Structural, spectroscopic, and multiconfigurational quantum chemical investigations of the electron-rich metal-metal triple-bonded Tc 2X4(PMe3)4 (X = Cl, Br) complexes

Frederic Poineau, Paul M. Forster, Tanya K. Todorova, Laura Gagliardi, Alfred P. Sattelberger, Kenneth R. Czerwlnski

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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 Tc2X82- 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.

Original languageEnglish (US)
Pages (from-to)6646-6654
Number of pages9
JournalInorganic Chemistry
Issue number14
StatePublished - Jul 19 2010

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