Effects of Non-reacting Ligands

Effects of Non-reacting Ligands. The two important effects of non-reacting ligands on the reactivity of platinum(n), as of other square-planar complexes are the trans effect and steric effects of bulky substituents. The latter lead to the so-called pseudo-octahedral behaviour wherein associative attack by the incoming nucleophile is prevented by the bulk of the nonreacting ligand. This results in a purely dissociative mechanism and a rate law 

Dithio-oxamide (dtoa) has a high affinity for platinum(n), so the following reaction, 

Peshchevitskii, G. D. Mal chikov, and R. L. Shchekochikhina, Izvest. sibirsk. Otdel. Akad. Nauk, Ser. khim. Nauk, 1972, 67 Chem. Abs., 1972, 77, 79 970n). 

An empirical equation has been established to correlate reactivities observed in reactions of [PtX4] , where X = Cl or Br, and of [PtLCls]-, where L = ammonia, methylamine, or pyridine, with a range of nitrogen bases.  

Isomerization and Inversion. The kinetics of the imcatalysed isomerization cw-- ru j-[Pt(PEt3)2(o-tolyl)Cl] have been studied over the temperature range 25—45 °C in methanol and in ethanol. Isomerization rates vary with chloride concentration the dependence is of the form kjil -f A [C1 ]). Photoisomerization of cw-[Pt(gly)2l is mentioned in the next section. 

An earlier investigation of phosphine-catalysed isomerization of platinum(n) complexes resulted in the proposal of a mechanism in which pseudorotation in a five-co-ordinate intermediate played a central role. The authors of a recent study of phosphine-catalysed isomerization of -[PtLaXa] prefer a consecutive displacement reaction, with an ionic intermediate [PtLaL X]+X . Their conclusion is based on n.m.r. determinations both of isomerization and of ligand-exchange rates, on solvent effects on rates, and on the stabilities of species [PtLL Xa] relative to [PtLaXJ and [PtL XJ. Although these results are convincingly consistent with a consecutive-displacement mechanism, they do not seem to rule out the alternative pseudorotation scheme. 

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Two further review articles discuss the factors which determine the catalytic reactivities of transition-metal complexes. The influence of the non-reacting ligands present in the complexes is described in detail, especially from the point of view of being able to tailor certain types of complex to give just the right catalytic properties. A specific example of an investigation of the effects of ligands on catalytic activity is provided in the study of the complexes MX(CO)La, where M = Rh or Ir, X = Cl, Br, or I, and L = a phosphine or phosphite. ... 

We have so far considered how the electrostatic effects of ligands cause the d electrons to prefer certain regions of space (i.c. certain orbitals) to others. We shall now look briefly at some ways in which the non-spherical distribution of the d electrons, caused by the environment, reacts back upon tile environment. 

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