Ligand Dissociation and Substitution

Chapter 13 introduced carbonyl dissociation reactions, in which CO may be lost thermally or photochemically. Such a reaction may result in rearrangement of the remaining molecule or replacement of CO by another ligand  

The second type of reaction shown above, involving ligand replacement, is an important way to introduce new ligands into complexes. Most thermal reactions involving replacement of CO by another ligand, L, have rates that are independent of the concentration of L they are first order with respect to the metal complex. This behavior is consistent with a dissociative mechanism involving slow loss of CO, followed by rapid reaction with L  

The first step is rate limiting, and has the rate law Rate = fei[Ni(CO)4]. Some ligand replacement reactions show more complicated kinetics. Study of the reaction 

The two terms in the rate law imply parallel pathways for the formation of Mo(CO)sL. The first term is consistent with a dissociative mechanism  

There is strong evidence that solvent is involved in the first-order mechanism for the replacement of CO however, because the solvent is in great excess, it does not appear in the above rate law this pathway exhibits pseudo-first order kinetics. The seeond term is consistent with an associative process, involving a bimolecular reaction of Mo(CO)6 and L to form a transition state that loses CO  

The second type of reaction, involving ligand replacement, is an important way to introduce new ligands into complexes and deserves further discussion. 

Loss of CO from the stable, 18-electron Ni(CO)4 is slow relative to the addition of L to the more reactive, 16-electron Ni(CO)3. Consequently, the first step is rate limiting, and this mechanism has the following rate law  

Some reactions show more complicated kinetics. For example, study of the reaction 

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Alternatively, bi- or multidentate ligands can also be used for support. As an additional benefit, the latter offer greater stability for the coordinatively bound metal center against leaching through ligand dissociation and substitution reactions. The first, somewhat remarkable, approach to this is shown in Figure 42.11, based on numerous examples of the support of bidentate phosphines on polymers [1-5]. 

I propose to develop and apply such methods, based on ultrafast X-ray absorption spectroscopy, to study the ultrafast molecular motions of organometallics in solutions. In particular, initial studies will focus on photo-induced ligand dissociation and substitution reactions of transition metal carbonyls and related compounds in various solvent systems. 

More recent mechanistic studies have been able to distinguish between pathways (b) and (c), and all results indicate that (c) is operative [87]. The initial ligand dissociation and substitution steps have been studied using NMR magnetization transfer experiments, NMR and UV-vis kinetics, and mass spectrometry [87,88]. These investigations indicate that both phosphine/phosphine (Fig. 4.31a) and phosphine/olehn (Fig. 4.31b) substitution reactions in (L)(PR3)(X)2Ru= CHR complexes proceed by a dissociative mechanism involving a 14-electron intermediate (L)(X)2Ru=CHR (A). Although this proposed intermediate has not been observed in solution, presumably due to its low concentration, it has been identihed in the gas phase [88]. 

Reactions involving the gain or loss of a ligand These reactions deal with the addition or subtraction of a ligand to or from the metal center and include ligand dissociation and substitution, oxidative addition, reductive elimination, and nucleophilic displacement. 

Nickel(II) complexes with cryptands are still rare. In general the encapsulation of nickel(II) in this type of macrocyclic ligand makes the complexes extraordinarily resistant to dissociation and substitution reactions. 

Every late-metal hydrogenation catalyst, whether homogeneous or heterogeneous, probably uses exactly the same catalytic cycle, although some catalysts, particularly sterically encumbered ones such as Wilkinson s catalyst, require that ligand dissociation or substitution occur before the catalytic cycle gets underway. In the case of metals supported on solids such as activated C, silica, and alumina, the support may participate in the reaction in ways that need not concern you here. 

Ligand association, dissociation, and substitution processes are facile, so the exact number of ligands on a metal center is usually not a major concern when mechanisms and catalytic cycles involving transition metals are drawn. 

Kinetic studies of H2 dissociation and substitution rates show that the potential energy surfaces for these reactions vary dramatically even with minor changes in ancillary ligands or for isomers (Table 7.7).5,69 Electronic effects, especially the influence of the trans ligand, appear to be more important than steric factors. For the Ir system, the ds isomer with H2 trans to Cl has a strongly bound H2 (dHH = 1.11 A) while the trans isomer with H2 trans to H contains a weakly bound H2 that dissociates nearly 10s times faster (see also Section 4.7.1). One of the few comprehensive quantitative studies of H2 substitution reactions shows displacement of H2 by L (MeCN, PhCN, ) fromCMHCHjXP) (M = Fe, Ru, Os) is first-order in concentration of complex and zero order in L, Le., a dissodative mechanism.70... 

Bagnall and Fischer (24) have studied the effect of ligand, metal and substitution on Cp on the formation of MCp XY complexes. With the large counter cation AsPh, [MCp CNCS) ] was isolated for M = U, Np, Pu. It was also found that U(MeCp) o(NCS) (MeCN), dissociates more readily than the Cp analogue. The investigation of the effect of an additional anion Y on the formation of [UCP3XY]" (X and/or Y = F", oh", CN", NCBH3 , NCO") showed that only NCS" and NCO are... 

The topics of four- and five-co-ordinate complexes are conveniently linked by a study of the kinetics of ligand dissociation and intramolecular rearrangement for a series of cations [MLs]+(M = Co, Rh, or Ir ) and [HMLJ+(M = NF, Pd , or Pt ), where L = PEtg or a trialkyl phosphite. The relevance to four-co-ordinate complexes is of course that these five-co-ordinate species can be regarded as models for transition states for associative substitution at four-co-ordinate species. Several quadridentate arsenic ligands are effective in stabilizing five-co-ordinate... 

The majority of ligand-dissociation reactions are thought to occur on the ultrafast time scale. A relatively rare case of a slow ligand dissociation from a relaxed excited state takes place in some amino complexes of Rh(III) and Ir(III), or in Co(II) polycyano complexes. In these complexes, the lowest ligand-field (dd) state is formed upon t2g eg excitation, and undergoes ligand dissociation and consequent ligand substitution reaction on a nanosecond time scale. 

For each of these reactions kinetic data were obtained. The reactions were first order in complex concentration, and zero order in isocyanide, as expected. The complex Ni(CNBu )4, and presumably other Ni(CNR)4 complexes as well, undergo ligand dissociation in solution. In benzene solution, a molecular weight determination for this compound gives a low value (110). This is in accord with the presumed mechanism of substitution. 

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