Molecular and electronic structure of Tc₂(O₂CCH ₃)₂Cl₄ studied by multiconfigurational quantum chemical methods

The molecular and electronic structure, as well as the electronic absorption spectrum of Tc₂(O₂CCH₃) ₂Cl₄ were studied by multiconfigurational quantum chemical methods. The computed ground state geometry is in excellent agreement with the experimental structure determined by single crystal X-ray diffraction (SCXRD). The total bond order (i.e., 3.20) is consistent with the presence of a moderately strong quadruple Tc-Tc bond and is the largest bond order reported so far for a multiple Tc-Tc bonded complex. Effective bond order analysis indicates stronger π and δ bonds for Tc₂(O ₂CCH₃)₂Cl₄ (i.e., 1.71 for π and 0.59 for δ) than for Tc₂Cl₈^2- (i.e., 1.68 for π and 0.47 for δ). The electronic absorption spectrum was recorded in dichloromethane and shows three distinct bands in the range 10 000-35 000 cm^-1. Assignment of the bands, as well as their excitation energies and intensities were performed at the CASSCF/CASPT2 level of theory. The lowest energy band corresponds to the δ → δ^{â̂ -} transition; the next higher energy bands are attributed to δ → π^{â̂ -} and π → δ^{â̂ -} transitions, respectively.