Influencing factors donor-acceptor substituent effects

The ability of heavy Group V ligands and transition metals to form stable L M—ER3 (E = P, As, Sb, Bi M = transition metal L = other ligands) is determined by the synergic interplay of their respective donor-acceptor properties, subtly modulated by steric influences. The electronic and steric factors determining the electron availability on the transition metal are determined by the oxidation state, coordination number, orbital geometry and the ligand effects of the other substituents in the coordination sphere. These factors will be discussed later. 

The introduction of a second substituent into an aromatic ring may have a very small or a very dramatic effect on the positions of the absorption bands. If both substituents are electron donors or electron acceptors, the effect of the second substituent will usually be rather small. If the two substituents are not identical, the electron donor with the highest-energy lone pair will dominate the spectrum. For example, the absorption spectrum of m-aminophenol will be very close to that of aniline. If the two substituents are electron acceptors, the one with the stronger electron-withdrawing influence will appear to be the dominating factor. The absorption spectrum of pyridine 3-aldehyde, for example, appears in very nearly the same place as the spectrum of benzaldehyde. If, however, one substitu-... 

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