Cobalt complexes ground state effects

The cobalt(II) complexes which undergo spin equilibrium are of several different types. Octahedral high-spin complexes with a T ground state are subject to Jahn-Teller distortion in the low-spin d1 2E state. This effect is best documented in structures of the Co(terpy)22+ spin-equilibrium complex. The high-spin isomer is nearly octahedral, with a difference in Co N bond lengths between the central and distal nitrogens of only 6 pm. In the Jahn-Teller distorted low-spin state this difference has increased to 21 pm (58). 

Theoretical aspects of the Co(NHj)g system have been described recently by Endres et al. They considered the involvement of various possible spin states of Co(III) and Co(II) and concluded that the most favorable exchange occurs between the ground state of Co(III), and a spin-excited state of Co(II), They find that the latter is stabilized by Jahn-Teller distortion, a factor which was not considered in earlier work where it was concluded that the reactants were electronic ground states for both oxidation states. The issue of spin-state effects in other cobalt complexes is discussed below. Endres et al. also have provided a brief summary of the weaknesses of Density Functional Theory when applied to such systems. If the spin-state change prediction is correct, then the agreement between calculated and observed values for the Co(NHj)5 system in Table 6.1 could be fortuitous. 

Low-spin ground states are found in planar compounds of cobalt(II) and nickel(III) like dithiolate complexes (c/2.1.4.1). The spectra are analyzed using the S= spin Hamiltonian Eq.(18). In [Co pc], e.g., quadrupole effects have been observed, the required spin Hamiltonian then being given by [78]... 

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