Tr Antibonding orbitals

The orbital interactions underlying this type of sulfur-mediated carbanion stabilization are somewhat subtle and are best described as a form of hyperconjugation. An examination of the highest occupied molecular orbital (HOMO) shows that the tr-antibonding orbitals for the C-S bonds parallel to the carbanion lone pair (shown in red below) are able to delocalize the negative charge ... 

Fig. 11.2 Effeci ol meial-lifand ir bonding the Cr—C bond order e increased and the C—O bond order is decreased, (a) VB viewpoint rr bond between J orbital on Cr atom and p orbital on C atom, (b) MO viewpoint tr bond between d orbital on Cr atom and antibonding orbital (rr ) on the CG ligund.

The observed stability order of alkenes is due to a combination of two factors. The first is hyperconjugation—a stabilizing interaction between the unfilled antibonding C=C tt bond orbital and a filled C-H tr bond orbital on a neighboring substituent (Figure 6.9). The more substituents that are present, the more opportunities exist for hyperconjugation, and the more stable the alkene. 

It appears that the metal-ligand bonds in carbonyl and dinitrogen complexes are similar but somewhat weaker in the dinitrogen complexes. Carbon monoxide is not only a better cr donor but also a better tr acceptor. This is to be expected on the basis of whatever polarity exists in the CO molecule (see Fig. 5.18) and the fact that the v antibonding orbital is concentrated on the carbon atom (see Fig. 5.20), which favors overlap with the metal orbital. The superior v accepting ability of CO also accounts for the instability of carbonyl dinitrogen complexes. Both CrfCOljNj and ds-... 

As a cycloaddition, singlet carbene addition to an alkene must obey the rules of orbital symmetry discussed in Chapters 35 and 36. We might consider the empty p orbital of the carbene (LUMO) interacting with the tr bond (HOMO) of the alkene or the lone pair of the carbene in its filled sp orbital (HOMO) interacting with the antibonding orbital of the alkene (LUMO). 

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