Halogen-bond geometry: a crystallographic database investigation of dihalogen complexes

X-ray crystal structures of 141 halogen-bonded complexes Y-X...B formed between homo- and heteronuclear dihalogens Cl-2, Br-2, I-2, IBr and ICl with O, S, Se, N, P and As Lewis bases show remarkable and constant geometrical features. The metrics of the halogen bond found in the gas phase for simple complexes [Legon (1999a). Angew Chem. Int. Ed. Eng. 38, 2686-2714] is supported (i) in the solid state, (ii) for new Lewis acids (I-2 and IBr), (iii) for new basic centers (Se, As and =N-) and (iv) for more complicated bases. The Y-X...B arrangement is more linear than the corresponding Y-H...B hydrogen bond and the axis of the Y-X molecule lies in the plane of the B lone pair(s), with a preference for the putative lone-pair direction within that plane. However, exceptions to this lone-pair rule are found for sterically hindered thiocarbonyl and selenocarbonyl bases. A bond-order model of the halogen bond correctly predicts the observed correlation between the shortening of the X...B distance and the lengthening, Deltad(Y-X), of the Y-X bond. The expectation that the solid-state geometric parameters d(X...B) and Deltad( Y-X) reflect the strength of the interaction is supported by their significant relationships with the solution thermodynamic parameters of Lewis acidity and basicity strength, such as the Gibbs energy of 1:1 complexation of Lewis bases with diiodine. This analysis of halogen-bonded complexes in the solid state reinforces the similarities already known to exist between hydrogen and halogen bonding.