Bonding in clusters. Part 9. The closo–nido relationship; an MNDO computational study

Abstract

The role of the four bridging hydrogens, 4H_b, 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 4H_b 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.