Role of the transition metal in metallaborane chemistry. Reactivity of (Cp*ReH2)(2)B4H4 with BH3 center dot thf, CO, and Co-2(CO)(8)

The reaction of Cp*ReCl4, [Cp*ReCl3](2), or [Cp*ReCl2](2) (Cp* = eta (5)-C5 Me5) with LiBH4 leads to the formation of 7-skeletal-electron-pair (7-sep) (Cp*ReH2)(2)(B2H3)(2) (1) together with Cp*ReH6. Compound 1 is metastable a nd eliminates Hz at room temperature to generate 6-sep (Cp*ReH2)(2)B4H4 (2) . The reaction of 2 with BH3(.)thf produces 7-sep (Cp*Re)(2)B7H7, a hypoele ctronic cluster characterized previously. Heating of 2 with 1 atm of CO lea ds to 6-sep (Cp*ReCO)(Cp*ReH2)B4H4 (3). Both 2 and 3 have the same bicapped Re2B2 tetrahedral cluster core structure. Monitoring the reaction of 2 wit h CO at room temperature by NMR reveals the formation of a 7-sep, metastabl e intermediate, (Cp*ReCO)(Cp*ReH2)(B2H3)(2) (4), which converts to 3 on hea ting. An X-ray structure determination reveals two isomeric forms (4-cis an d 4-trans) in the crystallographic asymmetric unit which differ in geometry relative to the disposition of the metal ancillary ligands with respect to the Re-Re bond. The presence of these isomers in solution is corroborated by the solution NMR data and the infrared spectrum. In both isomers, the me tallaborane core consists of fused B2Re2 tetrahedra sharing the Re-2 fragme nt. On the basis of similarities in electron count and spectroscopic data, 1 also possesses the same bitetrahedral structure. The reaction of 2 with C o-2(CO)(8) results in the formal replacement of the four rhenium hydrides w ith a 4-electron Co-2(CO)(5) fragment, thereby closing the open face in 2 t o produce the 6-sep hypoelectronic cluster (Cp*Re)(2)Co-2(CO)(5)B4H4 (5). T hese reaction outcomes are compared and contrasted with those previously ob served for 5-sep (Cp*Cr-2)(2)B4H8.