Ligand exchange reactions dissociative mechanism

Ligand exchange by dissociative activation now appears to be well established for some 16-electron square-planar compounds, though examples are not common. The detail of the intimate mechanism is crude by comparison with associative reactions, but all the evidence indicates the involvement of T-shaped intermediates. 

The question whether the exchange mechanisms are inter- or intramolecular can be answered without difficulty we found the coalescence temperatures and barriers to be independent of the complex concentration, within a range of eight-fold dilution. We are thus left with a choice of intramolecular exchange mechanisms. These can be divided into two main categories dissociative and nondissociative ligand exchange reactions. We next focus our attention to answer this question. 

A detailed mechanism of the process shown in Scheme 1.3 is unknown. Two general mechanistic pathways, dissociative and associative, have been proposed for the ligand exchange reactions of X -iodanes (Scheme 1.4) [26, 127]. The dissociative pathway seems to be less likely to occur, because of the low stability of the dico-ordinated iodonium ion [PhIL]+ involved in this mechanism [127]. Such iodonium 8-1-2 species, however, have been frequently observed in the gas phase, for example, in mass spectrometry studies of protonated iodosylbenzene, [PhIOH]+ [101], or in the mass spectra of all known iodonium salts, [ArIR]+. The presence of cationic iodonium species in aqueous solution has been confirmed by spectroscopic measurements and potentiometric titrations of PhI(OH)OTs and PhI(OH)OMs [198] however, all available experimental data show that the iodonium species in solution are coordinated with solvent molecules or with available counteranions. X-Ray diffraction analysis of the protonated iodosylbenzene aqua complexes [PhI(H20)0H]+ isolated from aqueous solutions revealed a T-shaped stmcture, ligated with one water molecule at the apical site of the iodine(III) atom of hydroxy(phenyl)iodonium ion, with a near-linear O-I-O triad (173.96°), which is in agreement with a regular X -iodane structure [178]. 

Ligand exchange reactions with labeled carbon monoxide performed by Basolo and Wojcicki (32) show that the carbonyls V(CO)ft, Cr(CO)6, Mn2(CO)io, and Fe(CO)s exchange CO groups only slowly, whereas Ni(CO)4 and Co2(CO)8 exchange rapidly. The kinetic lability of nickel carbonyl can in part be attributed to the thermodynamic weakness of the Ni—C bonds. The essential point, however, is that the exchange rate is independent of carbon monoxide concentration which supports a dissociative mechanism. 

Reactions of iron and cobalt dithioline complexes, [M(mnt)jX] mnt = [SjCa-(CN)al, X = a phosphine or phosphite, with unidentate nucleophiles (L) give five-co-ordinate products, [M(mnt)aL]", and with bidentate nucleophiles (L—L) give six-co-ordinate products, [M(mnt)2(L—L)]. In each case the iron complexes react more rapidly than those of cobalt and the mechanism involves both dissociative and, unusually for five-co-ordinate substrates, associative pathways. The kinetic equations are necessarily complicated but the experimental results are consistent with the predictions in limiting cases. Kinetic studies have been carried out on the ligand-exchange reactions ... 

Investigations of racemization and ligand exchange reactions of manganese complexes [Mn(7j -C5H4R)L L L ] have continued and more evidence to support a dissociative mechanism with a chiral three-co-ordinate intermediate has been obtained. Reaction (1) is so slow that chiral four-co-ordinate alkyl intermediates formed by carbonyl... 

Transmetalation, though, requires enhanced electrophilicity of the Pd. Additionally a free coordination site may be required, which may be freed by dissociation of either a neutral or an anionic ligand. The involvement of five-coordinate species and association-dissociation ligand-exchange mechanisms in the individual steps of Pd-catalyzed reactions also cannot be neglected (Scheme 3).384... 

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