Olefin transition-metal complexes geometry

The preceding perturbation theory analysis is supported by extended Hiickel calculations by Cusachs and his co-workers (166, 167, 237) on model platinum(II)- and platinum(0)-olefin and -acetylene complexes and Hoffmann and Rossi s extensive analysis of five-coordinate transition metal complexes (194). By using similar arguments, Hoffmann and Rosch (190) predicted that the planar conformation would be energetically preferred for d10 M(C2H4)3 complexes. This geometry has now been established by Stone (214) and his co-workers for the platinum-olefin complex shown in Fig. 12. 

The ship-in-a-bottle technique is perhaps the most common method for encapsulation of transition metal complexes. In this way the tetradentate Schiff base ligand SALEN (bis-salicylidene) ethylenediamine can diffuse through the 12 MR windows of faujasite. Then, when complexed with a previously exchanged metal ion, nearly square planar coordination geometry is formed inside the a-cages [97-100], Mn complexes with a chiral ligand, prepared by the ship-in-a-bottle technique inside Y and EMT zeolites, have enantioselectively carried at the epoxidation of olefins [101,102]. 

The active site in coordination catalysts for olefin polymerization is, therefore, a transition metal surrounded by ligands. The catalytic properties depend on the fine tuning between the transition metal and the ligands in terms of geometry and electronic character. In most cases the active site is produced by the activation of a complex called a precatalyst precursor. The pre[Pg.373]

Pertinent reviews published during 1980 cover cyclometallation of P-donor ligands, CO insertion into metal-carbon o-bonds, mechanistic features of catalytic CO hydrogenation reactions, and stoicheiometric reactions of transition-metal carbene complexes. Other articles of interest deal with the stability of metal-carbon bonds, " transition-state geometry for insertion of metals into C-H bonds, organic synthesis using Group VIII metal complexes, and C-H bond activation by transition metals. " Molecular orbital calculations on the interconversion of metal bis(olefin) and metallocyclo-pentane complexes have been reported. ... 

The simplest case which can be used to explore the factors influencing the difference in the energies of the diastereomeric transition states, which determine asymmetric induction, is the hydroformylation of (Z)-2-butene with the Rh/(—)-DIOP or Pt/ (—)-DIOP catalytic system. In this case, the asymmetric induction cannot be connected with enantioface discrimination in the step leading to the Tt-complex because this olefin has no enantiofaces 68). In the first step of the reaction it is assumed that a Tt-complex is formed by interaction between substrate and catalyst. This Tt-complex, depending on its geometry, can exist in two different conformations arising from the rotation of the olefin around the metal-olefin Tt-system-bond axis. 

---