Square-planar complexes, effect

The reductions are effected in nature by ferredoxin (p. 1102). This behaviour can be reproduced surprisingly well by simpler, model compounds. Some of the best known of these are obtained by the addition of axial groups to the square-planar complexes of Co with Schiff bases, or substituted glyoximes (giving cobaloximes) as illustrated in Fig. 26.7. The reduced Co species of these, along with vitamin... 

The preparation of the dinuclear complexes is readily achieved by taking advantage of the well-established trans effect in square-planar complexes. For example, the treatment of two equivalents... 

In addition to the indications of an octahedral trans effect presented, there exists structural information in the form of bond lengths and spectral data similar to that described earlier for square planar complexes. Although the trans effect in octahedral complexes is not the dominant influence that it is in square planar complexes, there is no doubt that there is such an effect. 

Isomerization involving a square planar complex is also known. Because of the trans effect, it is easier to synthesize the trans isomer of many complexes than it is to prepare the cis complex. The following reactions lead to the formation of an unusual platinum complex ... 

One of the effective catalysts for isomerization of alkenes is a square planar complex of Rh1 +. That... 

Mechanistic interpretation of activation volumes on square-planar complexes is complicated by the geometry. The sterically less crowded complexes may have loosely bound solvent molecules occupying the axial sites above and below the plane. Replacing them in the formation of a five-coordinate transition state or intermediate may result by compensation in relatively small volume effects. It is therefore difficult to distinguish between Ia and A mechanisms from the value of the activation volume. Nevertheless, the AV values are negative and together with the second-order rate laws observed, point to an a-activation for those solvent exchange reactions. 

Fio. 26. Energy level diagram of 3cP configuration (Cu" " ) in square planar complex or tetragonal crystal field (CF). Effect of bonding of the 3d-electrons with ligands is shown. 

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. 

When the apparently penta-coordinated diarsine complexes just described are dissolved in solvents more polar than nitrobenzene, they tend to dissociate into halide ions and bivalent cations, thus becoming 2 1 electrolytes (119). The effect is most marked with the platinum compounds. It has been shown that solvation effects might be less with platinum than with palladium, and so, other things in the equilibria being equal, it can also be concluded that the bonding of further ligands by a square-planar complex is much weaker with platinum than with palladium. Square-planar nickel complexes are of course the most ready to take up further ligands. 

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... 

There are several pathways by which one ligand may replace another in a square planar complex, including nucleophilic substitution, electrophilic substitution, and oxidative addition followed by reductive elimination. The first two of these are probably familiar from courses in organic chemistry. Oxidative addition and reductive elimination reactions will be covered in detail in Chapter 15. All three of these classes have been effectively illustrated by Cross for reactions of PtMeCItPMe-Ph),.-... 

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