Square-planar complexes, effect orbitals

Briefly, as Prof. Basolo points out in his paper, and Prof. Basolo and Pearson pointed out nicely in a review on the trans effect, the situation in the ir-bonding theory is a stabilization of the trigonal bipyramidal intermediate because there are more orbitals in the trigonal plane available for 7r-bonding in the trigonal bipyramid than in the square planar complex. The same is also true in the --system. The trans effect is directional, and in my opinion, it has to be orbital or quantum mechanical. 

The presence of empty dzb orbitals should play a decisive part in the reduction of O2, as they do in 2 transmission. In the square planar complexes (cf. Fig. 32) investigated by us, this condition is fulfilled only by the attachment of a 5th ligand. We therefore assume that the surface groups of the carbon take effect as... 

Fig. 1 Simple ligand field-splitting diagram for metal d orbitals in a square planar complex. By convention, the z axis is perpendicular to the plane of the complex and the M - L bonds lie along the x and y axes. Note that the exact ordering of the lower energy levels depends on the ligand set (e.g., relative importance of a- and 7r-effects) but the dX2-y2 is always unequivocally the highest...

NO BF3, SOo, tetracyanoethylene, and O2 to form a series of well characterized products. The variations in reactivity toward such molecules as SOo, Oo, and BF that is observed among the many known square-planar complexes were attributed to the following factors (15) (a) the electron affinity of the covalent molecule, (b) the nucleophilicity of the metal in the complex, and (c) the ability of d orbitals on the metal to overlap effectively with suitable orbitals on the electrophile. Molecules such as Oo and SO2 are most appropriately viewed as tt acids of considerable electron affinity, although they do have some ability to act as (T donors, and also as Lewis (cr-bonding) acids. 

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