O-bonding orbitals

When interactinj antibonding cr orbitals, the reader should beware that the ign of the overlap between adjacent o bond orbitals depends on the bond angle. 

Bonding orbitals in a metal complex may be thought of as molecular orhitals built from appropriate metal and ligand functions. In the case of an M-L o bond orbital, i/ , for example, we write... 

These compounds are often referred to as hypervalent. The apical bonds in a trigonal bipyramid are described by molecular orbitals constructed from a p-orbital on the central atom and o-bonding orbitals (p- or sp hybrid) on the apical ligands. The molecular orbitals can be drawn as ... 

The large downfield CSA for disilenes, and indeed the large isotropic chemical shift, is caused mainly by the great deshielding of one component of the tensor, o-,. Tossell and Lazzaretti propose that this deshielding results from a low-energy electronic transition between a o--bonding orbital in the molecular plane and the Si=Si 7r -orbital.45... 

Examples of 7r-bonding orbitals (A) and o--bonding orbitals (B) for d elec-... 

It follows from this analysis that any substituent, whether donor or acceptor, weakens the bond to which it is attached. Consider an acceptor substituent. Let M be the bond and N the substituent. The HOMO(M) is the o bonding orbital, which is depopulated by the M- N interaction. A similar reasoning shows that a donor will also weaken the bond because it will partially populate the o antibonding orbital. 

Is it better to place a substituent at each end of the bond, or to place both substituents upon the same atom A stepwise approach again provides the answer. If a donor D is present, the o bonding orbital becomes dissymmetric, with the larger coefficient at the nonsubstituted atom ... 

In addition to a relatively small inductive effect, two modes of stabilization of positive charge by the P-R3M substituent have been suggested. The first involves the classical cation 7, where the positive charge is stabilized by hyperconjugation (G-p conjugation) between the C-M o bonding orbital and the vacant carbocation... 

The NMR properties of B in three and four coordination with oxygen have also been studied (Tossell, 1986 Tossell and Lazzeretti, 1988c). For three-coordinate and four-coordinate B, the B 2/2-0 2p o-bonding orbitals make the dominant contribution to the paramagnetic shielding, o-p. Large-basis-set calculations on indicate that it has a less anisotropic... 

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