Bonding studies of derivatives of pentaborane(9) through self-consistent charge calculations

Abstract

Derivatives of pentaborane(9) have been studied by self-consistent charge calculations to examine multicentre bonding and the application of the ‘isolobal, pseudo-isoelectronic’ principles. The results reproduce previously reported photoelectron spectra reasonably well. Interpretation of the results suggests that the differences encountered in bonding apical and basal units are similar for BH, Fe(CO)₃, and Co(η⁵-C₅H₅) groups. The highest occupied molecular orbitals of B₄H₉ involve apical boron 2p orbitals and correlate well with cluster orbitals in 1-[B₄H₈Fe(CO)₃] and 1-[B₄H₈Co(η⁵-C₅H₅)] which largely involve metal 3d orbitals. However the next cluster orbital in B₅H₉ has no direct counterpart in the metal derivatives. The metallo-units appear to be bonded into the cluster by a one-electron, two-orbital combination rather than the two-electron, three-orbital method used by BH. Similarly for the 2-isomers. Replacement of a bridging hydrogen by a µ-(SiH₃) unit leaves the cluster substantially unaltered. However with a µ-[Cu(PH₃)₂] bridging unit the major bonding contribution combines boron s and p and a copper 4p orbital in an ‘open’ three-centre bond.