Octahedral complexes reaction rate

As with solvolysis reactions of octahedral complexes, the rate-determining step may be solvolytic or dissociative in any case, it is independent of the concentration of Y ... 

There have been extensive studies of the influence of an entering ligand on its rate of entry into a Pt(ll) complex.The rate constants for reaction of a large number and variety of ligands with trans-Pt(py)2C 2 have been measured (Table 4.13). The large range of reactivities is a feature of the associative mechanism and differentiates it from the behavior of octahedral complexes. The rate constants may be used to set up quantitative relationships. For a variety of reactions of Pt complexes in different solvents (Sec. 2.5.4) ... 

Hydroxide ion also may have an appreciable effect on the rate of hydrolysis of octahedral complexes. The rate constant for hydrolysis of [Co(NH3)5CllJ in basic solution is a million times that found for acidic solutions. Furthermore, the reaction is found to be second order and dependent on tbe hydroxide ion concentration ... 

Complexes of Ir(III) are kineticaHy inert and undergo octahedral substitution reactions slowly. The rate constant for aquation of prBr(NH3)3] " [35884-02-7] at 298 K has been measured at 2 x 10 ° (168). In many cases, addition of a catalytic reducing agent such as hypophosphorous acid... 

Combining volumes, law of, 26, 236 Combustion, heat of hydrogen, 40 Complex ions, 392 amphoteric, 396 bonding in, 395 formation, 413 geometry of. 393 in nature, 396 isomers, 394 linear, 395 octahedral, 393 significance of, 395 square planar, 395 tetrahedral, 394 weak acids, 396 Compound, 28 bonding in, 306 Concentration and equilibrium, 148 and E zero s, 213 and Le Chatelier s Principle, 149 effect on reaction rate, 126, 128 molar, 72... 

Several authors have suggested that the pathway may prove to be the most common mechanism in substitution reactions of octahedral complexes generally. However, the D path can be clearly demonstrated in some cases including at least two examples from Co(III) chemistry. The path (I - III - IV, Fig. 7) through the fivecoordinate intermediate would lead, in the case of rate studies in the presence of excess anionic ligand, to observed first-order rate constants governed by equation (13)... 

RATES OF HYDROLYSIS REACTIONS OF SOME OCTAHEDRAL COMPLEXES AT 25 °C... 

Chelating aldehydes such as 2-pyridine carbaldehyde and 2-dimethylamino benzaldehyde improve the stability of the aldehyde complexes via N,0 chelation. NMR studies show that the complexes are present in solution without an excess of aldehyde and can be formed in the presence of donor ligands. The X-ray structures showed longer and weaker Zn—O bonds when more than one chelating ligand was present. IR demonstrates the variation in C=0 bond strengths and how the environment of the zinc ion will influence potential catalytic activity via reaction rates or pathways. Tetrahedral chelate complexes, and octahedral bis- and tris-chelate complexes, were isolated.843... 

These parameters often parallel one another since they are related to similar characteristic of the system (ehange in number of particles involved in the reaction etc.). The catalyzed hydrolysis of CrjO by a number of bases is interpreted in terms of a bimolecular mechanism, and both AS and AK values are negative. In contrast the aquation of Co(NH2CH3)5L (L = neutral ligands) is attended by positive AS and AK values. The steric acceleration noted for these complexes (when compared with the rates for the ammonia analogs) is attributed to an mechanism.There is a remarkably linear AK vs AS plot for racemization and geometric isomerization of octahedral complexes when dissociative or associative mechanisms prevail, but not when twist mechanisms are operative (Fig. 2.15). For other examples of parallel AS and AF values, see Refs. 103 and 181. In general AK is usually the more easily understandable, calculable and accurate parameter and AK is... 

This problem requires a modified approach which Gray 16) has solved in the case of substitution in square planar complexes. He uses the fact that bases, like hydroxide, substitute very slowly but will immediately deprotonate, and hence stabilize, a protonic solvento intermediate. This elegant approach cannot be applied to the octahedral cobaltammines whose reaction rate with such bases is very high. 

A number of important structural aspects of zinc complexes as found in enzymes are introduced in this section to serve as background information for the subsequent sections. Aquated Zn(II) ions exist as octahedral [Zn(H20)6] + complexes in aqueous solution. The coordinated water molecules are loosely bound to the Zn + metal center and exchange rapidly with water molecules in the second coordination sphere (see Figure 1) with a rate constant of ca 10 s at 25 °C extrapolated from complex-formation rate constants of Zn + ions with a series of nucleophiles. The mechanism of the water exchange reaction on Zn(II) was studied theoretically, from which it was concluded that the reaction follows a dissociative mechanism as outlined in Figure 2. ... 

