Crystal structure of octabromoditechnetate(iii) and a multi-configurational quantum chemical study of the δ→δ* transition in quadruply bonded [M₂X₈]^2- dimers (M = Tc, Re; X = Cl, Br)

The technetium(iii) compound (n-Bu₄N)₂[Tc ₂Br₈] was prepared by metathesis of (n-Bu ₄N)₂[Tc₂Cl₈] with concentrated aqueous HBr in acetone and recrystallized from acetone-diethyl ether solution (2: 1 v/v). The acetone solvate obtained, (n-Bu₄N) ₂[Tc₂Br₈]·4[(CH₃) ₂CO] (1), crystallizes in the monoclinic space group P2₁/n with a = 13.8959(8) , b = 15.2597(9) , c = 15.5741(9) , β = 109.107(1)°, R₁ = 0.028, and Z = 4. The Tc-Tc distance (2.1625(9)) and the average Tc-Br distances (2.4734(7) ) are in excellent agreement with those previously determined by EXAFS spectroscopy. These and other experimental data on quadruply metal-metal bonded group 7 [M ₂X₈] ^2- dimers (M = Tc, Re; X = Cl, Br) are compared to the results of a set of multi-configurational quantum chemical studies. The calculated molecular structures of the ground states are in very good agreement with the structures determined experimentally. The theory overestimates the δ→δ* transition energies by some 1000 cm^-1, but mimics the trends in δ→δ* energies across the series.