Characterising secondary bonding interactions within triaryl organoantimony(V) and organobismuth(V) complexes

The thermodynamically preferred conformations of the five-co-ordinate trigonal-bipyramidal complexes SbPh3Cl2 1 and BiPh3Cl2 2, determined via van der Waals energy calculations, have been found to be in good agreement with the corresponding correlated propeller arrangements which are observed in the solid state. However, similar calculations upon the corresponding diesters SbPh3(O2CR)(2) 3 and BiPh3(O2CR)(2) 4 [R = C(OH)Ph-2 a or C6H4OH-o b] were at variance with the solid state, thereby indicating the participation of stabilising secondary bonding interactions between the metal centre and the carbonyl oxygen atom of the ester moiety. In accord with current Lewis acid-base bonding models, a strong correlation was found to exist between the pK(a) of the parent acid and the magnitude of the M ... O=C interaction. The complexes 3a,3b and 4a,4b were prepared in order to probe the effect of strong intramolecular hydrogen bonding upon the ability of ester ligands to stabilise sterically disfavoured conformations via sigma(nb) donation. Single crystal X-ray analyses of 3a,3b and 4a demonstrate that although significant attenuation of the M ... O=C interaction is achieved via strong intramolecular hydrogen bonding, sterically disfavoured arrangements predominate in the solid state.