Ligand thermodynamic argument

Polydentate ligands do not necessarily need to bind a metal at all possible locations. In some cases, only one site is bound to the metal and the other one or more are left dangling. Such complexes are rare, and it has been necessary to develop special synthetic methods to prepare them. Explain in terms of thermodynamic arguments why polydentate ligands will almost always form chelating complexes rather than leave dangling arms. 

Consider next how this general ligand binding argument relates specifically to the Timasheff mechanism for solute-induced protein stabilization and destabilization. Detailed, rigorous reviews of the Timasheff mechanism can be found elsewhere (e.g., [78,79]). For the purpose of the current review a brief summary, which purposely provides only a simplified explanation, will suffice. First, a descriptive overview will be given, followed by an examination in more detail of the most relevant thermodynamic equations. 

Only recently has direct evidence for the presence of a hydride ligand bound to FeMoco become available [268]. On the basis of thermodynamic arguments, Alberty contends that the highly reduced state of FeMoco required for N2 reduction leads to the incidental production of H2. Others have su ested that the reductive elimination of H2 is necessary to produce a more reduced form of the FeMoco center and an open coordination site for N2 binding [269]. 

Especially for the phosphine, the thermodynamics of complex formation follows very much the same pattern as has been found before for the likewise very soft cyanide ligand. Evidently the bonds formed by these ligands must be of a rather similar nature, all markedly covalent according to the arguments advanced above. 

This argument is logical and scientifically accurate with respect to the transfer of iron in the body. However, it does not address the potential importance of the thermodynamic stability constants of the two common valence forms of iron (II and III) with ligands in food. 

The fact that the equilibrium for the reaction favors the chelate Co(en)3 means that the chelate has thermodynamic stability. A similar argument could be made for any equilibrium involving the replacement of monodentate ligands by polydentate ligands. Reaction tends to favor the chelate. 

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