Explaining the singlet complexes detected for the reaction Zr(³F) + CH₃CH₃ through a non-spin flip scheme

Energy profiles for the lowest lying triplet and singlet electronic pathways that link the reactants Zr + CH₃CH₃ with the products observed under matrix-isolation conditions were obtained from DFT and CASSCF-MRMP2 calculations. The insertion of the metal into the C-H bond of the organic molecule to yield the oxidative addition product is not favorable for any of the investigated channels. However, the inserted structure H-Zr-CH₂CH₃ can be obtained from two sequential reactions involving the radical species ZrH and CH₂CH₃. According to this scheme, a first reaction produces the radical fragments from the ground state of the reactants. Then, the radicals can recombine themselves in a second reaction to form the inserted species H-Zr-CH₂CH₃. As the triplet and singlet radical asymptotes ZrH + CH₂CH₃ that vary only in spin of the non-metallic fragment are degenerate, the rebounding of the radicals can occur through both multiplicity channels. It is shown that the low spin channel leads to the most stable structures of the dihydride ZrH₂-(CH₂)₂ and the vinyl metal trihydride complexes ZrH₃-CH=CH₂ experimentally determined for this reaction under matrix-isolation conditions. The description attained for this interaction does not invoke interactions between the triplet and singlet electronic states emerging from the reactants, as proposed by other authors.