Structural, Spectroscopic, and Multiconfigurational Quantum Chemical Investigations of the Electron-Rich MetalMetal Triple-Bonded Tc₂X₄(PMe₃)₄ (X = Cl, Br) Complexes

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) Å, b = 9.1821(5) Å, c = 17.0090(9) Å, 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) Å) is similar to the one found in Tc2Br4(PMe3)4 (2.1316(5) Å). The calculated molecular structures of the ground states are in excellent agreement with the structures determined experimentally. Calculations of effective bond orders for Tc2X8 2and Tc2X4(PMe3)4 (X = Cl, Br) indicate strongerπ bonds in the Tc2 4þ core than in Tc2 6þ core. The electronic spectra were recorded in benzene and show a series of low intensity bands in the range 10 000-26 000 cm. 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* f σ* transition, the difference between calculated and experimental values being 228 cm for X = Cl and 866 cm for X = Br. The next bands are attributed to δ*f π*, δf σ*, and δf π* 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 π-acidic phosphines are more difficult to oxidize, and a correlation between oxidation potential and phosphine acidity was established.