Transition metal nucleophiles rates

Transition metal complexes are often very good nucleophiles and qualify as being supersoft under Pear son s HSAB classification for example, reaction with soft methyl iodide can be as much as 3 X 105 times faster than the reaction with hard methyl tosylate. Because soft nucleophiles are those with large a values in the Edwards equation, that rates for the transition metal nucleophiles are effectively correlated with oxidation potentials is not surprising. In the last chapter in this section, Chapter 16, Pearson uses recently obtained values of pKa for transition metal complexes to test the full Edwards... 

Rates of reaction of transition metal nucleophiles correlate both with oxidation potentials for MLn and with the pKa values of the corresponding acids, HMLn. Therefore, the two parameters, E° and H, in the Edwards equation are not independent parameters. The same result is found for other nucleophiles, if the donor atom is C,N, O, or F. However, for bases with heavier donor atoms, E° and H are not as correlated with each other. For transition metal complexes, soft ligands, L, increase acidity and decrease nucleophilic reactivity. 

Characteristically, transition metal nucleophiles react much faster with methyl iodide than with methyl tosylate. Rate constant ratios ranging from 30 to 3 X 105 have been found (3). Such behavior qualifies transition metal complexes to be called supersoft nucleophiles (5). Even larger ratios are found for reagents such as Co(CN)53, up to 109. Such large ratios are found only for free-radical pathways (6) and may be used as a mechanistic probe. 

A pioneering study of transition metal nucleophiles was made by Dessy et al. (7). These workers measured not only rates of reaction of various MLn with CH3I but also the oxidation potentials at a platinum electrode. A good linear relation was found when log k2 was plotted against EV2 for various nucleophiles. 

In an attempt to see if the mechanism of oxidative addition can be correlated with relative reactivities, Pearson and Figdore have measured rate constants and activation parameters for the reactions of methyl iodide and methyl tosylate (MeOTs) with a number of transition metal nucleophiles, mci and mcots both vary enormously (see Table 8.10). It is curious that the A// and A5 for the... 

The rate of oxidation/reduction of radicals is strongly dependent on radical structure. Transition metal reductants (e.g. TiMt) show selectivity for electrophilic radicals (e.g. those derived by tail addition to acrylic monomers or alkyl vinyl ketones - Scheme 3.89) >7y while oxidants (CuM, Fe,M) show selectivity for nucleophilic radicals (e.g. those derived from addition to S - Scheme 3,90).18 A consequence of this specificity is that the various products from the reaction of an initiating radical with monomers will not all be trapped with equal efficiency and complex mixtures can arise. 

Transition metal salts trap carbon-centered radicals by electron transfer or by ligand transfer. These reagents often show high specificity for reaction with specific radicals and the rates of trapping may be correlated with the nucleophilicity of the radical (Table 5.6). For example, PS radicals are much more reactive towards ferric chloride than acrylic propagating species."07... 

Few quantitative data are available on the relative nucleophilicities of L toward various alkyl carbonyls. The rates of the reaction of CpMo(CO)3Me with L in toluene (Table II) decrease as a function of the latter reactant P( -Bu)3 > P( -OBu)j > PPhj > P(OPh)j, but the spread is relatively small (<8). The above order is that customarily observed for 8 2 reactions of low-valent transition metal complexes (J, 214). Interestingly, neither CpMo(CO)3Me nor CpFe(CO)2Me reacts with 1 or N, S, and As donor ligands 28, 79). This is in direct contrast to the insertion reactions of MeMn(CO)5 which manifest much less selectivity toward various L (see Section VI,B,C,D for details). 

This type of map can be used to discuss the different types of nucleophilic displacement reaction. Using the simplified version shown in Fig. 2 we have already seen that SN1 reactions, for instance the solvolysis of triarylmethyl halides, go through the separated ions in the top right-hand corner (Swain et al., 1953 Ritchie, 1971). At the opposite extreme, nucleophilic substitution at centres where the number of ligands can be increased may proceed over the bottom left-hand corner of the diagram. Examples are acyl transfer reactions and substitution at tetrahedral phosphorus centres (Alder et al., 1971) as well as substitution at square planar transition metal compounds (Wilkins, 1974). The nucleophilic reactions studied by Ritchie (1976), for which the rate... 

Note The apparent enhanced nucleophilicity of the metal centre in transition metal BIMCA complexes paired with facile oxidative addition on the metal should make this pincer ligand system a prime candidate in those catalytic reactions where the oxidative addition is thought to be the rate limiting step (Suzuki, Sonogashira, Heck). 

Another way of bringing reactants into close proximity, which is encountered commonly in transition metal chemistry, is through metal ion complexation. The coordination of a reactant to a metal ion complex often activates its reactivity and can bring the reactant into close proximity with a second reactant or with a catalytic group. One example, shown in Fig. 6, is a zinc (11) complex of 1,5,9-triazacyclononane, as a model for the enzyme carbonic anhydrase, which contains a zinc (11) cofactor in its active site (4). In the aqua complex, the bound water molecule has a dramatically reduced pKa value of 7.3, which is similar to the pKa of the active site nucleophihc water. The corresponding cobalt (111) complex catalyzed ester hydrolysis at twice the rate because Co(lll) can coordinate both the hydroxide nucleophile and the ester carbonyl via a... 

Ethyl rhenlumpentacarbonyl has been reacted with various metal hydrides in acetonitrile 158). The observed products were heterobimetallic compounds, although a solvated rheniumtetracarbonyl acyl complex was detected [Eq. (47)]. If the metal hydride is in excess, the rate-determining step is formation of the propionyl complex. The reaction was subsequently found to be first order in both the propionyl complex and the metal hydride. The second-order rate constants were measured and were found to be the reverse of the order of the acidities of the transition metal hydrides, which implies that the hydrides react as nucleophiles with the propionyl complex. In a separate experiment, [Re(COEt)(CO)j] was found to react with [Re(H)(CO)j] only after carbonyl dissociation, implying that the metal and not the acyl carbonyl is the site of nucleophilic attack by transition metal hydrides on acyl complexes. 

---