中级无机化学北大10年第8讲配位催化反应.ppt

第8章 配位催化反应 二、过渡金属有机化合物的基本反应 Ligand substitution The substitution of ligands in organometallic complexes is very similar to the substitution of ligands in coordination complexes, with the additional constraint that the valence electron count at metal atom does not increase above 18; steric crowding of ligands increases the rate of dissociative processes and decreases the rate of associative processes. Oxidative addition and reductive elimination Oxidative addition occurs when a molecule X-Y adds to a metal atom to form new M-X and M-Y bonds with cleavage of X-Y bond; oxidative addition results in increasing the coordination number of metal atom by 2 and increasing the oxidation number by 2; reductive elimination is the reverse of oxidative addition. 1,1-Migratory insertion reactions 1,1-Migratory insertion reactions result from the migration of a species such as a hydride or alkyl group to an adjacent ligand such as carbonyl to give a metal complex with two fewer electrons on the metal atom, no change in the oxidation state. 1,2-Insertions and β-hydride elimination 1,2-Insertion reactions are observed with η2 ligands such as alkenes and result in the formation of anη1 ligand with no change in oxidation state of the metal; β-hydride elimination is the reverse of 1,2-insertion. 三、催化反应 Hydrogenation of alkenes Wilkinson’s catalyst, [RhCl(PPh3)3], and related complexes are used for the hydrogenation of a wide variety of alkenes at pressures of hydrogen close to 1 atm or less; suitable chiral ligands can lead to enantioselective hydrogenations. Hydroformylation The mechanism of hydrocarbonylation is thought to involve a pre-equilibrium in which octacarbonyldicobalt combines with hydrogen at high pressure to give a monometallic species that brings about the actual hydrocarbonylation reaction. Methanol carbonylation: ethanoic acid synthesis Both rhodium and iridium complexes are highly active and selective in the carbonylation of methanol to form acetic acid. Wacker oxid