Triplet electronic states in d2 and d8 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(F-3), and T-1g(P-3) Symmetry labels. CASSCF and CASPT2 calculations indicate the predominant electronic configurations for each triplet state. The two T-3(1g) states show strong configuration mixing in the d(8) complex [Ni(H2O)(6)](2+), but much weaker mixing occurs between these states in the d(2) compound [V(H2O)(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.