Metal ion stabilities correlate with electron affinity rather than hardness or softness

Abstract

Quantitative metal ion hardness values allow a test of their correlation with aqueous metal ion stability constants of four hard ligands: hydroxide, fluoride, acetate and ammonia. For all four ligands the correlation is weak and dependent on the metal ion charge. For all but fluoride the correlation is in the wrong direction. Metal ion electron affinity correlates much better than hardness or softness with metal ion stability constants of all four ligands across metal ions of differing charges. Positive deviations of stability constants from the general trend with electron affinity are often due to a decrease in coordination number from that of the aqueous ion. For metal ion stabilities the hard and soft precept needs to be modified along the following lines. With the proviso that the intrinsic strength of binding (including any change in coordination number) is first accommodated, then hard acids may prefer to coordinate with hard bases and soft acids with soft bases, however, in most cases metal ion stabilities are accounted for mainly by the intrinsic strength. In application to stability constants the hard and soft acids and bases concept is of limited utility and often misleading.