Ligand based redox series intris-bipyridyl complexes ofd⁶ metal ions andtris-o-semiquinone complex of Cr (III) follow a general pattern. It is demonstrated that it is possible to look for a isothermodynamic proton transfer series. In the present study orthophosphoric acid is singled out. Implications of such correlations are discussed.

A correspondence betweenab initio calculations, the principle of electronegativity equalisation and group electronegativity has been established within the framework of Mulliken population analysis. Using this we have calculated electronegativities of some 37 groups/atoms. These electronegativities show excellent linear correlation with¹J_CC coupling constants in monosubstituted benzenes and Inamoto’si scale and a satisfactory one with Wells’ group electronegativity data. The correspondence however required a scaling of charge (obtained byab initio calculations) and a proportionality between the electronegativity of the neutral group and its hardness. It is shown that using these electronegativity values it is possible to calculate group charges in molecules where groups under consideration interact with each other through σ bond only.

The electronegativities (χ_G) of some 78 groups/atoms(G) including some 11 ionic ones have been evaluated from the charge on the H atom in the corresponding H−G molecules. The charges are determined by the MNDO method of Dewar at the optimised geometries. For some 38 molecules/ions whereab initio molecular orbital calculations are available, a reasonable linearity (correlation coefficientr=0·900) has been observed between the MNDO charges and the charges obtained at the 6–31G^*//6–31G^* level. The very good linear correlation (r=0·950) obtained between the χ_G values and Inamoto’s i scale for some 72 groups/atoms including some 8 monocationic groups shows that the method works quite well for the monocationic groups. However the anionic groups probably cannot be tackled in the present approach. The¹J_cc (ortho-ipso) coupling constants in mono-substituted benzenes also correlate linearly with the χ_G values for some 33 groups/atoms to a satisfactory extent (r=0·917). It is mentioned that there does not exist a proper theoretical scale for group electronegativity which can accommodate ionic and neutral groups on a uniform basis.

It is shown that by a judicious mixing of ionisation potential and electron affinity, a constant can be devised for a monodentate ligand as a measure of the electronic effects exerted by the ligand. A qualitative MO treatment is presented to justify the approach.

Some aspects of the recent developments on the concept of hardness are discussed briefly with due emphasis to our own work. The usefulness of hardness in understanding the stability and reactivity of a chemical species in general is illustrated while certain deficiencies of the concept are also pointed out. Some attempts have been made here to prove Pearson’s hard-soft acid-base principle to the extent possible.

Drago’se, c andt parameters for a number of chemically important mono- and bivalent metal ions are estimated from the heterolytic bond dissociation energy data of their binary halides, hydrides and methides. An important result is that, for the cationic acids, thec andt parameters are not two independent variables. Attempts to correlate these parameters or any combination of these with Pearson’s chemical hardness have been unsuccessful so far.