17-electron complexes associative substitutions

Eighteen-electron complexes can also undergo associative substitution. Such complexes usually contain a ligand capable of rearranging and accepting an extra pair of electrons, so that the metal can avoid a 2(te configuration. Nitrosyls, with their... 

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. 

Redox reactions usually lead, however, to a marked change in the species, as reactions 4-6 indicate. Important reactions involve the oxidation of organic and metalloprotein substrates (reactions 5 and 6) by oxidizing complex ions. Here the substrate often has ligand properties, and the first step in the overall process appears to be complex formation between the metal and substrate species. Redox reactions will often then be phenomenologically associated with substitution. After complex formation, the redox reaction can occur in a variety of ways, of which a direct intramolecular electron transfer within the adduct is the most obvious. 

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. 

N-donor induced disproportionation of [Fe(CO)3(PR3)2]+ (R = Me, Bu, Cy, Ph) as well as halide induced disproportionation of [M(CO)3(PCy3)2]+ (M = Fe, Ru, Os) has been interpreted in terms of nucleophilic attack being rate determining.103 104 The rate data led to the conclusion that the reactivity of these 17-electron complexes is only weakly dependent on the metal, and the suggestion was made that periodic trends in 17-electron systems are generally attenuated in comparison to those for 18-electron analogues. However, it was noted previously that W > Cr by ca. 106 1 for substitution in [CpM(CO)3]. A direct comparison of the rate of associative ligand substitution at a 17-electron center as a function of the metal for a complete triad (Cr, Mo, W) was reported for the reaction in Eq. (20).14... 

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. 

Thermal substitution reactions of 18-electron complexes are often sluggish. By contrast, 17-electron species are highly reactive, e.g., V(CO)6 undergoes ligand substitution 1010 times faster than Cr(CO)6 via an associative mechanism. Shortlived 17-electron intermediates are known to participate in substitution reactions of Co2(CO)8 ... 

Although the IR spectra hardly changed in frequency between matrices, the visible absorption band of Cr(CO)5 was very sensitive to the matrix material (624 nm in Ne, 560 nm in SFe, 547 nm in CF4,533 nm in Ar, 518 nm in Rr, 492 nm in Xe, and 489 nm in CH4) (Fig. 3). These shifts were very large when compared with those of related 18-electron complexes, e.g., Cr(CO)5NH3, which were only of the order of 5 nm. In mixed matrices, such as Ne -I- 2% Xe, Cr(CO)5 showed two visible absorption bands in positions similar to those seen in the respective pure matrices and displayed substitutional photochemistry. Selective photolysis into one of these absorption bands caused a decrease in the intensity of that band, with a corresponding increase in the intensity of the other band. Furthermore, despite the small proportion of Xe in the mixed matrix, the band associated with the Xe matrix was much more intense than the one associated with the Ne matrix. These results demonstrated conclusively that the shift in visible absorption maximum was not due to... 

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 two dominant characteristics for substitution reactions of 17-electron complexes are very rapid reactions and associative mechanisms. Each of these features is in contrast to reactions of 18-electron complexes. The reactivity has been attributed to the formation of a three-electron bond between the entering nucleophile and the 17-electron complex. Electron density analysis supports stabilization of the 19-electron transition state as the primary source for the labilization. ... 

There are exceptions to Tolman s rule, however [24,25]. For example, if the ligands are very bulky, the 16-electron complex may be sterically hindered, making a 14-electron species the more stable one. The complex Pd[P(ferf-Bu)3]2 is a case in point [26]. Also, a solvent such as benzene can act as electron donor and thereby stabilize a nominally 14-electron complex as a 16-electron solvate [27]. A few reactions appear to proceed through paramagnetic, 17- or 19-electron complexes as intermediates [28,29]. 20-electron species are believed to be formed as intermediates in some associative ligand substitution reactions [30,31]. All such species are much less stable than the corresponding 16- or 18-electron complexes. 

It is generally believed that the oxidation of thiourea and related compounds by aqua-metal ions involves an inner-sphere electron-transfer process, whereas an outer-sphere mechanism is more commonly associated with substitution-inert complexes. The stoichiometry of redox reactions with one-electron oxidizing agents is different for acid and alkaline media. The oxidation of both thiourea and thioacetamide by [Mo(CN)g] in the range 0.02 < [HCIO4] < 0.08 M proceeds in a 1 1 ratio, yielding the disulfide as a product (108) ... 

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. 

The 18-electron rule makes it unlikely that coordinatively-saturated, 18-electron complexes would undergo substitution by an A mechanism. An exception to this conventional wisdom was formulated by Basolo,23 who postulated that substitution in 18-electron complexes may occur associatively if the metal complex can delocalize a pair of electrons onto one of its ligands. 

Equations 1.6-1.9 provide examples of associative substitution reactions on 18-electron complexes. In all cases, modification of the electron contribution of one of the ligands occurs. In reaction 7.6 the nitrosyl group rearranges from its 3-electron, linear form (an 18 e complex) to a 1-electron, bent form (a 16-e complex).26 This allows the phosphine to attack to give the 18 c complex Co(CO)3(PR3)(NO). Finally, CO departs to give the product. The overall rate of the reaction is related to the nucleophilicity of the phosphine and follows the order PEt2Ph PPh3 > P(OPh)3 (see Section 6-3). 

The calculated value for Eq. (e) agrees with the observed value, implying that association is favored on the NHj side of [Co(NH3)j(py)] . Similar reactivity patterns with substituted pyridine ligands support this assignment for the transition as well as for the ground state of the associated complexes. In contrast, when the oxidant is [Ru(NH3)5(py)] +, association (and electron transfer) are favored on the pyridine side. 

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