Racemization Pathway for MoO 2 (acac) 2 Favored over Ray-Dutt, Bailar, and Conte-Hippler Twists.

Chiral cis -MoO 2 (acac) 2 racemizes via four pathways that agree with and extend upon Muetterties' topological analysis for dynamic MX 2 (chel) 2 complexes. Textbook Ray-Dutt and Bailar twists are the least favored with barriers of 27.5 and 28.7 kcal/mol, respectively. Rotating both acac ligands of the Bailar structure by 90° gives the lower Conte-Hippler twist (20.0 kcal/mol), which represents a valley-ridge inflection that invokes the trans isomer. The most favorable is a new twist that was found by 90° rotation of only one acac ligand of the Bailar structure. The gas-phase barrier of 17.4 kcal/mol for this Dhimba-Muller-Lammertsma twist further decreases upon inclusion of the effects of solvents to 16.3 kcal/mol (benzene), 16.2 kcal/mol (toluene), and 15.4 kcal/mol (chloroform), which are in excellent agreement with the reported experimental values.