Charge-transfer spectra and bonding in tetrahedral MnVI, CrV, and VIV and MnVII, CrVI and VV oxo anions

Density functional theory (DFT) calculations on the tetrahedral Mn-VI, Cr-V, and V-IV(d(1)) oxo anions in their ground and lowest excited d-d and O --> M charge transfer (CT) states are reported and used to assign the electronic absorption spectra by reference to the spectra of the isoelectronic Mn-VII, Cr-VI, and V-V (d(0)) and the Mn-V and Cr-IV (d(2)) anions. Calculated geometrical shifts along the totally symmetric metal-ligand vibration (alpha(1)) for electronic excitations are in agreement with data deduced from experimental vibronic fine structures, supporting the proposed assignments. Using a CT model including las different from DFT) configuration interaction (CICT), it is shown that the CT excited states of MnO42- at 17 000, 23 300, and 28 200 cm(-1) are due to d(2) (3)A(2)(2e(2)), E-1(2e(2)), and (3)A(2)(2e(2)) final states combining with a single hole ((L) under bar) on the ligand 1t(1) and 4t(2) orbitals, respectively. The higher 10Dq and smaller B values for the d(2)(L) under bar(d(1)) states compared to those of the d(2) systems correlate with the shortening of the metal-ligand bond accompanying the removal of electrons from the antibonding d orbitals, leading to an increase in covalency and a change in the ordering of CT states for Cr-V with T-3(2)(2e(1)5t(2)(1))(L) under bar (10Dq) at a higher energy than E-1(2e(2))(L) under bar (8B + 2C) as compared to Cr-IV with nearly degenerate T-3(2)(2e(1)5t(2)(1)) and E-1(2e(2)) terms. This allows one to estimate the energy of the 3A2(2e2)(L) under bar --> E-1(2e(2))(L) under bar transition from the CT (d(2)(L) under bar) spectrum of Cr-V(d(1)), which could not be observed for Cr-IV. From a comparison of calculated and experimental oscillator strengths and Huang-Rhys factors (S) for the lowest CT band in the V-V, Cr-VI, and Mn-VII (d(0)) and the V-IV, CrV, and Mn-VI (d(1)) oxo anions, it is shown that the increase in covalency from left to right in this series is accompanied by a reduction in band intensity and S for the progression in the alpha(1) vibration. An explanation of this result in terms of ionic contributions to the metal-ligand bond increasing from Mn-VI to Cr-V and V-IV is proposed. Intensities of "d-d" transitions display the opposite trend; increasing covalency leads to stronger mixing between d --> d and CT excited states and thus an increase in intensity.