Trans effect mechanism

The more general use of Eq, (5-128) is to obtain the set of total interchange areas AS"- which con stitute a complete description of the effect of shape,. size, and emi.s.sivity on radiative flux, independent of the pre.sence or absence of other tran.sfer mechanisms. It may be shown that... 

A seminal paper [155] examined platinum-phosphorus NMR coupling constants in a series of cis- and trans-platinum(II and IV) complexes. The trans-influence had hitherto been explained in terms of d7r-p7r bonding, in other words, such a mechanism dominated with trans-effect... 

As already mentioned, a purely 7r-bonding mechanism cannot account for the position of hydride in trans-effect and trans-influence series. Overall, therefore, a major role (though not necessarily the only one) for <7-bonding is implied. 

Thiocarbamate (tc, RHNCSO-) is a monodentate ambidentate ligand, and both oxygen- and sulfur-bonded forms are known for the simple pentaamminecobalt(III) complexes. These undergo redox reactions with chromium(II) ion in water via attack at the remote O or S atom of the S- and O-bound isomers respectively, with a structural trans effect suggested to direct the facile electron transfer in the former.1045 A cobalt-promoted synthesis utilizing the residual nucleophilicity of the coordinated hydroxide in [Co(NH3)5(OH)]2+ in reaction with MeNCS in (MeO)3PO solvent leads to the O-bonded monothiocarbamate, which isomerizes by an intramolecular mechanism to the S-bound isomer in water.1046... 

The kinetics and mechanism of ligand substitution reactions of square-planar platinum(II) dimethyl sulfoxide complexes have been exhaustively studied (173), and these workers conclude that the cis and trans influences and the trans effects of Me2SO and ethylene are similar in magnitude whereas the cis effect of Me2SO is about 100 times as large as that of ethylene. The results for reaction (5), where the stability constants, Kt, are reported to be 1.5 x 108 (L = S-Me2SO) and 4.5 x 108 (L = ethylene) corroborate this analogy (213). 

Briefly, as Prof. Basolo points out in his paper, and Prof. Basolo and Pearson pointed out nicely in a review on the trans effect, the situation in the ir-bonding theory is a stabilization of the trigonal bipyramidal intermediate because there are more orbitals in the trigonal plane available for 7r-bonding in the trigonal bipyramid than in the square planar complex. The same is also true in the --system. The trans effect is directional, and in my opinion, it has to be orbital or quantum mechanical. 

The trans effect illustrates the importance of studying the mechanisms of complex substitution reactions. Before continuing with a discussion of mechanisms, the distinction between the thermodynamic terms stable and unstable and the kinetic terms labile and inert should be clarified. Consider the following cyano complexes [Ni(CN)4]2-, [Mn(CN)6]3-, and [Cr(CN)6]3-. All of these complexes are extremely stable from a thermodynamic point of view is yet kinetically they are quite different. If the rate of exchange of radiocarbon labeled cyanide is measured, we find that despite the thermodynamic stability, one of these complexes exchanges cyanide ligands very rapidly (is labile), a second is moderately labile, and only [Cr(CN)6]3 can be considered to be inert ... 

The mechanism proposed to account for the substitution reactions of the Ni(L)r complexes is shown in Fig. 60. A similar mechanism is proposed for substitution reactions with unidentate nucleophiles. Several of the five-coordinate intermediates shown in Fig. 60 were detected and their stabilities were estimated. It was suggested that the trans effect operates through the stability of the five-coordinate intermediate which in turn is correlated to the extent of involvement of the nickel 4pz orbital in n bonding. The implication of the 4pz orbital in n bonding should be considered cum grano salis, however, in view of recent theoretical calculations indicating that the involvement of the 4pz orbital in the n system is very small even in the Ni(MNT)2 complex (123). (See also Sect. 2E.)... 

In the kinetic trans effect, the departure of the trans ligand is probably aided by a stabilization of the transition state via the same mechanisms operative for the trans influence.114 Both associative and dissociative ligand substitution processes seem to be facilitated in this way.117... 

Recently, trans insertion of hexafluorobutyne into one of the M—H bonds in some metallocene hydrides, Cp2MH , was studied in some detail (47). Experiments carried out in the presence of various radical-sensitive reagents such as TV-phenyl-a-naphthylamine suggested that a free radical mechanism was unlikely. A stepwise ionic mechanism, involving a zwitter-ionic intermediate, Cp2(H2)M+—C(CF3)==CCF3, is improbable, since (i) the stereochemistry and the apparent rate are not influenced by the polarity of the solvents, (ii) no deuterium is incorporated in the reaction in EtOD, and (iii) the trend in reactivity (Mo > W) does not reflect the trend in v-basicity or M—C bond stability (W > Mo). An essentially concerted trans-insertion mechanism is inferred, which is supported inter alia by the low kinetic deuterium isotope effect (kH/k0 = 1). 

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