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Year 2007, Month 10 (Section Review, Page 1935)

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

I. P. Romm, Yu. G. Noskov, and A. A. Malkov

Numerous published data on the structure and thermodynamics of formation of molecular complexes are analyzed. The enthalpies of complexation (–∆Н ) are related to the characteristic parameter ∆r = [rDA–a1(rD+rA)], where rDA 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 a₁ is an empirical coefficient equal to 0.901±0.007. The relation between –∆Н and ∆r values has the form –∆Н = a₂/∆r (a₂ = 21.6±1.6 kJ Å 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 (∆r tends to zero). At ∆r >> 1, the strength of complexes is determined by weak van der Waals interactions between the complex components and the –∆Н values tend to zero.

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