Metal carbonyls substitution mechanisms

As mentioned in the chapter on the reaction mechanism, the anion, especially of Ni-salts, is important in affecting the reaction course. The catalytic efficiency of the nickel halides strongly increases in the series fluoride, chloride, bromide, iodide [374—376]. The molar ratio of cobalt or nickel to iodine is also very important [414]. As in the hydroformylation reaction, metal carbonyls substituted by phosphine ligands are very reactive [377, 1009], and especially modified rhodium and palladium catalysts [1021, 1045] allow reactions under mild conditions. Thus, the nickel bromide triphenylphosphine allyl bromide complex shows an increased reactivity in the carbonylation of acetylenes. On the other hand, carbonyls substituted by phosphine ligands are also readily soluble in the reaction mixture [345, 377]. 

As indicated in Chapter 8, the production of alkanes, as by-products, frequently accompanies the two-phase metal carbonyl promoted carbonylation of haloalkanes. In the case of the cobalt carbonyl mediated reactions, it has been assumed that both the reductive dehalogenation reactions and the carbonylation reactions proceed via a common initial nucleophilic substitution reaction and that a base-catalysed anionic (or radical) cleavage of the metal-alkyl bond is in competition with the carbonylation step [l]. Although such a mechanism is not entirely satisfactory, there is no evidence for any other intermediate metal carbonyl species. 

F. Basolo, Mechanisms of Substitution Reactions of Metal Carbonyls , Chem. Br., 1969, 5, 505 and refs, therein. 

Table VIII shows how metal-metal and metal-CO bond enthalpy contributions for Os3(CO), 2 compare with those for adjacent metal carbonyls and with Fe and Ru analogs. These values indicate that CO dissociation and M—M bond cleavage for Os3(CO)12 are more endothermic than for Ru3(CO)12 (288). Substitution reactions of Ru3(CO)12 have associative as well as dissociative rate terms (289), whereas Os3(CO)12 has substitution that is independent of the entering ligand. The first-order rate coefficient for reaction in decane at 92°C with PPh3 is 1.12 + 0.14 second-1 A// is 161 + 2 kJ mol-1 and AS is 101 + 6 J K-1 mol-1, and a simple dissociative mechanism seems likely (290).

Now for some of the reactions you have seen in the last few chapters. Starting with carbonyl substitution reactions, the first example is the conversion of acid chlorides into esters. The simplest mechanism to understand is that involved when the anion of an alcohol (a metal alkoxide RO ) reacts with an acid chloride. The kinetics are bimolecular rate = fc[MeCOCl] [RO ]. The mechanism is the simple addition elimination process with a tetrahedral intermediate. 

Fig. 2.14 Fred Basolo (1920-2007) was the Charles E. and Emma H. Morrison Professor of Chemistry at Northwestern University in Evanston in the US. He worked for his Ph.D. with one of the founders of coordination chemistry in the US, John C. Bailar, and received a doctorate from the University of Illinois in 1943. After working on then-classified projects for the war effort, he joined the chemistry department at Northwestern in 1946, where he was a force to be reckoned with for more than 60 years. Together with Ralph Pearson, he was one of the pioneers in the field of inorganic reaction mechanisms and one of the first studying the kinetics of substitution reactions of metal carbonyls. He coauthored two text books Mechanisms of Inorganic Reactions (with R. G. Pearson) and Coordination Chemistry (with R. C. Johnson). Fred was elected to the National Academy of Science in 1979, was the President of the American Chemical Society in 1983, and received the Priestley Medal, the highest award of the ACS, in 2001 (photo by courtesy from Professor Jim Ibers, Northwestern University)...

Substitution of several metal-carbonyl complexes Cr(CO)6 and Mn(CO)5 (amine) show a small dependence on the nature and concentration of the entering hgand. Under pseudo-first-order conditions, the rate laws for these substitutions have two terms, as shown for Cr(CO)6 (as for some substitution reactions with 16e complexes, see equation 5). The second-order term was always much smaller than the first-order term. A mechanism that ascribes the second-order term to dissociative interchange (U) has been suggested for the Mo(CO)5Am system (Am = amine) and involves a solvent-encased substrate and a species occupying a favorable site for exchange. Thus, the body of evidence for the simple metal carbonyls indicates that CO dissociation and is the mechanism of ligand substitution reactions. 

As seen from Table III, iron pentacarbonyl reacts satisfactorily in spite of its inertness towards carbon monoxide substitution under the normal conditions 189). In benzene at 80° C, however, Fe(CO)5 dissociates rapidly (190). The Fe(CO)4 generated displays a nucleophilic reactivity which should promote an A-type mechanism. In spite of the specificities discussed, Maitlis et al. 177) have proposed the following mechanism for the metal carbonyl exchange reactions. 

Kinetics and mechanism of ex- 41 change and substitution reactions (49) in metal carbonyls... 

As mentioned in Section 7-1, second-row complexes tend to react faster than first- or third-row complexes in simple ligand substitutions that occur by a dissociative mechanism.35 This trend has been well studied for Group 6 complexes and is often observed with d9 and d10 metals. For the series of metal carbonyls M(CO)6 where M = Cr, Mo, and W, the observed order of reactivity does not mirror the order of M-C bond energies, which are W > Mo > Cr. The order does seem to correspond to the calculated values for the force constants of M-C bonds in Group 6 metal carbonyls.36... 

In dinuclear complexes containing a metal-metal single bond, the major consequence of irradiation is homolytic cleavage to form reactive metal-carbonyl radicals. Many simple substitutions can be explained by a mechanism involving thermal substitution of these radicals. This mechanism is especially helpful in explaining the formation of a disubstituted product ... 

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