Bonding in clusters. Part 9. The closo–nido relationship; an MNDO computational study
Abstract
The role of the four bridging hydrogens, 4Hb, in the cluster bonding of B5H9 is compared with that of S in SB5H5, CH– in [CB5H6]–, and BH2– in [B6H6]2–, by analysis of MNDO calculations on the molecules. The four groups are shown to be very similar in their contributions to cluster bonding, and thus B5H9 can be considered as a subrogated closo system in the same manner as the two heteroborane clusters. This is consistent with the established pattern-recognition procedure for relating closo and nido boranes and with Wade's rules, simply introducing 4Hb as another subrogating moiety. This is further tested by analysis of calculations on the [B11H11]2––B10H14 and [B7H7]2––B6H10closo–nido pairs and found to be consistently applicable. This approach to the closo–nido relationship is compared to previous explanations.