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.