Olefin-ML4 complexes

The hydrogenation of olefins catalyzed by transition metals is a very important industrial reaction. The initial step in the catalytic cycle is the addition of H2 to a square planar complex to form an octahedral metal dihydride. There are two possible ways that H2 can approach a d ML4 complex. These are shown in reactions (I) and (2). 

The n orbital on the ethylene ligand, whose symmetry is SS in both conformations, can interact with (i) the empty orbital and (u) the occupied nonbonding orbital of the same symmetry on the ML4 fragment (4-2). The first of these interactions involves two electrons it constitutes the donation interaction of the Dewar-Chatt-Duncanson model (Chapter 3, 3.4.1.2) for the metal-olefin bond in this complex. It is easy to see that the overlap between these orbitals does not depend on the orientation of the olefin, due to the cylindrical symmetry of the empty orbital on the metal centre (4-4a and b). Things are essentially the same for the second interaction the overlap occurs mainly with the lobe of the d orbital that points towards the jt orbital, and this lobe also has cylindrical symmetry with respect to the metal-olefin axis (4-5a and b). We may conclude that since the interactions which involve the n orbital of the ethylene ligand are identical in the two conformations, they cannot contribute to a pronounced energetic preference for one of them. 

The greater electronic stabilization which follows (A c > A p, Figure 4.1) leads to a preference for this conformation, which is indeed adopted in allknown complexes of the type d -[ML4( j -olefin)]. The barrier to olefin rotation measured by nuclear magnetic resonance (NMR) is of the order of 10-15 kcalmol, ... 

The analogy between the C2v ML3 and 4 ML4 fragments can be carried much further. For example, the electronic features of the olefin insertion reaction [37,38], 18.50-18.51, in d complexes are very similar to the catalytic olefin hydrogenation step fori8.52-18.53 for d molecules [38,39]. The olefin insertion reaction is... 

Epoxy alcohols are the normal products of the [VO(acac)2]+(Me2C(CN)N a -catalysed oxidation of cyclic olefins by dioxygen however, cyclo-octene is oxidized exclusively to cyclo-octene oxide. The oxidation of sulphides and alkenes by peroxides with a [V(0)(acac)2] catalyst have been compared and the nature of the monoperoxovanadium(v) intermediate investigated. Complexation of a Cr(CO)3 unit to aromatic hydrocarbons enhances the benzylic positions towards attack by superoxide ion, e.g., diphenylmethane is readily converted into benzophenone. Metal porphyrin complexes ML4 continue to attract attention both as reversible oxygen-carriers (M = Fe) and oxidation catalysts (M = Mn, Fe, or Co ). For example [Mn (=0 IPh)(TPP)Cl] is believed to be involved in the oxidation of cyclohexene to cyclohexanol by PhIO in the presence of [Mn(TPP)]+ and a ferryl intermediate [Fe (0)L4] has been proposed in the oxygenation of triphenylphosphine with iron(ii) porphyrin. [M(TPP)]X (M=Mn, X = OAc M=Fe, X=C1 M = Co, X=Br) catalyses the epoxidation of styrene and cyclohexene with NaOCl under phase-transfer conditions. ... 

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