PARAMETRIC CORRELATION OF FORMATION-CONSTANTS IN AQUEOUS-SOLUTION .1. LIGANDS WITH SMALL DONOR ATOMS

It is shown that four-parameter equations proposed by previous authors have poor predictive powers for data of formation constants in aqueous solution. This relates partly to the paucity of data on complexes of ligiands such as ammonia which, with most metal ions, cannot exist in water because of hydrolysis. Equations previously proposed that relate the formation constants of polyamine and poly(aminocarboxylate) complexes to those of the ammonia and acetate complexes are used to calculate formation constants for these hydrolysis-prone ammonia complexes. An equation of the type log K1=EAEB- + CACB, where E and C are identified with the tendencies of the Lewis acid A and base B to undergo ionic and covalent bonding, was used to correlate the F-, OH- and NH3 formation constants of 27 Lewis acids to a standard deviation of 0.24 log unit. Hardness parameters and HA were defined as EA/CA and EB/CB for acids and bases, respectively and gave reasonable orders of hardness. It was found that for ligands with large donor atoms, such as CI- and for sulfur or phosphorus donor atoms, deviations from the predictions of this equation were observed that appeared to be related to the size of the acid, so that no deviations were observed for large cations such as Ag+ and Pb2+, with occurrence of large deviations for smaller cations such as Cu2+ or Ni2+, with the very largest occurring for the proton. These deviations were attributed to steric hindrance between the large donor atom and adjacent coordinated water molecules. © 1978, American Chemical Society. All rights reserved.