Transition Chatt-Dewar-Duncanson model

The reason for the ability of transition metals to bind to alkenes (and alkynes) lies in the fact that electrons can be donated in both directions, resulting in a synergistic effect (Figure 1.4). The Tr -orbital of the alkene can accept electrons from filled d-orbitals on the metal, while the filled Tr-orbital of the alkene can donate back to empty metal orbitals. This is known as the Chatt— Dewar— Duncanson model. 

Alkenes and alkynes coordinate to transition metal centers via the double or triple bond. The bonding has been described by the Chatt-Dewar-Duncanson model where a a bond is formed by electron donation from the filled n orbital on the alkene or alkyne into an empty orbital on the metal center, along with a synergistic back electron donation from a filled orbital on the metal center into the empty n orbital on the alkene or alkyne. Photoexcitation can therefore lead to either an LMCT or an MLCT state, with the latter being found at the lowest energy for... 

In low oxidation states, transition metals possess filled or partly filled d shells. The Dewar-Chatt-Duncanson model envisages some of that electron density in (local) d (e.g. d., d y) orbitals being donated into the empty n orbitals of the carbon monoxide ... 

Fig. 3. Schematic bonding pictures for transition metal atoms (M) binding to C2H4 via the Dewar-Chatt-Duncanson model.

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... 

Positively-charged fragments such as [ML,]+, CH , and H + are all strong electrophiles ( superelectrophiles in the extreme sense (13)) towards the Lewis basic H2, but transition metals can uniquely stabilize H2 and other cr-bond coordination by back donation from d-orbitals that main group analogues cannot do. This bonding is then remarkably analogous (14) to the Dewar-Chatt- Duncanson model (15) for ji-complexes (6). 

The bonding of transition metal fragments to a silicon surface can also be described with the Dewar-Chatt-Duncanson model. There is a striking similarity between the reactivity of the Si(100) surface and that of disilene (Si2H4)97. 

The description of the bonding of ethene to transition metals is known as the Dewar-Chatt-Duncanson model. This builds on the previous descriptions for CO and CH2 with the exception that we must now consider the frontier orbitals of the ligand to be molecular orbitals delocalized over the two (or more) donor atoms. The basic features are presented for ethene however, the bonding scheme applies in principle to the side-on coordination of any multiple bond to a transition metal. 

The bonding of a variety of unsaturated organic molecules when n-coordinated to a transition metal (Dewar-Chatt-Duncanson model)... 

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. 

The pi back-bonding model of Dewar (29), Chatt, and Duncanson (30) has been widely invoked as an explanation of a variety of features of transition metal complexes. While extended Huckel theory clearly shows such mixing of orbitals, ab initio calculations have found them more elusive (31,32). 

Schwartz s reasoning for optimizing these thermodynamic considerations led to the development of hydrozirconation. Hydride complexes of the late transition metals do not in general exhibit the hydrometallation reaction, probably because the alkene complexes are too stable. This may be understood from the Dewar-Chatt-Duncanson model for alkene bonding, wherein back donation of metal d-elec-trons to the alkene Tr -orbital is a major contributor. For metal centers with d -electron configurations, there should be substantial stabilization of (3) with respect to (2). Such metals are only found towards the left end of the Periodic Table, particularly Groups III A to VA. 

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. 

Powder as well as MAS studies are available for several r/ -bonded olefins. The spans of the chemical-shift tensors are reduced with respect to the free olefins, which is discussed in terms of the Dewar-Chatt-Duncanson model of fx-donation and tt- back-bonding." " " The bond lengths and the orientations of the shielding tensor elements are available from dipolar-chemical shift methods and 2-D spin-echo experiments on the doubly labelled oleflns." " 77 -cyclopentadienyl and 77 -benzene ligands of transition-metal complexes, but also some derivatives of alkali or main-group elements," exhibit single resonances and shielding tensors of axial symmetry at room temperature. Both observations point to relatively fast rotations around the respective 5- and 6-fold local rotor axis. ... 

The accepted mechanism for bonding of dihydrogen to transition metals is derived from the Dewar-Chatt-Duncanson model, which was originally developed to describe the binding of ethene and related ligands. The a orbital of H2 donates electron density to an empty d orbital of a symmetry. At the same time, this interaction is augmented by back donation from filled metal orbitals of principally d character, to the a orbital of H2 (Figure 12.28). 

The Dewar-Chatt-Duncanson model that takes into account this bonding mode, including the n backbonding with the CO and C2H4 ligands, is represented below for classic metal-carbonyl and metal-ethylene bonds. Of course, the other unsaturated hydrocarbons bind the transition-metals according to the same n backbonding model. 

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