Rhenium-oxo-bis(acetylene) anions: structure, properties, and electronic structure. Comparison of Re-O bonding with that in other rhenium-oxo complexes

Reduction of Re(O)I(RC[triple bond]CR)[sub 2] (2) or [Re(O)(RC[triple bond]CR)[sub 2]][sub 2] by two electrons gives Re(O)(RC=[triple bond]CR)[sub 2]Na (R = Me, 1a; Et, 1b; Ph, 1c). Compounds 1 are unusual oxo complexes, being highly nucleophilic and strongly reducing. X-ray structures of 1a-crypt and 1c-2MeCN reveal Re(O)(RC[triple bond])[sub 2] units, as isolated anions in the former but, in the latter, connected via Na-O-Na bridges into centrosymmetric dimers. The acetylene ligands lie in a plane that is roughly perpendicular to the Re - O bond, but the C[triple bond]C vectors are splayed rather than parallel. The bond lengths and angles about rhenium are quite similar in the two structures, and quite close to the values found for 2 in which the splaying occurs to accommodate the iodide ligand. Reduction of 2a to 1a[center dot]crypt causes a lengthening of the Re-O bond from 1.697(3) to 1.745(7) [angstrom] and a drop in [nu][sub ReO] from 975 to 869 cm[sup [minus]1], both indicative of a decrease in the Re-O bond order. The Re-C distances and C[triple bond]C stretching frequencies both decrease on reduction, indicating increased Re [yields] acetylene back-bonding in 1. Effective core potential calculations on Re(O)(HC[triple bond]CH)[sub 2][sup [minus]] (A), Re(O)H(HC[triple bond]CH)[sub 2] (B), Re(O)Cl[sub 4][sup [minus]] (C), and Re(O)F[sub 5] (D) have been performed with excellent agreement between the calculated structures and experimental crystallographic data (A and B are models for 1 and 2). 49 refs., 5 figs., 4 tabs.