Metal-Metal Multiply Bonded Complexes of Technetium. 1. Synthesis and Structural Characterization of Phosphine Complexes That Contain a Tc-Tc Multiple Bond

A series of triply metal-metal bonded ditechnetium(II) phosphine complexes with the general formula Tc{sub 2}Cl{sub 4}(PR{sub 3}){sub 4} (PR{sub 3} = PEt{sub 3}, PPr{sup n}{sub 3}, PMePh{sub 2}, PMe{sub 2}Ph) have been prepared from mononuclear Tc(IV) precursors and fully characterized. Two-electron reduction of the Tc(IV) bis(phosphine) complexes TcCl{sub 4}(PR{sub 3}){sub 2} (PR{sub 3} = PEt{sub 3}, PPr{sup n}{sub 3}, PMePh{sub 2}, PMe{sub 2}Ph) with finely divided zinc in aromatic solvents or tetrahydrofuran results in the formation of the corresponding electron-rich triply bonded compounds Tc{sub 2}Cl{sub 4}(PR{sub 3}){sub 4} in high yield. These are the first phosphine complexes of technetium that possess a metal-metal bond. The solid-state structures of the PEt{sub 3}, PMe{sub 2}Ph, and PMePh{sub 2} derivatives have been investigated by X-ray crystallography and are described in detail. Similar to the analogous dirhenium(II) complexes, the molecules adopt an eclipsed M{sub 2}L{sub 8} conformation with approximate D{sub 2d} symmetry. The Tc-Tc bond lengths are 2.133(3), 2.127(1), and 2.1384(5) {angstrom} for Tc{sub 2}Cl{sub 4}(PEt{sub 3}){sub 4}, Tc{sub 2}Cl{sub 4}(PEt{sub 3}){sub 4}, Tc{sub 2}Cl{sub 4}(PMe{sub 2}Ph){sub 4}, and Tc{sub 2}Cl{sub 4}(PMePh{sub 2}){sub 4}, respectively. Structural and spectroscopic evidence indicates that these dimers contain an electron-rich Tc-Tc triple bond with a {sigma}{sup 2}-{pi}-{sup 4}{delta}{sup 2}{delta}*{sup 2} ground-state electronic configuration. Electrochemical studies reveal that each compound undergoes two reversible one-electron oxidation processes, which presumably produce the corresponding Tc{sub 2}{sup 5+} and Tc{sub 2}{sup 6+} dinuclear species. {sup 1}H HMR, {sup 31}P({sup 1}H) NMR, and UV-vis spectroscopic data are presented for each compound.