Stability Constants of Metal(II) Complexes with Amines and Aminocarboxylates with Special Reference to Chelation

Abstract

Equations derived on the basis of mechanistic considerations have been utilized to calculate the stability constants of binary and ternary copper(II) complexes of amine(s) or aminocarboxylates. Calculated values of 185 copper(II) amine complexes have been compared with the observed. For 89 complexes including ammonia, ethylenediamine, cyclohexane-1,2-diamine, imidazole, pyridine, and some substituted analogs, the calculated values agree with the observed within 0.3 log unit. These complexes may be regarded as regular. Among the remaining non-regular complexes we find some complexes with lower stability constants due to steric hindrance, and some others with higher constants resulting from aromatic ³³ interactions and/or hydrophobic interactions between coordinated ligands. Mechanistic considerations have also been successfully utilized to predict stability constants of polyamine and aminocarboxylate chelates of nickel(II), cobalt(II), and copper(II) according to Adamson's approach.In our earlier reports 1 too much emphasis has been put on the successful application of mechanistic considerations to the prediction of stability constants of binary and ternary metal complexes from the stability constants of the 1:1 complexes. For amines and/or aminocarboxylates (A or L) the stability constant of metal complexes [MAL] and [ML n ] is given by the following equations.In the above equations and in the following, we denote K OS : the formation constant of the outer-sphere complex indicated; n w : the number of water molecules coordinated to the species indicated; ¤ ij : the effect of the donor atom i in the incoming ligand on the donor atom j in the coordinated ligand(s); log n: the statistical correction term for the number of ways that [ML n ] can dissociate. Stability constants are defined as follows:Because of the distorted octahedral structure of the copper(II) complexes some additional factors should be taken into account in their formation. In the reaction of copper(II) ion with ligands occupying less than four coordination sites terms with n w need not be taken into consideration: the dissociation of the water molecule at the axial position of the distorted octahedron of the hydrated copper(II) complexes is the rate determining step followed by rapid pseudo-rotation leading to the coordination of the incoming ligand on the equatorial position. In the coordination of non-charged amine nitrogen the second term involving the formation constant of outer-sphere complexes in eqs 1 and 2 1a may be dropped, and we have simpler eqs 4 and 5.Donor atomdonor atom interaction terms ¤ ij for copper(II) are given in Table 1. In this work we calculated 185 copper(II) amine complexes using eqs 4 and 5 and compared with the observed values. From the difference between the calculated and observed values we classified the ligand as "regular" complexes and "non-regular" complexes and have discussed the higher or lower stability constants from the reaction mechanism of the metal complex formation, steric hindrance, hydrophobic intera...

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