Crystal field theory covalence adjustments

The model just described, which is derived on the basis of pure crystal field theory, can be modified rather simply to include the effects of electron delocalization. Since the ab initio calculation of crystal field terms is at best totally unreliable, we follow the usual practice of treating these terms simply as adjustable parameters. Thus the question of paramount importance is how the spin-orbit coupling constant i is affected by covalent bond formation. 

The basic difficulty with the CFT treatment is that it takes no account of the partly covalent nature of the metal-ligand bonds, and therefore whatever effects and phenomena stem directly from covalence are entirely inexplicable in simple CFT. On the other hand, CFT provides a very simple and easy way of treating numerically many aspects of the electronic structures of complexes. MO theory, in contrast, does not provide numerical results in such an easy way. Therefore, a kind of modified CFT has been devised in which certain parameters are empirically adjusted to allow for the effects of covalence without explicitly introducing covalence into the CFT formalism. This modified CFT is often called ligand field theory, LFT. However, LFT is sometimes also used as a general name for the whole gradation of theories from the electrostatic CFT to the MO formulation. We shall use LFT in the latter sense in this Chapter, and we introduce the name adjusted crystal field theory, ACFT, to specify the form of CFT in which some parameters are empirically altered to allow for covalence without explicitly introducing it. 

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