Some factors influencing mixed complex formation

Abstract

The tendency toward mixed complex formation may be expressed quantitatively in two different ways. The most common approach is to compare the stability of the mixed complex with the two pure 2:1 complexes where two like ligands are bound to a single metal ion. It is shown that extraneous factors that enter into formation of the pure 2:1 complexes, such as steric effects, may give a misleading indication of the stability of a mixed complex. Moreover, unless both ligands are of the same charge, there is an appreciable electrostatic contribution to mixed complex formation that is ionic strength dependent. The alternative formulation that relates the stability of the mixed complex to 1:1 complexes provides a better measure of the intrinsic tendency toward mixed complex formation. When one of the ligands is uncharged as is the case with amines, there is no electrostatic contribution in this formulation. For most ligands the visible circular dichroism magnitudes of mixed Cu(II) complexes containing l-alanine or R-1,2-diaminopropane as one ligand are not half that of the 2:1 complex with one of the optically active ligands. This result suggests profound electronic interactions within the tetragonal plane. In addition four aromatic amines, bipyridyl, phenanthroline, bipyridylamine and histamine, when present as the other ligand in a mixed Cu(II) complex invert the sign of the visible CD from that of the 2:1 complex of the optically active ligand. Steric effects are ruled out and transmission of electronic effects through π systems is suggested to account for the inversions. Most Cu(II) complexes with two bidentate ligands are at least five-coordinate in solution, but axially bound water is not specifically considered in mixed complex formation. So-called binary complexes are actually ternary complexes with water as the second ligand. Viewed in this way ratios of the first to second stability constants provide an untapped fund of data on tendencies toward formation of mixed complexes.