A Decade of Dinuclear Technetium Complexes with Multiple Metal-Metal Bonds

Transition metal complexes with multiple metal-metal bonds exhibit interesting catalytic and biological properties. One element whose metal-metal bond chemistry has been poorly studied is technetium. Currently, only 25 technetium complexes with multiple metal-metal bonds have been structurally characterized. The nature of metal-metal interactions in these complexes, as well as the influence of ligands on the bonding in the Tc-2(n+) unit (n = 6, 5, 4) are not well understood. In order to better understand the influence of ligands on the Tc-Tc bonding, a study of the solid-state and electronic structure of dinuclear complexes with the Tc-2(n+) unit (n = 6, 5, 4) has been performed. Dinuclear technetium complexes (nBu(4)N)(2)Tc2X8, Tc-2(O2CCH3)(4)X-2, Tc-2(O2CCH3)(2)Cl-4, cesium salts of Tc2X83-, and Tc2X4(PMe3)(4) (X = Cl, Br) were synthesized; their molecular and electronic structures, as well as their electronic absorption spectra, were studied by a number of physical and computational techniques. The structure and bonding in these systems have been investigated by using multiconfigurational quantum calculations. For all these complexes, the calculated geometries are in very good agreement with those determined experimentally. Bond order analysis demonstrates that all these complexes exhibit a total bond order of approximately 3. Analysis of individual effective bond order (EBO) components shows that these complexes have similar s components, while the strength of their p components follows the order Tc2X4(PMe3)(4) > Tc2X83- > Tc-2(O2CCH3)(2)Cl-4 > Tc2X82-. Calculations indicate that the delta components are the weakest bond in Tc2X8n- (n = 2, 3) and Tc-2(O2CCH3)(2)Cl-4. Further analysis of Tc2X83- and Tc2X4(PMe3)(4) (X = Cl, Br) indicates that the electronic structure of the Tc-2(5+) and Tc-2(4+) units is insensitive to the nature of the coordinating ligands. The electronic absorption spectra of Tc2X8n- (n = 2, 3), Tc-2(O2CCH3)(2)Cl-4, and Tc2X4(PMe3)(4) (X = Cl, Br) were studied in solution, and assignment of the transitions was performed by multiconfigurational quantum chemical calculations. For the Tc2X8n-(n = 2, 3; X = Cl, Br) anions and Tc-2(O2CCH3)(2)Cl-4, the lowest-energy band is attributed to the delta ->delta* transition. For Tc2X4(PMe3)(4), the assignment of the transitions follow the following order in energy: delta* -> sigma* < delta* -> pi* < delta -> sigma* < delta -> pi*.