Hypervalent bonding, orbital diagram

In 1969, Musher (5) termed such electron rich species as hypervalent. Musher suggested that the three-center four-electron (3c-4e) bond could be viewed as a linear combination of an atomic /7-orbital from each of the three atoms involved in the hypervalent bond. The combination of these orbitals leads to three molecular orbitals that approximate hypervalent bonding. The orbital diagram indicates two of... 

This model implies that the TBP M atom could use its nsp2 orbitals for bonding with the equatorial ligands to form two-center bonds, and its npz orbital could be involved in the interaction with an appropriate orbital of the axial substituent X and a lone electron pair of the donor atom to form a hypervalent, 3c-4e bond in the axial moiety D — M—X. The simplest MO diagram of a 3c-4e bond may be represented by three molecular orbitals bonding (b), nonbonding (nb) and antibonding (ab) (Scheme 1). 

The description of 3c-4e bonds, the sort which Musher (35) has called hypervalent, in terms of linear combinations of orbitals of the three colinear atomic centers is pictured for the product sulfurane (2i ) at the right below. The 5c-6e bond i stu-lated for transition state 23 is illustrated in similar terms in the MO diagram to the left. Arduengo and Burgess (36) have discussed related multicenter bonds. 

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