Chatt's ligand constants: applicability and correspondence with Bursten's ligand additivity

Abstract

Recently Chatt has assigned a parameter P_L, closely analogous to the Hammett σ, for each of the common monodentate ligands L to indicate its electron-donating power. These P_L values correlate linearly with the oxidation potential E^o_L of various low-spin d⁶ complexes, where L can be varied through a three-parameter relation of the type E^o_L=E_S+βP_L(E_s and β are two constants). Considering low-spin d⁶ complexes where two ligands can be varied, I have shown that these P_L constants are almost additive in nature. The concept of P_L is extended to bidentate ligands. P_L values for ethylenediamine (en), 2,2′-bipyridyl (bipy), and 2-phenylazopyridine are found to be –1.57, –1.14, and –0.76 V respectively; interestingly, P_en≈ 2P_NH3, and P_bipy≈ 2P_py(py = pyridine). The additivity of P_L does not hold if one (or both) of the substituting ligands is (are) strongly electron-withdrawing or -donating in nature. Possible applicability of P_L to d⁸ systems is indicated by the observed variation of the rate of the substitution of L in [Pt(dien)L]²⁺(dien = diethylenetriamine) by py with P_L. Bursten has recently given a semiempirical relationship for predicting the oxidation potentials of low-spin d⁶ complexes of type [ML_nL′_6–n]. A correspondence between the two approaches of Chatt and Bursten towards parameterisation of the ligand effect is established. Meanings of P_L, E_s, and β in the light of Bursten's model of ligand additivity are explored. It is observed that Chatt's P_L values should be applicable only in cases where the substituting ligand(s) is (are) much less π acidic than the ligands constituting the unchanged (core) fragments of the complexes.