin isomer must be more rapid than the spin equilibrium relaxation in order for the spin equilibrium to be perturbed. 

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