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

[Image: see text] 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.