Abstracts 1998

Abstract of Publication No. 308

M. Atanasov, T. C. Brunold, H. U. Güdel and C. Daul
Charge-Transfer Spectra and Bonding in Tetrahedral MnVI, CrV, and VIV and MnVII, CrVI, and VV Oxo Anions
Inorg. Chem. 37, 4589-4602 (1998)      Full Text (PDF)      DOI-Link     

Abstract: Density functional theory (DFT) calculations on the tetrahedral MnVI, CrV, and VIV(d1) 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 MnVII, CrVI, and VV (d0) and the MnV and CrIV (d2) anions. Calculated geometrical shifts along the totally symmetric metal-ligand vibration (a1) for electronic excitations are in agreement with data deduced from experimental vibronic fine structures, supporting the proposed assignments. Using a CT model including (as different from DFT) configuration interaction (CICT), it is shown that the CT excited states of MnO42- at 17000, 23300, and 28200 cm1 are due to d2 3A2(2e2), 1E(2e2), and 3A2(2e2) final states combining with a single hole (L) on the ligand 1t1 and 4t2 orbitals, respectively. The higher 10Dq and smaller B values for the d2L(d1) states compared to those of the d2 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 CrV with 3T2(2e15t21)L (10Dq) at a higher energy than 1E(2e2)L (8B + 2C) as compared to CrIV with nearly degenerate 3T2(2e15t21) and 1E(2e2) terms. This allows one to estimate the energy of the 3A2(2e2)L 1E(2e2)L transition from the CT (d2L) spectrum of CrV(d1), which could not be observed for CrIV. From a comparison of calculated and experimental oscillator strengths and Huang-Rhys factors (S) for the lowest CT band in the VV, CrVI, and MnVII (d0) and the VIV, CrV, and MnVI (d1) 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 a1 vibration. An explanation of this result in terms of ionic contributions to the metal-ligand bond increasing from MnVI to CrV and VIV 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.

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