THEORETICAL-STUDY OF THE INTRAMOLECULAR CIS-TRANS ISOMERIZATION MECHANISM IN CR(CO)5CR(CO)5CO, CR(CO)5PH3, CR(CO)5PPH3

The reaction mechanism for intramolecular cis–trans isomerization in Cr(CO)5X (X = CO, PH3, PPh3) was explored. The approximate molecular orbital method PRDDO (partial retention of diatomic differential overlap) was used to optimize the geometries of Cr(CO)6, Cr(CO)5PH3, and Cr(CO)5PPh3 and to estimate the transition-state structures resulting from nondissociative cis–trans isomerization mechanisms. The Bailar twist, Ray and Dutt, and bicapped tetrahedron nondissociative mechanisms were investigated. With monodentate ligands, all three mechanisms lead to identical transition states, clearly identifiable as trigonal prisms. Ab initio theory including MP2 perturbation theory was used to evaluate the energy barrier for Cr(CO)6 and Cr(CO)5PH3. The calculated barrier in both systems is ~40 kcal/mol. PRDDO calculations for the same systems yield a slightly higher value of ~47 kcal/mol. For Cr(CO)5PPh3, PRDDO predicts a barrier of ~40 kcal/mol. Although the predicted energy of activation for a nondissociative mechanism occurring via a trigonal-prism transition state in Cr(CO)5PPh3 is 7 kcal/mol lower (at the PRDDO level) than that observed in the two previous systems, the barrier is still slightly higher than the triphenylphosphine ligand dissociation energy of 32 kcal/mol. © 1990, American Chemical Society. All rights reserved.