18-electron complexes dissociative substitution reactions

As a result, activation enthalpies of many dissociative substitution reactions of 18-electron complexes are relatively high and close to the M-L BDE. The weak M-CO bond energy of Ni(CO) (25 2 kcal /mol) allows this complex to react rapidly at room temperature, but the higher M-L bond energies of the olefin in CpRhCethylene) and CO in Cr(CO)g (31 kcal/ mol and 37 2 kcal/mol, respectively) cause the reactions of these complexes to require temperatures of > 100 °C and 80-140 °C, respectively, to occur at reasonable rates. 

For many species the effective atomic number (FAN) or 18- electron rule is helpful. Low spin transition-metal complexes having the FAN of the next noble gas (Table 5), which have 18 valence electrons, are usually inert, and normally react by dissociation. Fach normal donor is considered to contribute two electrons the remainder are metal valence electrons. Sixteen-electron complexes are often inert, if these are low spin and square-planar, but can undergo associative substitution and oxidative-addition reactions. 

The associative mechanism resembles a conventional radical (hydrogen atom) substitution reaction where the 7T-bonded benzene molecule is attacked by a hydrogen atom formed by the dissociative adsorption of water or hydrogen gas. The activation energy in this process is essentially due to the partial localization of one tt electron in the transition complex 21, 31). The transition state differs, however, from conventional substitution reactions by being 77-bonded to the catalyst surface ... 

In terms of the development of an understanding of the reactivity patterns of inorganic complexes, the two metals which have been pivotal are platinum and cobalt. This importance is to a large part a consequence of each metal having available one or more oxidation states which are kinetically inert. Platinum is a particularly useful element of this pair because it has two kinetically inert sets of complexes (divalent and tetravalent) in addition to the complexes of platinum(O), which is a kinetically labile center. The complexes of divalent and tetravalent platinum show significant differences. Divalent platinum forms four-coordinate planar complexes which have a coordinately unsaturated 16-electron d8 platinum center, whereas tetravalent platinum is an 18-electron d6 center which is coordinately saturated in its usual hexacoordination. In terms of mechanistic interpretation one must therefore consider both associative and dissociative substitution pathways, in addition to mechanisms involving electron transfer or inner-sphere atom transfer redox processes. A number of books and articles have been written about replacement reactions in platinum complexes, and a number of these are summarized in Table 13. 

In the above examples, the nucleophilic role of the metal complex only comes after the formation of a suitable complex as a consequence of the electron-withdrawing effect of the metal. Perhaps the most impressive series of examples of nucleophilic behaviour of complexes is demonstrated by the p-diketone metal complexes. Such complexes undergo many reactions typical of the electrophilic substitution reactions of aromatic compounds. As a result of the lability of these complexes towards acids, care is required when selecting reaction conditions. Despite this restriction, a wide variety of reactions has been shown to occur with numerous p-diketone complexes, especially of chromium(III), cobalt(III) and rhodium(III), but also in certain cases with complexes of beryllium(II), copper(II), iron(III), aluminum(III) and europium(III). Most work has been carried out by Collman and his coworkers and the results have been reviewed.4-29 A brief summary of results is relevant here and the essential reaction is shown in equation (13). It has been clearly demonstrated that reaction does not involve any dissociation, by bromination of the chromium(III) complex in the presence of radioactive acetylacetone. Furthermore, reactions of optically active... 

For di- and triethylamine, the dissociative electron transfer is again the only exist channel, as for fluorobenzene. These experiments show that the substitution reaction can be observed for 1-1 complexes the efficiency of reaction of these 1-1 complexes is correlated with a greater reactivity in bimolecular collision experiments. 

It has been known for many years that normally slow CO substitution in 27+ becomes rapid in the presence of a catalytic amount of reducing agent NEt3.145 This result and others suggest the possibility that many substitution reactions of 18-electron complexes thought to occur by conventional dissociative or associative pathways may in fact take place by an ETC catalyzed mechanism initiated by trace amounts of adventitious reductants in solution. 

Kinetic studies at different phosphine concentrations indicated that the substitution reactions occur totally by a dissociative mechanism, while the ring expansion reaction is by an associative mechanism. The associative reaction could proceed by an 18-electron transition state involving either a bent NO or an rj to rj ring slippage mechanism. The bent NO mechanism seems more likely, because the ring slippage mechanism is known to result in the formation of oxocyclobutenyl product with ring expansion and because the isoelec-tronic cobalt complex does not react by a parallel associative pathway. 

Substitution reactions at Os(CO)4( -alkene) or Os(CO)4 ()] -alkyne) take place through initial dissociation of a ligand. The complexes Os(CO)4(jj -alkyne), where the alkyne is CF3CSCCF3 or HC=CH, compounds are more reactive than Os(CO)5 in substitution and insertion reactions. The acetylene complex is 10 times more reactive than the hexafluoro-2-butyne complex. This is probably due to the ability of the alkyne to act as a four electron donor and stabilize electron-deficient intermediates. The reaction of Os(CO)4( -HC CH) with excess PMes gives a CO insertion product 0s(C0)2(PMe3)2 C(H)=C(H)-C(0) while reaction with the bulkier phosphine PBuj gives a double insertion product, 0s(C0)3(PBu ) C(0)-C(H)=C(H)-C(0) (Scheme 9). ... 

The most common reaction of carbonyl complexes is CO dissociation. This reaction, which may be initiated thermally or by absorption of ultraviolet light, characteristically involves a loss of CO from an 18-electron complex to give a 16-electron intermediate, which may react in a variety of ways depending on the nature of the complex and its environment. A common reaction is replacement of the lost CO by another ligand to form a new 18-electron species as product—a substitution reaction. The following are examples 12... 

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