Carbon monoxide back bonding

IR spectroscopy of adsorbed carbon monoxide has been used extensively to characterize the diluted, reduced Cr/silica system [48-54,60,76,77]. CO is an excellent probe molecule for Cr(ll) sites because its interaction is normally rather strong. The interaction of CO with a transition metal ion can be separated into electrostatic, covalent a-dative, and 7r-back donation contributions. The first two cause a blue shift of the vco (with respect to that of the molecule in the gas phase, 2143 cm ), while the last causes a red shift [83-89]. From a measurement of the vco of a given Cr(II) carbonyl complex, information is thus obtained on the nature of the Cr(II)- CO bond. 

Rh-Rh vector may donate electron density to the empty Rh-Rh (T -orbital and/or overlap through back-bonding with either of the fiUed n MOs [103]. Infrared measurements of Rh2f4-CO adducts demonstrate clearly a red shift in the C=0 stretching frequency relative to imhgated carbon monoxide [104]. These results are consistent with electron delocalization from the Rh-Rh n into the low-lying Similarly, back-... 

Table I shows that there is a general tendency for the C—0 force constant to fall as the constant on the other side of the carbon increases. In all these compounds (except ketene) the symmetry is such that there must be two equivalent ir-bonds between carbon and oxygen decrease of the force constant is a measure of the weakening of these bonds, as compared to free carbon monoxide, by the presence of a third atom which can also form ir-bonds. Table II shows that the bonds are also lengthened slightly by the process. The position of nickel carbonyl in the series shows that the back-...

The production of two moles of carbon monoxide and the 18-electron rule lead us to predict that the acetylene molecule is acting as a four-electron donor. In fact this is just one of many complexes in which alkynes bind in this fashion.81 For example, the structure of the diphenylacetylene complex in Fig. 15.26 shows that the positions of the two rhodium atoms are such as to allow overlap with both tr orbitals in the carbon-carbon triple bond.82 The extent of back donation into the antibondirg orbitals determines the lengthening of the C—C bond and the extent to which the C—H bonds are bent away from the complex. Bond length values vary greatly from system to... 

Munakata and coworkers (1991) have recently characterised copper complexes with ethylene, acetylene, carbon monoxide and cod and studied the structures by X-ray analysis to investigate the bonding involved. Ethylene and acetylene are sideways bonded to copper (Fig. 5-18). Sigma donation from ethylene to copper predominates and ti-back donation is very weak. 

A simple view of the mode of bonding of the carbon monoxide molecule to the surfaces of the transition metals and in their carbonyl complexes is well known and is thoroughly described in the literature.4 These metals have unfilled d-orbitals (or holes in their d-band), and the molecule in the linear form is held by a push-pull bond in which charge is transferred from the 5a orbital of the molecule into the metal s d-band, while there is back-donation of charge from the top of the d-band into the molecule s vacant... 

Carbon monoxide and ethylene are common substrates involved in homogeneous catalysis. The bonding of carbon monoxide to a transition metal has been depicted in Fig. 4.4. The bonding of alkenes to transition metals is described by the Chatt-Dewar-Duncanson scheme involving c donation by the filled it orbital of the alkene, and n back donation from the metal into the n orbital of the alkene (see Fig. 4.6). 

The general conclusion was that rather weak bonding was established between the cation and the carbon monoxide molecule. The scheme of the bonding was depicted as essentially a transfer of-the carbon lone pair to the cation empty orbitals with a variable extent back-donation from the cation filled orbitals to the antibonding II orbitals of the CO molecule. This bonding scheme generally resulted in an electron deficient carbonyl carbon especially in view of the weak back-donation to the II molecular orbitals of CO. This makes the carbon particularly suitable for nucleophilic attacks by electrophiles. 

Carbon monoxide is an excellent ligand for transition metals. It coordinates via the free electron pair of the carbon atom as a rr-donor (and weakly as a n-do-nor) to the metal, back bonding shifts electrons from the transition metal into the n antibonding orbital of carbon monoxide, making it susceptible to the catalytic chemistry discussed here (Scheme 5.35). 

---