Monodentate ligands thermodynamic stability

The thermodynamic stability of a species is a measure of the extent to which this species will be formed from other species under certain conditions, provided that the system is allowed to reach equilibrium. Consider a metal ion M in solution together with a monodentate ligand L, then the system may be described by the following stepwise equilibria, in which, for convenience, coordinated water molecules are not shown ... 

Whilst the synthesis of new transition metal-olefin and -acetylene complexes continues unabated, only a relatively small amount of data has accumulated on the thermodynamic stability of these complexes and these are restricted almost exclusively to complexes of the unsatured species acting as monodentate ligands. Metals able to coordinate strongly with unsaturated ligands are restricted to those in a small triangle around the centre of the periodic table, and designated class (b) acceptors by Ahrland et al., 0>. Class (b) acceptors include Cu(I), Rh(II), Ag(I), Pt(II) and Hg(II). However the majority of such metals form inert complexes which are either very readily oxidised or involve solubility problems. If thermodynamic stability constants are to be measured reliably, the equilibrium should be reached reasonably quickly, the reaction should be clean and the stoichiometry should be known or easily deduced. Furthermore, the equilibrium must be followed by means of suitable electrodes or changes in some physical property of the reaction mixture. The solvent is therefore important. 

For a given metal ion, the thermodynamic stability of a chelated complex involving didentate or polydentate ligands is greater than that of a complex containing a corresponding number of comparable monodentate ligands. This is called the chelate ejfect. 

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. 

Thermodynamic data for the formation of Ag(TSC)3 are given in Table 58.437 Based on the similarity of the stability of this complex and the analogous thiourea complex, the ligand was proposed to be monodentate. The IR spectra of [Ag2(TSC)3](NC>3)2438 and of Ag(TSC)Cl439 were also interpreted in terms of silver-sulfur bonding only. 

Platinum (II) complexes of the type (COD)PtRX (COD = jj -l,5-cyclooctadiene R, X = alkyl, aryl, halide) are known for many combinations of R and X, and then-reaction chemistry is well developed see Platinum Organometallic Chemistr. The lability of the COD-Pt interaction renders these compounds convenient sources of organoplatinum fragments for coordination to Lewis bases such as phosphines, according to equation (27). In an effort to better understand the factors that influence the stability of bis(phosphine)platinum(II) species, a thermodynamical study of the reaction in equation (27) was undertaken for a series of monodentate and bidentate phosphine ligands of varying steric and electronic character. 

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