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

The technetium(III) compound (n-Bu(4)N)(2)[Tc(2)Br(8)] was prepared by metathesis of (n-Bu(4)N)(2)[Tc(2)Cl(8)] with concentrated aqueous HBr in acetone and recrystallized from acetone-diethyl ether solution (2:1 v/v). The acetone solvate obtained, (n-Bu(4)N)(2)[Tc(2)Br(8)] x 4 [(CH(3))(2)CO] (1), crystallizes in the monoclinic space group P2(1)/n with a = 13.8959(8) A, b = 15.2597(9) A, c = 15.5741(9) A, beta = 109.107(1) degrees, R(1) = 0.028, and Z = 4. The Tc-Tc distance (2.1625(9) A) and the average Tc-Br distances (2.4734(7) A) are in excellent agreement with those previously determined by EXAFS spectroscopy. These and other experimental data on quadruply metal-metal bonded group 7 [M(2)X(8)](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 delta-->delta* transition energies by some 1000 cm(-1), but mimics the trends in delta-->delta* energies across the series.