THE OLEFIN INSERTION REACTION INTO A METAL-HYDROGEN BOND FOR 2ND-ROW TRANSITION-METAL ATOMS, INCLUDING THE EFFECTS OF COVALENT LIGANDS

The olefin insertion into a transition metal-hydrogen bond has been st udied for the entire sequence of second row transition metal atoms. En ergies including correlation of all valence electrons have been obtain ed for geometry optimized structures of the reactants, of the pi-bonde d olefin complexes, of the transition states, and finally of the produ ct metal-alkyl systems. To study the effects of covalent ligands the c alculations were repeated with one and two additional hydride ligands. The main differences between the metals and also the main effects of the hydride ligands can be explained by a dominant role played by repu lsion between nonbonding metal electrons and the olefin electrons. Cle ar consequences of this repulsion can be seen, for example, on the lar ge barrier height increase between the niobium and molybdenum systems and also on the strikingly low barrier for insertion into the bond of the diatomic RhH. The systems to the left including the niobium comple xes have low barriers because they can use empty 4d-orbitals to reduce the repulsion toward the olefin. The rhodium atom in RhH can reduce-t his repulsion by using the s0-state for binding at the transition stat e. No relations can be noted between barrier heights and the bond stre ngths of the initial metal hydride or the strength of the pi-bonded co mplexes.