Influence of L-type ligands on the relative stability and interconversion of cis-trans-[Ru(phen)(2)L-2](n+) type complexes. A theoretical study

Polipyridine complexes of type [Ru(N-N)(2)L-2](2+) are useful for the conve rsion of light into usable energy for which the trans geometry would be pre ferable. In this paper, the effect of distortion on trans structure stabili ty was studied theoretically. A formal separation of the contributions was carried out, and it explained why the hydrogen repulsions are an important factor. A study of the main aspects related to the cis-trans isomers and al so their mutual interconversion was analyzed. Calculations were carried out at the PM3, ZINDO and ab initio level. Energy differences between trans an d cis isomers after full geometry optimizations for different L-ligands sho w a dependence on the L-ligand Jorgensen field strength parameter f. A driv ing coordinate was performed in order to determine the kinetic preferences for isomer formation. The analysis showed that the entrance of the second p henanthroline is favoured for the cis geometry in regard to the trans geome try. The activation barrier is also dependent on the L-ligand strength. The thermal trans-cis interconversion seems to be very improbable adiabaticall y (considering only one potential energy surface). (C) 2000 Elsevier Scienc e B.V. All rights reserved.