MECHANISMS OF DIHYDROGEN ACTIVATION BY ZIRCONOCENE ALKYL AND HYDRIDE DERIVATIVES A MOLECULAR ORBITAL ANALYSIS OF A 'DIRECT HYDROGEN TRANSFER' REACTION l\'10DE

The formation of an adduct between a dihydrogen and a dO species of the general type (CSR5)2Zr(R')2 (R,R' = H or CH3 ) is suggested to involve a process analogous to that in CO adduct formation, and is studied by an extended Huckel molecular orbital analysis. Back donation into a* Hz orbitals arises from high-lying M-R' bonding orbitals and determines the stability of alternative adduct geometries. Two isomeric H2 adducts can interconvert by an intramolecular hydrogen shift. With D 2 , this reaction sequence leads to hydrogen isotope exchange. The transition state of this reaction mode contains a three-centre (H, .. H· .. H)ligand configuration with bonding properties similar to those of an allyl ligand; it does not involve an oxidative change in the charge on the metal nor generation of a positively charged, protonic hydrogen species by heterolytic H2 cleavage. The term 'direct hydrogen transfer' is proposed for this type of elementary reaction. For H2-induced alkane elimination from group IV metallocene alkyl derivatives, an analogous l'eaction mechanism, involving formation of a H2 adduct and a subsequent intramolecular hydrogen shift toward the alkyl ligand, is predicted to proceed with moderate activation energies.