Thermodynamic trans effect

B. Chemical equilibria Thermodynamic trans effect - Dynamic trans effect... 

Basolo555 noted that reactions of Rh111 amine complexes were not dramatically accelerated by hydroxide ion, but did show that substitutions in base do follow the standard kobs = k, + k2[OH" ] format, with k, representing the first-order aquation observed in acidic solution, and k2 representing the second-order base-catalyzed path. Poe has studied the kinetics of the base hydrolysis of a variety of frans-[Rh(en)2XY]"+ complexes (Table 41). Studies on the base hydrolyses of trans-[Rh(en)2(OH)X]+ (X = Cl, Br, I) showed that the coordinated hydroxide has an intrinsic kinetic tram effect comparable to that of Cl" but that its position in a thermodynamic trans effect series is much higher.635 For oms-[Rh(en)2X2]+ (X = Cl, Br), virtually complete tram -+ cis isomerization occurs upon hydrolysis in base, and ca. 50% isomerization is observed when X = I.653 No such... 

The sources of the trans effect are conunonly separated into two factors. The weakening of the bond to the leaving group by the bans ligand in the ground state of the reactant is now called the trans influence, or the thermodynamic trans effect. It can be assessed by strur ural, spectroscopic and thermodynamic properties of the reactant. The kinetic factor relates to the stabilization of the transition state by the trans ligand and is referred to here as the kinetic trans effect. It is determined from the kinetic properties of the system. 

The dispersion is brought into play by two different orbitals d in the bent and p, in the linear form. Again, the s orbital of the metal is the critical one for overlap interaction and it is involved in both geometries. As a result, the cis and trans forms of (PH3)2PtH2 are found to be nearly equienergetic That is to say there is no thermodynamic trans effect in this case. 

The trans-effect is, therefore, a kinetic labilizing effect rather than a thermodynamic one. An approximate series is ... 

Very recently, the kinetics and thermodynamics of a variety of axial ligation reactions have been investigated with Fe11 and Co11 porphyrins involving the small molecules CO, 02, and NO as ligands 27-30, 40, 92, 93). These experiments lead to the conclusion that the dynamic trans effects observed in these systems cannot alone be explained by the interaction models D and F (Fig. 1). Especially imidazole and its derivatives do not hold the place in various series of trans effects that they should take on the ground of their proton basicities. Therefore, besides their usual a-donor-tr-acceptor function, these unsaturated molecules are ascribed an additional 7r-donor function. 

Br HjO [PtBrjlNHjU] [PtBr Cl ] - [PtBr lNO U - [PtBr lCNU] - potentiometric Studied replacement of one Br by HjO. Results show that thermodynamic cis effect is small. Order of increasing cis influence parallels trans influence order. h... 

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 characterization and calorimetric results for a series of displacement reactions (equations 231-233) show little difference between CeHg and CH2C12 solvent.644 The relative displacement energies are P(OPh)3 ethylene > cyclooctene > cis-butene > styrene > cyclopentene > nitrostyrene > cyclohexene. These data are used to give a thermodynamic description of the trans effect. 

As described earlier, the trans effect is indicated by the fact that the products obtained when NH3 and N02 are substituted for Br- in [PtBr4]2- depend on the order of addition. If NH3 is added first followed by N02 , the product is cA-[PtBr2NH3N02] , but if N02 is added first, the product is trans- PtBr2NH3N02r. Clearly the stereochemistry of the product is the result of a trans effect. If this trans directing influence is manifested in this way, it should also be evident from other properties of square planar complexes. In fact, there should be kinetic as well as thermodynamic and structural evidence to indicate the difference in trans influence exerted by different ligands. In the case of square planar complexes, much... 

Poe and co-workers have extensively studied the thermodynamics, kinetics and the trans effect in the reactivity of such complexes.623-634 635 638,64Cl-650-653 The thermodynamics of the successive anations of [Rh(en)2X(H20)]"+ by halogens suggests that the Rh center is a marginally soft acid, and that its softness is increased by the coordination of a soft base (such as I- ) in a position trans to the reaction site a halide ligand weakens the metal-ligand bond trans to itself, and an I- causes a more dramatic weakening than a Br-, which has more effect than a coordinated q-.634,638-640 The spectrophotometrically obtained thermodynamic parameters are summarized in Table 38. 

Carbenes and carbynes can exert both a strong trans influence and trans effect. A trans influence is a ground-state destabilization (thermodynamic) of the trans M-L bond. The trans effect is a kinetic phenomenon wherein the lability of the trans ligand is increased either by ground-state destabilization or transition-state stabilization. 

In addition to carbenes exerting a strong trans influence (thermodynamic), which may be manifest as a trans effect (kinetic) in ligand substitution reactions, heteroatom-functionalized carbene ligands (with their associated dipolar resonance contributor, Figure 1.16) may assist in the stabilization of transition states of reduced coordination number by electron donation to the metal. 

Another class of mthenium aUcene complexes contains those derived from the hexaaqua ion [Ru(H20)6] +. The thermodynamically stable complex [(cod)Ru(H20)4] + (74) forms directly from [Ru(H20)6] + and cod in alcohol at ambient temperature (equation 14). In (74), the redox potential of Ru has shifted more positive for the oxidation to Ru and more negative for the reduction to Ru or Ru°, so as to impose a high stability towards disproportionation see Disproportionation) (in contrast to the readily disproportionating aqua ion [Ru(H20)6] +). The X-ray crystal structure see X-ray Crystallography) of the Tosylate (Ots) salt disclosed quite different R-OH2 distances of 2.095(2) and 2.156(2) A for water gauche or trans to the alkene double bond, showing the structural trans effect see Trans Effect) of the latter on a a- (and tv-) donor ligand trans... 

The experimental approach to the study of cis and trans effects is to compare the relative effect of a series of ligands by following the effect on some property of ligand Y as ligand X is varied. In this review the ligand which is used as the probe in ground-state effects is labeled Y. In thermodynamic effects we consider the equilibrium between two ligands... 

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