Substitution reactions in octahedral complexes may proceed by D, Idt Ia, or A mechanisms and it is often difficult to distinguish between them because the rate law by itself does not allow the distinction to be made. Consider the replacement of water by ligand L under neutral conditions ... 

Because of the inertness of Co(III) and Cr(III) complexes, their substitution reactions were the first among those of octahedral complexes to be extensively studied. Most evidence supports the fd mechanism for substitution in Co(fll) complexes. First, there is little dependence of reaction rates on the nature of the incoming ligand, if bond making were of significant importance, the opposite would be expected. Data are presented in Table 13.4 for the anation reaction of penta-ammineaquacobaltdll) ... 

There is growing evidence that substitution reactions in Co(lll) complexes may not be typical of octahedral transition metal complexes. Early studies of substitution reactions for Cr(III) complexes revealed a rather strong dependence of reaction rate... 

Certain octahedral complexes, particularly the acido—amine complexes of cobalt(III), undergo substitution in protonic solvents at rates that are proportional to the concentration of the conjugate base of the solvent (e.g. OH- in water) or inversely proportional to the concentration of the conjugate acid of the solvent (e.g. retardation by H30+ in water or NH4+ in liquid ammonia). Such reactions have received considerable attention since systematic studies of ligand substitution commenced, and figured amongst the earliest kinetic studies in the field.298 The subject has been... 

In 1959, the coordinated mercaptide ion in the gold(III) complex (4) was found to undergo rapid alkylation with methyl iodide and ethyl bromide (e.g. equation 3).9 The reaction has since been used to great effect particularly in nickel(II) (3-mercaptoamine complexes.10,11 It has been demonstrated by kinetic studies that alkylation occurs without dissociation of the sulfur atom from nickel. The binuclear nickel complex (5) underwent stepwise alkylation with methyl iodide, benzyl bromide and substituted benzyl chlorides in second order reactions (equation 4). Bridging sulfur atoms were unreactive, as would be expected. Relative rate data were consistent with SN2 attack of sulfur at the saturated carbon atoms of the alkyl halide. The mononuclear complex (6) yielded octahedral complexes on alkylation (equation 5), but the reaction was complicated by the independent reversible formation of the trinuclear complex (7). Further reactions of this type have been used to form new chelate rings (see Section 7.4.3.1). 

Complexes of (( Ir(III) are kinetically inert and undergo octahedral substitution reactions slowly. The rate constant for aquation of [IrBr(NH3)5]2+ [35884-02-7] at 298 K has been measured at -2 x 10-10 s-1 (168). In many cases, addition of a catalytic reducing agent such as hypophosphorous acid greatly accelerates the rate of substitution via a transient, labile Ir(H) species (169). Optical isomers can frequently be resolved, as is the case of ot-[IrCl2(en)2]+ [15444-47-0] (170). Ir(III) amine complexes are photoactive and undeigo rapid photosubstitution reactions (171). Other iridium complexes... 

Kochi and Buchanan53 state that reaction (41) is rate-determining, and hence equilibrium (40) must be set up rapidly. Although this might be feasible for an octahedral complex of chro-mium(II), it does not seem feasible if (XIV) is a derivative of chromium(lll) since octahedral complexes of chromium(III) are invariably kinetically inert towards nucleophilic substitution (see ref. 54). 

Recently, mechanistic studies have been extended to the substitution reactions of octahedral complexes of Pt(IV). It has been found, for example, that the organic base pyridine replaces chloride in the complex Pt(NH3)3ClJ at a rate proportional to the concentration of pyridine ... 

Another possible two-electron mechanism involves the direct transport of two electrons from a mononuclear transition metal complex to a substrate (S). Such a transport alters sharply the electrostatic states of the systems and obviously requires a substantial rearrangement of the nuclear configuration of ligands and polar solvent molecules. For instance, the estimation of the synchronization factor (asyn) for an octahedral complex, with Eq. 2.44 shows a very low value of asyn = 10 7to 10 8 and, therefore, a very low rate of reaction. The probability of two-electron processes, however, increases sharply if they take place in the coordination sphere of a transition metal, where the reverse compensating electronic shift from the substrate to metal occurs. Involvement of bi- and, especially, polynuclear transition metal complexes and clusters and synchronous proton transfer in the redox processes may essentially decrease the environment reorganization, and, therefore, provide a high rate for the two- electron reactions. 

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