HIGH-YIELD SYNTHESIS AND CHARACTERIZATION OF CHROMABORANES - COMPARISON OF THE GEOMETRIC, ELECTRONIC, AND CHEMICAL-PROPERTIES OF AN ELECTRONICALLY UNSATURATED (ETA(5)-C(5)ME(5))(2)CR2B4H8 CLUSTER WITH ITS SATURATED DERIVATIVE (ETA(5)-C(5)ME(5))(2)CR-2(CO)(2)B4H6

The reaction of BH3 . THF with [Cp*CrCl](2), Cp* = eta(5)-C(5)Me(5), leads to the isolation of Cp*2Cr2B4H8 (1) in very good yield. This green-brown, diamagnetic chromaborane cluster exhibits a nido geometry based on a pentagonal pyramidal deltahedron with metal atoms occupying the axial vertices and boron atoms four of the five equatorial vertices. Reaction with CO leads cleanly to brown, diamagnetic Cp*Cr-2(2)(CO)(2)B4H6 (3) possessing the same cluster core structure as 1. Fenske-Hall molecular orbital calculations show that both 1 and 3 have a significant Cr-Cr bonding interaction and that the cluster geometry is properly described as a bicapped tetrahedron. On this basis, 3, with 6 cluster bonding pairs, is an electronically saturated cluster whereas 1, with only 5 cluster bonding pairs, is unsaturated. The calculations show that it is the high energy of the Cr 3d orbitals that leads to the electronic unsaturation of 1 and that the back-bonding of the CO ligands in 3 selectively stabilizes some of the Cr metal orbitals producing an additional filled skeletal orbital. The reaction of LiBH4 with [Cp*CrCl](2) results in the formation of the green-brown, paramagnetic [Cp*Cr(BH4)](2) borohydride complex, 2, which has been characterized spectroscopically. Chromaborane 2 reacts with stoichiometric amounts of CO at low temperature to yield brown, diamagnetic [Cp*Cr(CO)(2)(BH4)](2) (4). Based on the 18 electron rule, 2 is electronically unsaturated whereas 4 is saturated. The intercomparison of these four compounds is used to illustrate the structural expression of an electronically unsaturated metallaborane cluster bonding network and to relate this description to analogous organometallic compounds.