Oxidative-Addition Reactions of d" Complexes

The most interesting work on the isocyanide complexes of the elements in this subgroup has been done with rhodium and iridium. For the most part, the work is involved with the oxidative addition reactions of d square-planar metal complexes. 

B. D. Dombeck, and R. J. Angelici, Electrophilic and Oxidative Addition Reactions of Tungsten Thiocarbonyl Complexes, Inorg. Chem. 15, 2397-2402 (1976). 

Recognition of >j -H2 complexes as likely intermediates in oxidative addition reactions to d ions or reductive eliminations from d metal ions prompts examination of another possible complication to the intimate mechanism of d square-planar metal-ion reactions. Although none have been isolated, it seems reasonable to expect such species as intermediates in reductive eliminations from d complexes also, and a number of site fluxionality cases have been explained this way. 

The first report of the oxidative addition reaction of Ir with a B—bond of a complexed carbaboranylphosphine has appeared. The reaction scheme is shown in Figure 9, and the attack at a B—H and not a C—bond was substantiated by the use of isotopically (H and D) substituted carbaborane derivatives. 

DFT calculations indicate that the oxidative addition reactions of a G-F bond in GeFe at Ni(H2PGH2GH2PH2) and Pt(H2PGH2GH2PH2) proceed initially via exothermic formation of an 77 -arene complex. The G-F oxidative addition reaction is more exothermic at nickel than at platinum. The barrier for exothermic oxidative addition is higher at Pt than at Ni because of strong d-p repulsions in the transition state. Similar repulsive interactions lead to a relatively long Pt-F bond with a considerably lower stretching frequency in the oxidative addition product than for... 

Equation 6.2 shows binuclear oxidative addition, in which each of two metals changes its oxidation state, electron count, and coordination number by one unit instead of two. This typically occurs in the case of a 17e complex or a binuclear 18e complex with an M—M bond where the metal has a. stable oxidation state more positive by one unit. Table 6.1 systematizes the more common types of oxidative addition reactions by d" configuration and position in the periodic table. Whatever the mechanism, there is a net transfer of a pair of electnms from the metal into the [Pg.160]

Cobalt, Rhodium, and Iridium.—Much work has been published during the period covered by this Report concerning oxidative addition reactions of complexes of rhodium and iridium, particularly those of the d type. 

LS = -18 cal deg mol. In comparison with other oxidative-addition reactions of methyl iodide to d complexes this is a relatively small negative value for AS, and the authors claim that steric effects contribute significantly to values of AS in these systems. Thus since [Rh(CN)J will be smaller than a complex with bulky phosphines, transition states involving [Rh(CN)J will have a greater effect on the entropy of the solution. Some data have also been obtained on the reaction of methyl iodide with [Rh(CN)3(CO)] and [Rh(CN)2(CO)a] . Some acyl product [Rh(COMeXCN)s] can result from these reactions, possibly by a Ry-catalysed interconversion of a Rh species. ... 

Third, in analogy with the discussion presented above, oxidative addition represented by Pattern 11 may not be readily observable, and oxidative addition must also proceed mostly through more complex processes, such as that shown in Scheme 1.6 [31], More readily observable are various types of a-bond metathesis reactions of d° Cp2Zr(IV) species (Pattern 13). 

There are of course borderline cases when the reacting hydrocarbon is acidic (as in the case of 1-alkynes) a direct attack of the proton at the carbanion can be envisaged. It has been proposed that acyl metal complexes of the late transition metals may also react with dihydrogen according to a o-bond metathesis mechanism. However, for the late elements an alternative exists in the form of an oxidative addition reaction. This alternative does not exist for d° complexes such as Sc(III), Ti(IV), Ta(V), W(VI) etc. and in such cases o-bond metathesis is the most plausible mechanism. 

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