Triplet electronic states in d(2) and d(8) complexes probed by absorption spectroscopy: a CASSCF/CASPT2 analysis of [V(H2O)6]3+ and [Ni(H2O)6]2+

Octahedral complexes of transition metal ions with d(2) and d(8) electron configurations have triplet electronic states with identical T(2g), A(2g), T(1g)((3)F), and T(1g)((3)P) symmetry labels. CASSCF and CASPT2 calculations indicate the predominant electronic configurations for each triplet state. The two (3)T(1g) states show strong configuration mixing in the d(8) complex Ni(H(2)O)(6)](2+), but much weaker mixing occurs between these states in the d(2) compound V(H(2)O)(6)](3+). Calculated vibrational frequencies and equilibrium geometries for the triplet states are used to obtain theoretical absorption spectra that are in agreement with the experimental data.