Dewar model, metal-olefin bond

The crystal structures of Zeise s salt and of two analogous palladium-olefin complexes were published,shortly after the publication of the Chatt-Duncanson paper. They confirmed the structural proposals made by Chatt, but none of these papers cites Dewar,though metal-olefin bonding models were not discussed. Two short reviews on the history of Zeise s salt, (one part of a more general discussion of the history of organometallic chemistry) also only refer to Chatt s contribution, though neither specifically address questions of bonding. 

The metal-dihydrogen bonding is similar to the Dewar-Chatt-Duncanson model for metal-olefin bonding [29-31]. There is a donor bond from the electron pair in the bonding orbital of the H2 ligand into an empty metal orbital (Figure 1, left-hand side). 

Figure 4. Classic Dewar—Chatt—Duncanson model of metal—olefin bonding.

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. 

Figure 2.7 shows that the ethylene is coordinated to the titanium center at the vacant octahedral position through sigma bonding involving the ethylene 7t-orbitals and the titanium d orbital. The 7t-bond involves titanium d orbital overlapping with empty Tt-antibonding orbitals on ethylene. The coordination of ethylene to the titanium active site is known as the Dewar-Chatt-Duncanson model of the metal-olefin bond [31]. The... 

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 course of modern organometallic chemistry has been greatly influenced by three simple generalizations the Dewar-Chatt-Duncanson synergic bonding model for metal-olefin complexes (40, 72) Pauling s electroneutrality principle (174), and the 18-electron or inert gas rule (202). In this section the impact of recent theoretical calculations on these important generalizations will be evaluated. 

G. J. Kubas, Metal-Dihydrogen and cr-Bond Coordination The Consummate Extension of the Dewar-Chatt-Duncanson Model for Metal-Olefin n Bonding, J. Organomet. Chem. 635, 37-68 (2001). 

The Dewar-Chatt-Duncanson Bonding Model of Transition Metal-Olefin Complexes... 

In this paper we present a summary of three theoretical investigations of TM olefin complexes, which show that the original bonding model of Dewar and its ingenious application to metal-olefin interactions by Chatt and Duncanson ... 

G.J. Kubas, Metal-dihydrogen and sigma-bond coordination the consummate extension of the Dewar-Chatt-Duncanson model for metal-olefin pi bonding . Journal of Organometallic Chemistry, 635, 37 (2001). 

The current understanding of the nature of the metal-( 72-H-X) bonding interaction for various types of cr-complexes is based on the traditional Dewar-Chatt-Duncanson model for the well-known 7r-olefin complexes which emphasizes both ligand-to-metal a bonding and metal(d7r)-to-ligand(a )... 

Remarkably, one of the simplest conceivable H2 complexes, [Ru(H20)s(H2)]2+, can be formed by displacement of an aquo ligand from the hexaquo complex by pressurized H2.14 It seems astonishing that interaction of a bonding electron pair could be on a par with that for a lone pair. What is unique about the three-center bonding in M-H2 and other bond complexes that stabilizes them and sets them apart from species such as carbocations is BD, i.e., donation of electrons from a filled metal d orbital to the a orbital of the H-H bond, similar to metal donation to n orbitals in the Dewar-Chatt-Duncanson model for olefin coordination. 

---