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Numerous published data on the structure and thermodynamics of formation of molecular complexes are analyzed. The enthalpies of complexation (-∆Н ) are related to the characteris tic parameter ∆r = [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 -∆Н and ∆r values has the form -∆Н = a 2 /∆r (a 2 = 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.Key words: molecular complex, enthalpy of formation, donor acceptor bond, bond length.One of the most important characteristics of molecu lar complexes is the enthalpy of formation (-∆H ), a mea sure of the strength of the donor acceptor bond. Unlike the -∆H values for covalent bonds, the energies of formation of intermolecular donor acceptor bonds (-∆H DA /kJ mol -1 ) vary over a rather wide range from a few to a hundred and more kJ mol -1 .Numerous attempts to relate the -∆H parameter to other characteristics of molecular complexes, e.g., posi tions of the charge transfer bands in electronic absorption spectra (EAS), dipole moments of complexes, changes in entropy of complexation, and the chemical shift of coor * Dedicated to the memory of E. N. Guryanova (1911Guryanova ( -2004, a prominent scientist who formulated the basic principles elabo rated by her disciples in this review.

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