Interaction back-donation

In many of their complexes PF3 and PPI13 (for example) resemble CO (p. 926) and this at one time encouraged the belief that their bonding capabilities were influenced not only by the factors (p. 198) which affect the stability of the a P M interaction which uses the lone-pair of elecU"ons on p and a vacant orbital on M, but also by the possibility of synergic n back-donation from a nonbonding d , pair of electrons on the metal into a vacant 3d , orbital on P. It is, however, not clear to what extent, if any, the a and n bonds reinforce each other, and more recent descriptions are based on an MO approach which uses all (cr and n) orbitals of appropriate symmeU"y on both the phosphine and the metal-containing moiety. To the extent that a and n bonding effects on the stability of metal-phosphorus bonds can be isolated from each otlier and from steric factors (see below) the accepted sequence of effects is as follows ... 

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. 

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. 

P 162 x 1CT4 cm-1, k0 0.5, c2 0.9, and fry 0.9 (= V23 0.5), indicating only a small degree of metal-ligand interaction for the level. It was suggested by Rettig etal. (65) that the metal orbitals (5 ) of Mn(Cp)2 were more involved in back donation to the ring than in Fe(Cp)2+ (q.v.) by virtue of the smaller apparent k values found for the former (ca. 0.5 vs. ca. 0.8). However, since the calculated parameter is actually fcy 23 this conclusion cannot be substantiated since only the product of k and V23 is currently known for Fe(Cp)2+. 

As a typical case, olefin-metal complexation is described first. Alkene complexes of d° transition metals or ions have no d-electron available for the 7i-back donation, and thus their metal-alkene bonding is too weak for them to be isolated and characterized. One exception is CpfYCH2CH2C(CH3)2CH=CH2 (1), in which an intramolecular bonding interaction between a terminal olefinic moiety and a metal center is observed. However, this complex is thermally unstable above — 50 °C [11]. The MO calculation proves the presence of the weak metal-alkene bonding during the propagation step of the olefin polymerization [12,13]. 

In most palladium-catalyzed oxidations of unsaturated hydrocarbons the reaction begins with a coordination of the double bond to palladium(II). In such palladium(II) olefin complexes (1), which are square planar d8 complexes, the double bond is activated towards further reactions, in particular towards nucleophilic attack. A fairly strong interaction between a vacant orbital on palladium and the filled --orbital on the alkene, together with only a weak interaction between a filled metal d-orbital and the olefin ji -orbital (back donation), leads to an electrophilic activation of the alkene9. 

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