Dinuclear (arene)ruthenium hydrido complexes: Synthesis, structure, and fluxionality of (C(6)Me(6))(2)Ru2H3(BH4)

In aqueous solution, the cationic complex [(C(6)Me(6))Ru(H2O)(3)](2+) reacts with sodium borohydride to give the known dinuclear cation [(C(6)Me(6))(2)Ru2H3](+) (1(+)), which can be isolated as the hexafluorophosphate salt. Under biphasic conditions (water/diethyl ether), 1(+) reacts with; additional sodium borohydride to afford the neutral dinuclear hydride complexes (C(6)Me(6))(2)Ru2H4 (2) and (C(6)Me(6))(2)Ru2H3(BH4) (3), which are extracted into the organic phase. The solid-state structure of 3 has been solved by a single-crystal X-ray structure analysis. It contains a single-bonded ruthenium-ruthenium backbone with one mu-hydride and two terminal hydrides; the BH4 bridge is bonded through two H atoms to the two ruthenium atoms. In solution, two independent fluxional processes can be distinguished by variable-temperature H-1 NMR spectroscopy, one of which involves the bridging and the terminal hydride ligands and the other the coordinated and the noncoordinated H atoms of the BH4 bridge. In addition, deuteration experiments reveal that the terminal hydrides and the mu-hydrides at the Ru atoms as well as the boron-bound hydrides of the BH4 ligand undergo complete hydride scrambling, a process that is slow with respect to the NMR time scale.