Structural, spectroscopic, and multiconfigurational quantum chemical investigations of the electron-rich metal-metal triple-bonded Tc ₂X₄(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 trlmethylphosphlne. The Tc(II) dlnuclear 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 multlconfiguratlonal (CASSCF/CASPT2) quantum chemical studies.The compound Tc₂Cl₄(PMe₃)₄ crystallizes in the monoclinlc space group CZ/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 Tc ₂Br₄(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 Tc₂X₈^2- and Tc₂X ₄(PMe₃)₄ (X = Cl, Br) indicate stronger it bonds in the Tc₂⁴⁺ core than in Tc₂ ⁶⁺ 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* -π*, δ* -π, and δ -πit* transitions. The cyclic voltammograms exhibit two reversible waves and indicate that Tc ₂Br₄(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.