Antibonding orbitals back bonding into

FIGURE 5. View of the P=C bond as a o bond and a n bond formed by back-bonding into an antibonding orbital of e symmetry on the phosphine moiety... 

The K additives in CO hydrogenation result in strong reduction of the metal catalyst, increasing the density of electronic states (Fermi level) available to form bonds with adsorbed CO by back bonding into the antibonding n CO orbitals. This effect increases the adsorption energy in the same manner as it strengthens the bond between the metal and carbonyls in metal-carbonyl complexes (see Chap. 9). For instance, on a rhodium (111) surface, CO is adsorbed but does not dissociate at room temperature. When K is added in an optimal amount, CO dissociation occurs at the rhodium surface. ... 

Figure Schematic representation of the two components of the ij -Hi-metal bond (a) donation from the filled (hatched) CT-H2 bonding orbital into a vacant hybrid orbital on M (b) jr-back donation from a filled d orbital (or hybrid) on M into the vacant a antibonding orbital of Hj.

Two possible reasons may be noted by which just the coordinatively insufficient ions of the low oxidation state are necessary to provide the catalytic activity in olefin polymerization. First, the formation of the transition metal-carbon bond in the case of one-component catalysts seems to be realized through the oxidative addition of olefin to the transition metal ion that should possess the ability for a concurrent increase of degree of oxidation and coordination number (177). Second, a strong enough interaction of the monomer with the propagation center resulting in monomer activation is possible by 7r-back-donation of electrons into the antibonding orbitals of olefin that may take place only with the participation of low-valency ions of the transition metal in the formation of intermediate 71-complexes. 

A quantitative treatment of tt complex formation is, however, more complicated, since it is generally recognized that all three wave functions are necessary for an accurate description of the bond. For instance, it has been pointed out by Orgel (27) that n complex stability cannot solely be the result of n electron donation into empty metal d orbitals, since d and ions (Cu+, Ag+, Ni , Rh+, Pt , Pd++) form some of the strongest complexes with poor bases such as ethylene, tt Complex stability would thus appear to involve the significant back-donation of metal d electrons into vacant antibonding orbitals of the olefin. Because of the additional complication of back-donation plus the uncertainty of metal surface orbitals, it is only possible to give a qualitative treatment of this interaction at the present time. 

According to Dewar (41) the metal-to-olefin bond in such complexes consists in part of the overlap of the 7T-electron density of the olefin with a cr-type acceptor orbital of the metal atom and in part of the back-donation of electrons from filled metal or other d-n-pn hybrid oribitals into the antibonding orbitals on the carbon atoms. 

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