In square-planar complexes

Splitting of d-orbitals in square planar complexes of copper(II), nickel(II) and cobalt(II). Y. Nishida andS. Kida, Coord. Chem. Rev., 1979, 27, 275-298 (94). 

The basic thiosemicarbazone ligands coordinate in a N,S bidentate manner, generally via the azomethine N-donor and the thione or deprotonated amidothione group. Designating the ligands as HL, this usually results in square planar complexes [NiL2],1221-1227 [Ni(HL)2]X2,12 2,1228... 

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... 

One of the fascinating characteristics of substitution in square planar complexes is illustrated by the following equations ... 

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. 

A completely empirical LFER can also be constructed with recourse only to kinetic data. This has been the case in the setting up of a scale of nucleophilic power for ligands substituting in square-planar complexes based on the Swain-Scott approach. The second-order rate constants Ay for reactions in MeOH of nucleophiles Y with tra 5-Pt(py)2Cl2, chosen as the standard substrate... 

Fig. 4.9 Simplified reaction profiles for various situations in the associative mechanism for substitution in square planar complexes, focusing attention on the replacement M-X-l-Y —> M-Y + X(4.93).

As with substitution in octahedral complexes, in chelation in square planar complexes the formation of the first bond is usually rate-determining (see (4.101)). 

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. 

This problem requires a modified approach which Gray 16) has solved in the case of substitution in square planar complexes. He uses the fact that bases, like hydroxide, substitute very slowly but will immediately deprotonate, and hence stabilize, a protonic solvento intermediate. This elegant approach cannot be applied to the octahedral cobaltammines whose reaction rate with such bases is very high. 

Fig. 11. Metal-metal interaction in square-planar complexes. Axes are taken between the ligands.

Pearson, in elaborating upon these ideas, distilled the essence of the tr competition theory to two soft ligands in mutual irons positions will have a destabilizing influence on each other when attached to class (h) (soft) metal atoms. He also provided additional examples of the rule that symbiosis prevails in octahedral complexes. antisymbiosis in square planar complexes. Tetrahedral complexes are expected to show antisymbiosis but on a much reduced scale compared with the square planar complexes.92... 

Mechanism of Nucleophilic Substitution in Square Planar Complexes... 

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