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)

Abstract

The technetium(III) compound (n-Bu₄N)₂[Tc₂Br₈] was prepared by metathesis of (n-Bu₄N)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 groupP2₁/n with a = 13.8959(8) Å, b = 15.2597(9) Å, c = 15.5741(9) Å, β = 109.107(1)°, R₁ = 0.028, and Z = 4. The Tc–Tc distance (2.1625(9) Å) and the average Tc–Br distances (2.4734(7) Å) are in excellent agreement with those previously determined by EXAFS spectroscopy. These and other experimental data on quadruply metal–metal bonded group 7 [M₂X₈]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 1 000 cm⁻¹, but mimics the trends in δ→δ* energies across the series.