Dewar-Chatt-Duncanson theory

J. Chatt and L. A. Duncanson in a seminal paper in 1953 and the Dewar-Chatt-Duncanson theory forms the basis of most subsequent discussion. ... 

The classical Dewar-Chatt-Duncanson model for metal-alkene bonding has been revisited with a combination of X-ray structural data (see Diffraction Methods in Inorganic Chemistry) and DFT calculations (see Molecular Orbital Theory), particularly on complexes of the type (acac)Rh(alkene)2. These indicate the existence of distortions from idealized geometry involving a twist (127), where the axis of the double bond is no longer perpendicular to the molecular plane and a roll (128), where the line... 

The key to our present understanding of the bonding in Zeise s salt and all other alkene complexes stems from the perceptive suggestion by M. J. S. E>ewar in 1951 that the bonding involves electron donation from the n bond of the alkene into a vacant metal orbital of o symmetry this idea was modified and elaborated by J, Chatt and L. A. Duncanson in a seminal paper in 1953 and the Dewar-Chatt-Doncanson theory forms the basis for most subsequent discussion. The bonding is considered to arise from two interdependent components as illustrated schematically in Fig. 19.20 (a) and (b). In the first part, a overlap between the filled -k orbital of... 

Like in the case of olefin complexes, the chemical shift t of the coordinated CHR group increases by 2-3 ppm compared to the free allene. This is also in agreement with the Dewar-Chatt-Duncanson model of the metal-allene bond (rehybridization sp -> sp ) according to the valence bond theory. 

None of the theories proposed before 1951 to explain the nature of the bonding in metal-olefin complexes was entirely satisfactory (35). Chatt (S3) suggested that, in addition to the ordinary coordinate bond, some sort of bond involving the filled d-orbitals of the metal atom was essential for coordination of the olefin, but such a bond was difficult to formulate until Dewar (64) described it in terms of molecular orbitals. The structure which he proposed for the silver-olefin complexes, and that subsequently proposed for the platinum-olefin complexes by Chatt and Duncanson (35) are shown schematically in structures (I) and (II). The type bond, which has also been called a ji-bond (64, 4), is formed by the overlap of the filled bonding... 

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). 

G. Wilkinson, M. Rosenblum, M. C. Whiting and R. B. Woodward, 1952) and the jr bonding of ethylene complexes (M. J. S. Dewar 1951, J. Chatt, and L. A. Duncanson, 1953). The constricting influence of classical covalent-bond theory was finally overcome when it was realized that carbon in many of its compounds can be 5-coordinate (Al2Me6, p. 258),... 

In 1951, Michael J. S. Dewar (1918-97) published a molecular orbital (MO) theory of bonding between unsaturated compounds and transition metals later augmented by Joseph Chatt (1914-94) and Leonard A. Duncanson. It recognized o-type overlap of an occupied 7t-orbital on the ligand (e.g., ethylene, benzene, cyclopentadienide) with a vacant d-orbital of appropriate symmetry on the transition metal, coupled with back-donation through 7t-type overlap of an occupied d-orbital on the metal with a vacant (antibonding) 7t -type orbital on the ligand. 

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