Associative substitutions complexes

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. 

Considerable investigation of the octahedral carbonyl complexes has been carried out. To a certain degree this is because definitive evidence for associative substitution in the case of type A complexes has been conspicuously lacking whereas for the type B compounds there seem to be several well-substantiated examples. A general summary of the main types of octahedral substitutions which have been kinetically examined is given in Table 15. 

The study of rapid, intermolecular ligand exchange between square-planar complexes trans-Ir(CO)L2X (X = C1 or Me, L - PPh3, P(p-tolyl)3, or PMePh2) by variable-temperature 31P NMR spectroscopy indicates that the reaction proceeds through dissociation of phosphine from the metal center and a subsequent associative substitution with other complexes 559,560 Ligand exchange between square-planar Ir and Pt complexes is slow. 

Associative substitution (A) in organometallic octahedral complexes involving 7i-type ligands is well established but not common. F. Basolo, Inorg. Chim. Acta 100, 33 (1985). See Also Ref. 19. 

This is expected to be favoured for metal-centred excited states for example, in d-d states of d or d complexes, where excitation often involves promotion of an electron from an essentially non-bonding orbital to one with appreciable sigma antibonding M-L character (e.g. in CrfNHalsCl Eq. 3). The net effect is lengthening of the M-L bond, which predisposes the complex to dissociation or associative substitution. The incoming ligand is often the solvent (e.g. as in Eq. 3) or counterion of an ion pair (Eq. 4). 

It has always been of some interest to examine the extent to which associative activation dominates the mechanism of substitution of four-coordinate planar cP metal complexes. The coordination unsaturation of these formally 16-electron valence shell complexes predicts that a substitution pathway with increased coordination number (18-electron valence shell) will be favoured over one with a reduced coordination number (14 electrons). This was well understood by workers in the field438 long before Tolman94 published his rules. The first attempt to force a dissociative mechanism was made by Basolo and Baddley513>514 who reasoned that since the steric requirements of associative substitution (rates reduced by steric hindrance from the cis position) were opposite to those of a dissociative mechanism (rates increased or unchanged by increased steric hindrance), sufficient congestion in the substrate should reduce the rate of the associative process to the point where dissociative activation took over. If this did not produce a change in mechanism it could at least indicate a lower limit to the difference of the two modes of activation. 

Another polyhalophenyl gold(I) complex, [Au(3,5-C6Cl2F3)(tht)], has been found to be a very efficient catalyst for the isomerization of fra/w-[Pd(3,5-CftCI2F3)2(tht)21 to c/s-[Pd(3,5-C6Cl2F3)2(tht)2].40 The reaction takes place through a novel reversible aryl exchange between Pd(II) and Au(I). The mechanism involves associative substitution of... 

The principal IR absorptions of 3,20,23 5, and 913,18 (which compounds have been recommended for the identification of alcohols24 25 and primary or secondary amines18) and of 2-substituted saccharins (10)20,23 recorded under identical conditions appear as sets of relatively constant frequencies. For structures with an acidic hydrogen, e.g., compounds (l)16 (2), (9), self-association and complex formation is expected, but the dipolar 3-dialkylamino derivatives (5b) also tend to associate in various solvents.14 Saccharin (1) has been reported to form 1 1 complexes with purines, e.g., theophylline and caffeine, and with amides and... 

Higher substitution products of Co2(CO)g are available through CO dissociation followed by ligand association, as described in equations (1) and (2). However, in cases where valence disproportionation (see Disproportionation) has occurred, valence conproportionation can also result in more substituted complexes (equations 11 and 12). Reduction of a cation solution of Co(CO)3L2+ results in a two-electron reduction from the cation to the monoanion, which combines with excess cation in solution with loss of CO to form the product. An example that has been obtained by this procedure is Co2(CO)4(PMe3)4. The preparation of these highly substituted species proves that the Co-Co bond is retained upon phosphine substitution. 

The 17-electron species see Seventeen Electron Configuration) formed can undergo rapid substitution since associative pathways of low activation energy exist for them. Recombination of the substituted flagments can yield the monosubsti-tuted, disubstituted, or more fully substituted complexes. The rate-determining step would be cleavage of the metal-metal bond since all other steps are relatively fast. This pathway may be preferred for photochemical substitution. Clearly, this pathway is not open to mononuclear analogs. The reactivity of low valent metal radical complexes has been reviewed. ... 

The activation parameters and dependence on L are shown in Table 13. These data are fully consistent with an associative reaction. The 17-electron complex V(CO)6 has an associative substitution reaction rate that is > 10 ° more facile than for the 18-electron Cr(CO)6 complex. The vanadium complexes are among the most inert of the 17-electron complexes. Table 14 shows the rate constants for substitution of several complexes. As expected from size considerations, substituting a phosphine ligand for a CO decreases the rate for an associative reaction. 

The neutral Ni l or Ni-R complexes undergo associative ligand substitution reactions (Scheme 5, see Associative Substitution Mechanisms of Reaction of Organometallic Complexes), and react with methylaluminoxane (MAO) to generate intermediates that polymerize ethylene to high MW poly(ethylene), and alkynes to cis, tran5 otW-poly(alkynes) (Scheme 6). ... 

Kinetic studies ou reactions of Pt(ll) complexes with biologically relevant nucleophiles have been reported. The substitution of both coordinated water molecules by a series of nucleophiles (namely, thiourea (tu), L-methionine (L-Met), and gnanosine-5 -monophosphate (5 -GMP)) was investigated as a fimction of concentration, temperature, and pressure using stopped-flow techniques and was found to occur in two subsequent reaction steps. The activation parameters for all reactions suggest an associative substitution mechanism. 

The X-ray study 170, 171) established a planar structure for the cyclobutadiene ring with C-—C distance equal to 1.46 A and angles of 90°. All the M—C distances are equivalent and close to those observed in ferrocene. The phenyl and methyl substituents are distorted from the ring plane and bent towards the metal atom. If one assumes that cyclobutadiene occupies two coordination sites then in the known tetraphenylcyclobutadiene-nickel and -palladium complexes the metal atom has a coordination number of 5. This suggests coordinative unsaturation for the metal and a priori one may expect an associative substitution for such complexes. 

Coordinatively-unsaturated 16-electron complexes typically undergo associative substitution. Here the mechanism involves a slow bimolecular step where the incoming ligand and 16-electron complex combine to form a coordinatively saturated 18-electron intermediate. The intermediate rapidly expels the leaving group to give the new substituted 16-electron product. This is outlined in equation 7.4. 

Associative substitution is usually found with square planar cP metal complexes such as those of Ni(II), Pd(II), Pt(II), Ir(I), and Au(III). Substitution reactions of these complexes have been thoroughly investigated.19 As in the... 

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