金属有机化学教案7.pdf

第十一章烯和炔络合物 Alkenes act as neutral 2e donors (per C=C double bond). Due to the presence of empty π* antibonding orbitals, there is the possibility of some π-backbonding: C C Dewar-Chatt- M M Duncanson bonding C C model (1953) σ-donation via the π-back donation via the ∗ filled alkene π -system empty alkene π -system The greater the electron density back-donated into the π* orbital on the alkene, the greater the reduction in the C=C bond order. An alternative way of stating this would be to say that the hybridization of the alkene carbon changes from sp2 to sp3 as back-donation increases. Either formalism describes two limiting structures: a planar olefin adduct and a metallocyclopropane. X-ray crystallographic studies confirm that the as the C-C bond length increases, the CH2 plane is distorted from the ideal planar geometry of an alkene: π-complex The strongest alkene-metal bonds occur with third row metals (as with almost all ligands) and when one can get more π- backbonding to occur. The amount of π-backbonding depends strongly on how electron-rich the metal center is and whether or not there are electron-withdrawing groups on the alkene to make it a better acceptor ligand. NC H H H H NC Cl PR3 PR3 Pt Cl Pt Pt Cl H H H H PR3 NC PR3