The emission spectra of the solids [n-Bu₄N]₂Tc₂X₈ (X = Cl, Br) have been investigated at room temperature and 77 K. In each case, the emission originates in the ¹Î´â€“δ* excited state, as with the rhenium homologues, but has a shorter lifetime.

The compounds Tc₂Cl₄(PMe₃)₄ and Tc₂Br₄(PMe₃)₄ were formed from the reaction between (n-Bu₄N)₂Tc₂X₈ (X = Cl, Br) and trimethylphosphine. The Tc(II) dinuclear species were characterized by single-crystal XRD, UV−visible spectroscopy, and cyclic voltammetry techniques, and the results are compared to those obtained from density functional theory and multiconfigurational (CASSCF/CASPT2) quantum chemical studies. The compound Tc₂Cl₄(PMe₃)₄ crystallizes in the monoclinic space group C2/c [a = 17.9995(9) Ã…, b = 9.1821(5) Ã…, c = 17.0090(9) Ã…, β = 115.4530(10)°] and is isostructural to M₂Cl₄(PMe₃)₄ (M = Re, Mo, W) and to Tc₂Br₄(PMe₃)₄. The metal−metal distance (2.1318(2) Ã…) is similar to the one found in Tc₂Br₄(PMe₃)₄ (2.1316(5) Ã…). The calculated molecular structures of the ground states are in excellent agreement with the structures determined experimentally. Calculations of effective bond orders for Tc₂X₈^2− and Tc₂X₄(PMe₃)₄ (X = Cl, Br) indicate stronger Ï€ bonds in the Tc₂⁴⁺ core than in Tc₂⁶⁺ core. The electronic spectra were recorded in benzene and show a series of low intensity bands in the range 10000−26000 cm−1. Assignment of the bands as well as computing their excitation energies and intensities were performed at both TD-DFT and CASSCF/CASPT2 levels of theory. Calculations predict that the lowest energy band corresponds to the δ* → σ* transition, the difference between calculated and experimental values being 228 cm^−1 for X = Cl and 866 cm^−1 for X = Br. The next bands are attributed to δ* → Ï€*, δ → σ*, and δ → Ï€* transitions. The cyclic voltammograms exhibit two reversible waves and indicate that Tc₂Br₄(PMe₃)₄ exhibits more positive oxidation potentials than Tc₂Cl₄(PMe₃)₄. This phenomenon is discussed and ascribed to stronger metal (d) to halide (d) back bonding in the bromo complex. Further analysis indicates that Tc(II) dinuclear species containing Ï€-acidic phosphines are more difficult to oxidize, and a correlation between oxidation potential and phosphine acidity was established.

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 group P2₁/n with a = 13.8959(8) Ã…, b = 15.2597(9) Ã…, c = 15.5741(9) Ã…, β = 109.107(1)°, R1 = 0.028, and Z = 4. The Tc–Tc distance (2.1625(9) Ã…) and the average Tc–Br distances (2.4734(7) Ã…) 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 * transition energies by some 1000 cm^-1, but mimics the trends in δ – δ* energies across the series.