Theoretical studies on mechanisms of some transition metal organometallic reactions

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The mechanism of’ three important organometallic reactions catalyzed by transition metals have been studied with semi-empirical PM3(tm) molecular orbital MO) method. The stationary state structures and energetics relevant to the mechanistic pathway are discussed. In the nickel catalyzed Homo-Diels -Alder reactions of Bicyclo [2.2. 1 ] hepta-2,5-diene with electron deficient olefins the PM3(tm) calculations gave geometrical parameters comparable with previous Density Function Theory (DFT) calculations. The calculations indicate that the most energetically favourable reaction pathway is a three step mechanism which involves the direct conversion of the reactant complex 1 to the it—homoaliylic complex 3 followed by electronic rearrangement to give intermediate complex 6 which subsequently undergo reductive elimination to give the deltacyclane product. The activation energies obtained IOI’ the conversion ol reactant complex 1 to the π-homoallylic complex 3, TS (1-3), the conversion ol the 2t—homoallylic complex 3 to intermediate complex 6, TS (3—6). and conversion o[ intermediate complex 6 to the polycyclic adduct, 1’S (6—Pdt.) are 4X. 1 keal molt, 15.9 kcal mol’ and 74.3 kcal mol-1 respectively. In ihe homogeneous polymei-ization ol’ ethylene with ethylene bridged group IV metallocenc catalysts (Q,lI1FICpM — Me and C7H11n7M — Me) the PM3(tm) calculation gave geometric and energetic parameters comparable with previous ab intho and l)ensitv Functional results. The activation energies for ethylene insertion in C,H.41n,M - Me were calculated to be 10.3 kcal mold, 5.0 kcal mol4, 15.0 keal inol and 8.81 kcal mol’, 14.4 keal mol’ and 1 5.8 kcal moV’ for Zirconium, ‘l’itanium and Hafnium respectively. The ö—agostic resting state was found to he the most stable chain orientation in both CH4FlZr! — Bu and C2H4In2Zr - Hu For l)inilrogen cleavage 1w three coordinate Molybdenum (Ill) and Tungsten HI complexes the PM3(tm) calculation yielded geometrica.l parameters and energetics comparable with the results of Density Functional Theory ealcitlati ons by Morokuma et a! and representative experimental parameters. lnergy harriers ol’ 38.8 kcal irol and 51 .6 kcal mol’ were predicted lor 1-I and (.1 ligands respectively i’or the cleavage oithe N—N bond in molybdenum (111).
A thesis submitted to the Board of Postgraduate Studies, Kwame Nkrumah University of Science and Technology, Kumasi, in partial fulfilment of the requirement for the award of the Degree of Master of Science in Inorganic Chemistry, 1994