The reaction of BH3·THF with [Cp*CrCl]2, Cp* = η5-C5Me5, 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*2Cr2(CO)2B4H6(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.
ASJC Scopus subject areas
- Colloid and Surface Chemistry