The strength and length of donor-acceptor bonds in molecular complexes

Abstract

Numerous published data on the structure and thermodynamics of formation of molecular complexes are analyzed. The enthalpies of complexation (−ΔH) are related to the characteristic parameter Δr = [r DA−a 1(r D+r A)], where r DA is the donor-acceptor bond length determined by microwave spectroscopy and X-ray analysis, r D and r A are the tabulated values of the homopolar covalent radii of the heteroatoms that form the donor-acceptor bond, and a 1 is an empirical coefficient equal to 0.901±0.007. The relation between −ΔH and Δr values has the form −ΔH = a 2/Δr (a 2 = 21.6±1.6 kJ A mol−1), with a mean relative error of approximation of about 15% and a correlation coefficient of 0.97. As the strength of the complex increases, the donor-acceptor bond length approaches the sum of the heteropolar covalent radii of the atoms involved in the bond (Δr tends to zero). At Δr ≫ 1, the strength of complexes is determined by weak van der Waals interactions between the complex components and the −ΔH values tend to zero.