Assisted Ligand Substitution Reactions

Because dissociation of CO from many metal-carbonyl complexes is slow, methods for ffie catalytic or stoichiometric assistance of ligand substitution reactions have received considerable attention. These methods are presented in ffie following sections. 

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In addition to carbenes exerting a strong trans influence (thermodynamic), which may be manifest as a trans effect (kinetic) in ligand substitution reactions, heteroatom-functionalized carbene ligands (with their associated dipolar resonance contributor, Figure 1.16) may assist in the stabilization of transition states of reduced coordination number by electron donation to the metal. 

Oxygen atom transfer (OAT) reactions catalyzed by oxorhenium(v) compounds have been comprehensively reviewed. Some recent developments include a focus on nucleophilic assistance in this process, and a study in which the ligand-substitution reaction that precedes OAT is examined in detail. In the former report, oxygen atom transfer from pyridine A -oxide to ttiphenylphosphine catalyzed by [CH3ReO(mtp)PPh3] (ReX), where mtpH2 is 2-(mercaptomethyl)thiophenol, has been monitored. Reaction was carried out in benzene and following the absorbance of pyridine A -oxides, (PyO), by UV spectrophotometry. The rate law was established to be ... 

The effects predicted are qualitative at best. There are other factors that must be taken into account when predicting how various characteristics of the metal and ligand affect substitution reactions. For example, increasing the size of the metal ion is predicted to assist the formation of the transition state in SN1,... 

The parent TMM complex (190 R = H) undergoes photochemical ligand substitution with trifluorophosphine or trimethylamine Al-oxide assisted substitution with tertiary phosphines or t-butyl isocyanide (Scheme 5A) Trimethylamine A-oxide assisted substitution using isoprene as the incoming ligand results in C-C bond formation to afford the bis-TT-allyl complex (197). An intramolecular version of this reaction is also known.The parent complex (190 R = H) reacts with electrophiles. Addition of HCl or Br2 gives the methallyl complexes (192) and (198), respectively. Tetrafluoroethylene adds across the Fe bond to afford (199) under photochemical conditions. Complex (190) undergoes Friedel-Crafts-type acylation with... 

As an example, consider reaction (6.1). Here, the Cu2+aq ion has water ligands in its coordination sphere substituted by a stronger ligand, in this case ammonia, to form [Cu(NH3)4]2+. Multiple ligand substitution is assisted by the use of an excess of the incoming ligand to drive equilibria towards the fully-substituted compound. 

Relatively few rate and mechanistic studies on tetrahedral complexes have been reported. These complexes are less common than octahedral complexes and their substitution reactions are frequently so fast that their study requires techniques such as stopped flow, temperature jump, or nuclear magnetic resonance. That the reactions are fast indicates that associative processes are occurring, since the activation energy for a reaction will be reduced when an incoming group can assist in the cleavage of a metal—ligand bond. 

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