Synthesis and Characterization of Hypoelectronic Rhenaboranes. Analysis of the Geometric and Electronic Structures of Species Following Neither Borane nor Metal Cluster Electron-Counting Paradigms.

The reaction of (Cp*ReH2)2B4H4 with monoborane leads to the sequential formation of (Cp*Re)2BnHn (n = 7-10, 1-4). These species adopt closed deltahedra with the same total connectivities as the closo-borane anions [BnHn]2-, n = 9-12, but with flattened geometries rather than spherical shapes. These rhenaborane clusters are characterized by high metal coordination numbers, Re-Re cross-cluster distances within the Re-Re single bond range, and formal cluster electron counts three skeletal electron pairs short of that required for a canonical closo-structure of the same nuclearity. An open cluster, (Cp*ReH)2B2H9 (5), is isolated that bears the same structural relationship to arachno-B2H15 as 1-4 bear to the closo-borane anions. Chloroborane permits the isolation of (Cp*ReH)2B5Cl5 (6), an isoelectronic chloroanalogue of known open (Cp*WH2)2B5Cl5 and (Cp*Re)2B6H4Cl2 (7), a triple-decker complex containing a planar, six-membered 1 ,2-B6H6C12 ring. Both are putative five- and six-boron intermediates in the formation of 1. Electronic structure calculations (extended Hückel and density functional theory) yield geometries in agreement with the structure determinations, large HOMO-LUMO gaps in accord with the high stabilities, and 11B chemical shifts accurately reflecting the observed shifts. Analyses of the bonding in 1-4 reveal that the Cp*Re...Cp*Re interaction generates fragment orbitals that are able to contribute the "missing" three skeletal electron pairs required for skeletal bonding. The necessity of a Re...Re interaction for strong cluster bonding requires a borane fragment shape change to accommodate it, thereby explaining the noncanonical geometries. Application of the debor principle of borane chemistry to the shapes of 1-4 readily rationalizes the observed geometries of 5 and 6. This evidence of the scope of transition metal fragment control of borane geometry suggests the existence of a large class of metallaboranes with structures not found in known borane or metal clusters. [ABSTRACT FROM AUTHOR]