EMPIRICAL FORCE-FIELD CALCULATIONS OF STRAIN-ENERGY CONTRIBUTIONS TO THE THERMODYNAMICS OF COMPLEX-FORMATION .3. CHELATE EFFECT IN COMPLEXES OF POLYAMINES

Empirical force field calculations are used to show that the steady decrease in enthalpy change on complex formation per nitrogen donor atom in complexes of polyamine ligands H(NHCH2CH2)xNH2, with NiII as x increases from 1 (en) to 4 (tetren), is due to cumulative ring strain. At the same time, this effect is partly offset by the increase in Ni-N bond strength in the absence of strain effects as the nitrogen donor changes from primary to secondary to tertiary. The opposing effects of cumulative ring strain and increasing bond strength of secondary relative to primary nitrogen donors coordinated to NiII produce a net decrease in ΔH on complex formation that almost exactly matches a term in ΔS present in the free energies of formation of the unidentate ammine complexes. This matching of terms makes analysis of the chelate effect in these polyamines appear much simpler than when the separate enthalpy and entropy contributions are examined. © 1979, American Chemical Society. All rights reserved.