Dewar-Chatt-Duncanson mode

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. 

As expected, there is generally a large dependence of the modes for bound H2 on both the metal center and coligands. Vibrational analysis of a M-T 2-H2 system is further complicated by the three-center, two-electron bonding. The bonding is of the Dewar-Chatt-Duncanson type present in metal-olefin complexes, where there is a strong M —> H2 G component, FBD, to the bonding in addition to electron donation, Ed, to the empty metal d-orbital from the H2 electron pair (Scheme 5.18). 

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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