Coordinately unsaturated complexes catalysis

Perhaps the most important metal metal bonded reaction from the point of view of catalysis is addition of hydrogen. In spite of years of study, the exact mechanism of hydrogenation of even dicobalt octacarbonyl is not ftdly understood. Oxidative addition see Oxidative Addition) of hydrogen to metal-metal bonds is generally slower than oxidative addition to mononuclear complexes that have vacant sites. Two principal mechanisms have been proposed in this chemistry. The first relies on cleavage of the metal-metal bond to generate radicals (equation 75). The second mechanism involves dissociation of CO to generate a coordinatively unsaturated complex that retains the metal metal bond (equation 76). In spite of the apparent simphcity of these two possibilities, the reaction mechanism is unknown and it is likely that parallel reaction mechanisms occur. 

Complexes of nitric oxide are receiving increasing attention as potential catalysts. The origin of this interest lies in the proposal,2 since verified, that NO might exhibit a bent mode of coordination in which the nitrogen possesses a nonbonding lone pair. Since bent NO donates two fewer electrons to a metal than the linear isomer does, linear-bent tautomerism raises the possibility of coordinative unsaturation and catalysis. 

Commercially available Pd°(PPh3)4 also catalyzes the process depicted above, but observations indicate that the reaction rate is significantly depressed over those seen with Ph(TH2Pd (PPh3)2Cl. This is true even though the same coordinately unsaturated complex, Pd°(PPh3)2, is responsible for catalysis i both cases through the well-known dissociative process shown in equation (82). ... 

Pincer ligands, that is, tridentate Hgands that enforce meridional geometry upon complexation to transition metals, result in pincer complexes which possess a unique balance of stability versus reactivity [3]. Transition-metal complexes of bulky, electron-rich pincer ligands have found important appHcations in synthesis, bond activation, and catalysis [4, 5]. Among these, pincer complexes of Pr-PNP (2,6-bis-(di-iso-propylphosphinomethyl)pyridine), Bu-PNP (2,6-bis-(di-terPbutyl-phosphinomethyl)pyridine), and PNN ((2-(di-tert-butylphosphinomethyl)-6-diethyl-aminomethyl)pyridine), PNN-BPy (6-di-tert-butylphosphinomethyl-2,2 -bipyridine) ligands exhibit diverse reactivity [6-8]. These bulky, electron-rich pincer ligands can stabilize coordinatively unsaturated complexes and participate in unusual bond activation and catalytic processes. 

In nickel catalyzed toluene oxidation the route of formation major oxidation products, benzaldehyde (BAL), benzyl alcohol (BZA) and benzoic acid (BA) is changed from successive (catalysis by coordinated unsaturated complexes Ni(II)(acac)2 MP on parallel (catalysis by complexes Ni2(OAc)3(acac)-MP ( A ). 

Coordinatively Unsaturated Cobalt Carbonyls Relevant to Hydro-formylation. The negative effect of carbon monoxide partial pressure on the rate of hydroformylation was the first indication of the participation of coordinatively unsaturated cobalt carbonyls in the catalysis of aldehyde formation and of the accompanying olefin isomerization. The retarding effect of carbon monoxide has also been observed in cobalt-catalyzed olefin and aldehyde hydrogenation and in various other reactions of cobalt carbonyls as well. It was assumed that in these reactions in fast reversible carbon monoxide dissociation highly reactive coordinatively unsaturated complexes are formed in very low concentrations, undetectable by conventional analytical methods. By using sophisticated new methods, in some cases the detection and characterization of these elusive species has become possible. 

Reactions of binuclear complexes 29 Stereochemical changes at an unsaturated carbon centre 30 Reactions of coordinatively-saturated complexes 30 Reactions of coordinatively-unsaturated complexes 31 Stereochemical changes at the metal 32 The iodide catalysis effect 38 The reactions of binuclear complexes 39 The reactivity of the carbon centre 41 The reactions of a,o>-dihalogenoalkanes 44 Activation parameters 47... 

Exceptions to this rule are not many but do exist. Thus, more than 18 valence electrons can be accommodated in some cases. The more important cases are those where the number is less than 18, leading to what are known as coordinatively unsaturated complexes. Some of the latter (the 14- and 16-electron complexes) are particularly useful in catalysis, resulting in the important rule described later. A procedure for determining the stabilities of complexes based on this rule is illustrated below for two complexes (see Kegley and Pinhas, 1986, for details). 

Catalysis by Metals. Metals are among the most important and widely used industrial catalysts (69,70). They offer activities for a wide variety of reactions (Table 1). Atoms at the surfaces of bulk metals have reactivities and catalytic properties different from those of metals in metal complexes because they have different ligand surroundings. The surrounding bulk stabilizes surface metal atoms in a coordinatively unsaturated state that allows bonding of reactants. Thus metal surfaces offer an advantage over metal complexes, in which there is only restricted stabilization of coordinative... 

Vacancies were later called coordinately unsaturated sites (cus). This is more in line with terminology used in organometallic chemistry. In view of the present understanding of the nature of the active sites, SBMS or Co(Ni)-Mo-S, the following discussion describes mechanisms in terms of catalysis by organometallic complexes. The references available on this topic are too numerous to mention, and the mechanisms are very well understood. A particularly useful reference is the book by Candlin, Taylor, and Thompson (90), although there are many others that can be consulted. 

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