Synthesis and characterization of the octahydrotriborate complexes Cp*V(B3H8)2 and Cp*Cr(B3H8)2 and the unusual cobaltaborane cluster Cp*2Co2(B6H14).

The new compounds CpV(B(3)H(8))(2), CpCr(B(3)H(8))(2), and Cp(2)Co(2)(B(6)H(14)) have been synthesized by treating the pentamethylcyclopentadienyl complexes [CpVCl(2)](3), [CpCrCl(2)](2), and [CpCoCl](2) with NaB(3)H(8). X-ray crystallography shows that CpV(B(3)H(8))(2) and CpCr(B(3)H(8))(2) have the same ligand sets but different molecular structures: the vanadium compound contains two bidentate B(3)H(8) ligands (i.e., bound to the metal center via two vicinal hydrogen atoms), whereas the chromium compound has one bidentate B(3)H(8) ligand and one B(3)H(8) ligand bound in an unprecedented fashion via two geminal hydrogen atoms. The "gem-bound" B(3)H(8) group itself has an atypical structure consisting of a BH(2)-BH(2)-BH(3) triangle with one additional hydrogen atom bridging the unique BH(2)-BH(2) edge. The B-B distances are nearly identical within experimental error at 1.790(5), 1.792(5), and 1.786(6) Angstrom. The relationship between the electronic and molecular structures of the V and Cr compounds is briefly discussed. The structure of Cp(2)Co(2)(B(6)H(14)) can be viewed in two different ways: as a dicobalt complex in which two CpCo units are each bound to four adjacent boron atoms of an S-shaped B(6)H(14) ligand, or as an eight-vertex hypho cluster compound. In the former case, the B(6)H(14) ligand is best regarded as a dianionic bi-borallyl group H(3)B(mu-H)BH(mu-H)BHBH(mu-H)BH(mu-H)BH(3) in which one hydrogen at each end of the chain is involved in an agostic interaction. From a cluster point of view, the structure of Cp(2)Co(2)(B(6)H(14)) can be generated by removing three adjacent high-connectivity vertices from the eleven-vertex closo polyhedron. The Co-B distances vary from 2.008(5) to 2.183(4) Angstrom, and the B-B distances within in the S-shaped chain range from 1.734(8) to 1.889(6) Angstrom. Finally, a new synthesis of the known molybdenum compound Cp(2)Mo(2)(B(5)H(9)) is described; its structure as established by X-ray crystallography closely resembles that of the previously described (C(5)H(4)Me) analogue.