Bonding in metal-alkene complexes

A number of X-ray crystal structures of nickel(O) complexes containing both alkenes and phosphines have been reported to date with the aim of gaining more information on the bonding in metal alkene complexes. Structural data for the most relevant nickel(O) phosphine alkene complexes are reported in Table 9. 

The bonding in metal-alkene complexes is described by a Chatt-Dewar-Duncanson model. This model considers a a (alkene to platinum) bond to be formed by donation of electron density from the filled pn orbital on the alkene to the empty hybrid... 

Figure 7.3 A representation of bonding in metal alkene complexes (a) the o bond in which a n MO on the alkene overlaps with a metal orbital (6) the rbond in which the Ji antibonding MO of the alkene overlaps with a metal d orbital.

Figure 3.55 Dewar Chatt Duncanson bonding model in metal alkene complexes.

S.3.4 Metal-Carbon Bond Strengths in Metal-Alkene Complexes... 

Fig. 8.6. Representation of it bonding in a alkene-metal cation complex.

The bonding in these two complexes is very different. In the first there is a simple alkyl group as in a Grignard reagent R-MgBr and this type of complex is called a a complex. In the alkene complex, bonding is to the p orbitals only. There are no a bonds to the metal, which sits in the middle of the ft bond in between the two p orbitals. This type of complex is called a ft complex. 

The bonding in monometal alkyne complexes is usually interpreted in terms of the Dewar-Chatt-Duncanson model (293), since the alkyne molecule has a pair of n and n molecular orbitals which lie in the plane of the metal and the two carbon atoms. These two orbitals are denoted n and n, and are analogous to those in jr-bonded alkene complexes (394). There is also a pair of n and n molecular orbitals which lie perpendicular to the metal-carbon plane, denoted nL and n . These orbitals are illustrated in Fig. 14. Both sets of n and n orbitals have the correct symmetry to interact with metal d orbitals. The interaction... 

The alkene metathesis reaction see Alkene Metathesis) exchanges alkylidene groups between different alkenes, and is catalyzed by a variety of high oxidation state, early transition metal species (equation 40). The reaction is of interest because it is the strongest bond in the alkene, the C=C bond, that is broken during the reaction. It is also commercially important in the Shell higher olefins process and in the polymerization of cycloalkenes. It is relevant to this article because carbenes are the key intermediates, and the best-known catalyst, (1), is a carbene complex. 

Experimental evidence and computational analysis point to a mechanism in which the alkene (or alkyne) carbons and the M-H bond must be nearly coplanar to react. Once the metal alkene complex has achieved such geometry, 1,2-insertion can occur. During insertion, the reactant proceeds through a four-center transition state. 14The reaction involves simultaneous breakage of the M-H and C-C n bonds, as well as the formation of an M-C a bond and a C-H bond at the 2-position of the alkene (or alkyne). The result is a linear compound, L M(CH2CH3), in the case of ethene insertion. The reverse reaction, (3-elimination, follows the same pathway starting from a metal-alkyl complex with an open coordination site. 

Figure 1.14. Bonding in transition metal-alkene complexes, (a) Orbitals involved (b) valence bond representations.

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