Coordination and Exchange of Ligands

In many transition metal complexes, the coordination number is variable. Especially in solution or as the result of thermal dissociation, ligands can be released from the complex or undergo exchange, or free coordination sites can be occupied by solvent molecules. Therefore, most complexes do not react in their coordinatively saturated form, but via an intermediate of lower coordination munber with which they are in equilibrium. For example, triphenylphosphine platinum complexes are involved in the following equilibrium reactions [T12]  

In aromatic solvents, the first equilibrium constant Ki indicates rapid dissociation, but the second equUibrimn constant is very small. However, the extremely high reactivity of [Pt(PPh3)2] compensates for this concentration effect, and complete reaction occurs with it-acidic molecules such as CO and NO  

The rapid dissociation of many complexes is explained in terms of steric hindrance of the ligands. With increasing space requirements of the phosphine or phosphite ligands, the rate of dissociation increases. A semi-quantitive measure for steric demand is tlie cone angle of the ligand (Table 2-1), as introduced by Tohnan [20]. 

Accordingly, the stericaUy most demanding ligands should exhibit the fastest dissociation. This is demonstrated by the dissociation constants for complexes of nickel. For the reaction 

For ligand dissociation/association processes, Tohnan introduced the 16/18-electron rule [19] (see Section 2.2.1). For each covalently bonded ligand, two electrons are added to the number of d electrons of the central transition metal atom (corresponding to its formal oxidation state) to give a total valence electron count. An example for a complex involved in a thermal dissociative equilibrium is the well-known Wilkinson s catalyst  

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