Dewar-Chatt-Duncanson olefin binding

The Dewar-Chatt-Duncanson model of the binding of an olefin in a transition metal complex involves two types of interactions. Transfer of electron density from the relatively high-lying olefinic ic-orbital to the metal (cf. 20) represents a Lewis acid Lewis base interaction (a-bonding). A metal-olefin jr-bond due to interaction... 

The matrix isolation experiments using epr, ir, uv-visible and other spectroscopic techniques on transition metal-olefin complexes [8,49] have naturally attracted the attention of theoretical chemists and calculations on the Ni-C2H4 system were reported in one of the first theoretical-experimental papers mentioned in the introduction [16]. These results were later supplemented with a larger (double-zeta) basis set [3Q] and also [31] extended for a Ni(C2H4)2 system. The main conclusions are that a net charge transfer of almost 1/5 of an electron from the metal to the ethylene is evident and that a donation and back donation mechanism consistent with a classical Dewar-Chatt-Duncanson model exists. The Ni-ethylene binding energy is 12.8 kcal/mol. 

A possible explanation for the calculated trend in the metal ion binding energies to ethylene, which will be discussed for the triply bonded substrates, could lie in a consideration of the Dewar-Chatt-Duncanson donor-acceptor model for bridging-type metal-olefin complexes. Their proposed two-way interaction involves mixing of the olefin n electrons with a metal (n + l)sp a hybrid atomic orbital (L —> M, for short) and simultaneous back donation (M L) of metal nd electrons of appropriate symmetry into the olefin k molecular orbital MO. For the monocation metal ions the latter-type interaction should be less favourable due to stabilizaion of the nd electrons by the charge on the metal. L M should be favoured for the same reason stabilization of the (n + l)s and (n+ l)p orbitals by the + 1 charge. 

From the earliest days of metal-catalyzed olefin polymerization, the olefin complexation step, eq 1, has been thought to be important for catalysis. For Ni(II) and Pd(II) catalysts, " 4, the olefin binding event is well precedented with numerous crystal structures of olefin complexes. In fact, the resting state of the catalyst is thought to be an olefin complex. " The binding for these d complexes is properly explained in the terms of the classic Dewar— Chatt—Duncanson model. Figure 4. In this model, bonding consists of a donation of electron density from the olefin ji orbital into an empty o orbital on the metal (forward coordination), and simultaneous donation from a filled metal dji orbital into the empty 71 orbital of the olefin (back-donation). 

Orbital interactions in the Chatt-Dewar-Duncanson bonding model of olefin binding. 

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