Arene hapticity in (C ₆H ₆)Cr(CO) _n (n = 1-5) complexes: A DFT study of singlet and triplet energy surfaces

DFT calculations were employed to investigate the mechanism of formation and the interconversion of reduced-hapticity (η ^x-C ₆H ₆)Cr(CO) _n complexes (n = 1-5, x = 1-6) on both singlet and triplet energy surfaces. A variety of functionals (mPW1k, B3LYP, B97-1, and BP86) and basis sets (SDD, SDB-cc-pVDZ, and LANL2DZ+P) were used. The same structures were consistently found for all functional/basis set combinations. The structures of the different singlet and triplet complexes and comparisons between their energies are described. On the basis of the structures and energies of the intermediates and the transition states found for singlet reduced-hapticity complexes, a mechanism for the decomposition of the (η ⁶-C ₆H ₆)Cr(CO) ₃ complex to Cr(CO) ₆ is suggested. This mechanism, which is closely related to the arene exchange reaction mechanism, involves the formation of (η ¹-C ₆H ₆)Cr(CO) ₄ and (η ²-C ₆H ₆)Cr(CO) ₅ complexes as ring-slippage intermediates along the pathway for the complete replacement of the benzene ring by carbonyl ligands. The barriers found for the ring-slippage process are accessible under mild conditions. Triplet reduced-hapticity complexes have different structures; however, they are similar in energy to the analogous singlet complexes. Formation of triplet reduced-hapticity complexes might take place in reactions of (η ⁶-C ₆H ₆)Cr(CO) ₃ initiated by photolysis, which have been reported to show no observable decomposition to Cr(CO) ₆.