Orbital interactions Dewar-Chatt-Duncanson

C) Synergistic H2 coordination. When the metal has both donor and acceptor orbitals, interaction with the cthh and ohh orbitals of H2 can act synergistically (cooperatively) in the Dewar-Chatt-Duncanson mode. Such self-reinforcing charge flow enables a significantly stronger interaction than does simple dative coordination of H2. 

The bonding between these two fragments can be understood using the Dewar-Chatt-Duncanson model of donation and backdonation [79, 80], The frontier orbitals responsible for these interactions between 15 and 16-R are drawn to scale in Figure 15. 

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 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... 

Whereas transition metal complexes of alkenes and their chemistry have been well explored, comparatively little is known about the structure and reactivity of n complexes obtained from strained olefins. The stability of transition metal complexes of alkenes in general is preferably discussed in terms of the Dewar-Chatt-Duncanson model (171). A mutual er-type donor-acceptor interaction accounts for the bonding overlap of the bonding 71-MO of the olefin with vacant orbitals of the metal together with interaction of filled d orbitals with the 7r -MO of the double bond (back bonding) leads to a partial transfer of. electron density in both directions (172). The major contribution to the stabilizing interaction is due to back-bonding. 

Figure la shows a schematic representation of the Dewar-Chatt-Duncanson (DCD) model. The pivotal idea is that the olefin serves as a donor and an acceptor at the same time. There is ligand metal donation and metal -> ligand back-donation. The former interaction involves a donor orbital of the ligand which has n symmetry in the free ligand but cr symmetry in the complex. The metal acceptor orbital is mainly the d 2 orbital of the metal. Quantum chemical calculations have shown that the valence s orbital of the metal is less important as an acceptor orbital than the d 2 orbital. The metal ligand back-donation takes place via a d( r) orbital of the metal and the n orbital of the olefin. 

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. 

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. 

Figure 3.9. Diagram for the interaction between the d orbitals of a d ML fragment and the X and jr orbitals on an ethylene ligand (the Dewar-Chatt-Duncanson model).

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. 

In this system, the donation interaction of the Dewar-Chatt-Duncanson model involves the occupied n orbital and the empty z orbital, both with SS symmetry, in the two conformations. As the z orbital has cylindrical symmetry, the overlap between n and z does not depend on the orientation of the ethylene ligand, so this interaction cannot lead to any conformational preference. The back-donation interaction involves the empty tt orbital and the occupied yz orbital, both with symmetry SA, in the two conformations. This second... 

To clarify these points, we shall consider the carbene as an L-type ligand (4-36a). It therefore acts as a r donor, using its lone pair described by the tier orbital, which interacts with an empty orbital on the metal (e.g. z, 4-38a). In this model, the Tip orbital is empty, so the carbene acquires a r-acceptor character (single face) (4-38b). The interaction scheme is similar to that in the Dewar-Chatt-Duncanson model (Chapter 3, 3.4.1) used, for example, to describe ethylene complexes or molecular hydrogen complexes ( 4.1.4). 

Backbonding is of pivotal importance with respect to the metal-olefin interaction and the so-called Dewar-Chatt-Duncanson model , in which donation of electron density from the filled olefin 7r-orbital (an L function) is supplemented by backbonding into the empty tt -orbital (a Z function). Depending upon the extent of backbonding, which depends critically on the nature of the metal center, the compound may be described as either a metal-olefin... 

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