Reactions Involving Gain or Loss of Ligands

Many reactions of organometallic compounds involve a change in metal coordination number by a gain or loss of ligands. If the oxidation state of the metal is retained, these reactions are considered addition or dissociation reactions if the metal oxidation state is changed, they are termed oxidative additions or reductive eliminations. In classifying these reactions, it is often necessary to determine oxidation states of the metals the donor pair method (Chapter 13) can be used. 

Type of Reaction Change in Coordination Number Change in Formal Oxidation State of Metal 

We will first consider ligand dissociation reactions. These reactions provide an avenue to replace ligands such as carbon monoxide and phosphines. 

In classifying these reactions, it will frequently be necessary to determine formal oxidation states of the metals in organometallic compounds. In general, method A (the donor pair method) described in Chapter 13 can be used in assigning oxidation states. Examples will be given later in this chapter in the discussion of oxidative addition reactions. 

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A) Reactions involving only one coordination position (the Co—C bond), where the only role of the surrounding solvent or medium is to facilitate the gain or loss of a proton by the organo-ligand. 

Reactions involving the gain or loss of a ligand These reactions deal with the addition or subtraction of a ligand to or from the metal center and include ligand dissociation and substitution, oxidative addition, reductive elimination, and nucleophilic displacement. 

Fig. 5. Dual-pathway square scheme mechanism for electron transfer involving the Cun/I([9]aneS3)tt system. The vertical reactions involve ligand gain or loss while the horizontal reactions represent electron transfer. Reproduced from Ref. (6) by permission of the Royal Society of Chemistry.

This section deals with reactions which involve only the organo-ligand, whether unmodified or modified by the loss or gain of a proton, and the cobalt. After considering established and possible mechanisms (Section B,l) we list examples to show how reactivity is influenced by different functional groups and substituents (Section B,2), and by changes in the axial and equatorial ligands (Section B,3). 

It was also demonstrated that both 16 and 17 are in equilibrium, which involves loss or gain of chloride ligand. Compound 16 is very reactive toward alkenes and alkynes in water, whereas it is absolutely unreactive in organic solvents. The reaction which takes place with alkynes is shown in Eq. (2). 

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