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 Cp*V(B3H8)(2), Cp*Cr(B3H8)(2), and Cp*Co-2(2)(B6H14) have been synthesized by treating the pentamethylcyclopentadienyl complexes [Cp*VCl2](3), [Cp*CrCl2](2), and [Cp*CoCl](2) with NaB3H8. X-ray crystallography shows that Cp*V(B3H8)(2) and Cp*Cr(B3H8)(2) have the same ligand sets but different molecular structures: the vanadium compound contains two bidentate B3H8 ligands (i.e., bound to the metal center via two vicinal hydrogen atoms), whereas the chromium compound has one bidentate B3H8 ligand and one B3H8 ligand bound in an unprecedented fashion via two geminal hydrogen atoms. The "gem-bound" B3H8 group itself has an atypical structure consisting of a BH2-BH2-BH3 triangle with one additional hydrogen atom bridging the unique BH2-BH2 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*Co-2(2)(B6H14) can be viewed in two different ways: as a dicobalt complex in which two Cp*Co units are each bound to four adjacent boron atoms of an S-shaped B6H14 ligand, or as an eight-vertex hypho cluster compound. In the former case, the B6H14 ligand is best regarded as a dianionic bi-borallyl group H3B(mu-H)BH(mu-H)BHBH( A-H) BH(mu-H)BH3 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*Co-2(2)(B6H14) 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*Mo-2(2)(B5H9) is described; its structure as established by X-ray crystallography closely resembles that of the previously described (C5H4Me) analogue.