Substitution reactions octahedral

In Section 6.4, it was noted that the rate of water exchange decreases markedly as the charge on the metal ion increases and its size decreases. An increased charge-to-radius ratio is expected to strengthen metal-ligand bonds 

The rate of replacement of coordinated water by 864, 8203, EDTA, and other species has also been measured for a variety of metal ions. It is believed that the mechanism for many of these reactions involves the very rapid association of the hydrated ion with the entering anion to form an ion pair (in which the anion is in the second coordination sphere of the metal ion), equilibrium (26). This ion pair then reaets to give the observed product in 

Probably, the most widely studied coordination compounds are the ammine complexes of cobalt(III). Their stability, ease of preparation, and slow reactions makes them particularly amenable to kinetic study. 8ince work on these complexes has been done almost exclusively in water, the reactions of the complexes with the solvent water had to be considered first. In general, ammonia or amines coordinated to cobalt(ni) are observed to be replaced so slowly by water that only the replacement of ligands other than amines is usually considered. 

The rates of reactions of the type of reaction (28) have been studied and 

Two of the many types of experiments which have provided good mechanistic evidence are presented here. The rate of hydrolysis (replacement of one chloride by water) of tra 5-[CoCl2(NH3)4] is approximately 10 times faster than that of [CoCl(NH3)5]. Since a decrease in rate is observed as the charge on the complex increases, a dissociative process seems to be operative. 

The AH- term is generally not very informative because the enthalpies of activation (which deal with the energy of bond breaking) for associative and dissociative mechanisms are similar. However, the AS term is very useful. When AS Is large and negative, the implication is that there are less particles present in the TS than in the reactants. This would indicate an associative TS, where the transition metal increases its coordination number in the activated complex. On the other hand, if the AS term is large and positive, the TS will have more particles than the reactants, implying a dissociative mechanism. 

The volume of activation (AV ) can also be measured by changing the overall pressure of the reaction vessel. This can be accomplished using a technique known as pressure-tuning spectroscopy (PTS) or by other methods. The relationship between the rate constant as a function of pressure and A is shown in Equation (17.9), which is simply a modification ofthevan t Hoff equation. If AV is independent of the pressure. Equation (17.9) can be integrated at constant T to yield Equation (17.10). The volume of activation can then be determined from the slope of the line when ln(k) is plotted versus the pressure. 

A positive volume of activation implies that the TS is occupying more volume than the reactants, indicating a dissociative mechanism. On the other hand, a negative volume of activation implies that the TS is occupying less volume than the reactants, indicating an associative mechanism. The determination of the activation parameters is therefore an invaluable tool in the study of inorganic reaction mechanisms. 

One of the most common types of reactions of coordination compounds is a simple substitution reaction in which one of the ligands is replaced by another, as shown by the example given in Equation (17.1 I)  

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Octahedral substitution reactions (e.g. those involving cobalt(III) complexes) may proceed by both Sf l or 8 2 reactions. In the S l case a slow dissociative mechanism (bond breaking) may take place. Reaction with the substituting... 

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... 

Stereochemical changes during octahedral substitution reactions. R. D. Archer, Coord. Chem. Rev., 1969, 4, 243-272 (148). 

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... 

OCTAHEDRAL SUBSTITUTION REACTIONS. LABILE AND INERT COMPLEXES... 

The iridium(III) pentaamine complexes Pr(X)(NH3)5]2+ undergo octahedral substitution reactions in aqueous solution, similar to Co111 and Rh111 analogues.221 [Ir(X)(NH3)5]2+ (X = N03, Cl, Br, I) undergoes aquation (reaction 63) with the reactivity increasing in the order... 

C tereochemistry has played a major role in the development of chemistry, and it continues to be most significant. Werner made extensive use of the information available to him on the stereochemistry of metal complexes in developing his coordination theory. He made the first meaningful attempt to understand the mechanisms of substitution reactions of these systems on the basis of the stereochemical changes accompanying such reactions. The paper 49) he wrote in 1912 is a real milestone and should be read by anyone interested in octahedral substitution reactions. It is valuable because of the large amount of experimental data it contains on reactions of cis and [Pg.408]

Although Werner s interpretation of the cause of stereochemical changes during substitution and how they take place adequately explains the experimental facts, it has almost no predictive value. This is not intended as a criticism for we will see in the discussion which follows that we are still largely unable to predict the steric course of a substitution reaction of an octahedral metal complex. This is true despite the availability of much more experimental data and more sophisticated theories of bonding. Excellent reviews (27) have been written on the stereochemistry of octahedral substitution reactions. The discussion that follows deals almost exclusively with cobalt (III) complexes, but the principles involved are generally applicable to other octahedral systems. 

Before considering the results of previously studied complexes, it would seem wise to consider a general scheme for classifying the products of octahedral substitution reactions in general. 

Figure 1, A general scheme for octahedral substitution reaction products assuming a dissociative mechanism...

The planar quinquedentate ligand 2,6-diacetylpyridine bis(semicarbazone), D APSC, has been synthesized and used to prepare the complex [(H20)2(DAPSC)Cr]-0H(N03)2,H20. The two water molecules occupy axial positions in the pentagonal bipyramid. It is suggested that the characterization of this stereochemistry for chro-mium(iii) may imply a possible role for seven-co-ordinate species as intermediates in octahedral substitution reactions. ... 

This promise has been only partially fulfilled because of the difficulty of interpreting anation mechanisms where second order kinetics, first order in entering anion and first order in complex, are often found because of ion association which contributes a term in anion concentration to the rate law. A further difficulty, emphasised by Archer in his recent review on the stereochemistry of octahedral substitution reactions, is found in cobalt(III) chemistry because of the difficulty in isolating trans solvent-containing species. This results in continued doubt in the study of such systems as ... 

Substitution react ions are generally classified as having either dissociative or associative mechanisms. We will use the general octahedral substitution reaction (23)... 

The majority of octahedral substitution reactions are believed to occur by a D or Ij type mechanism, although it can be difficult to distinguish between these two mechanisms based on the similarity of their rate laws in the absence of the observation of any intermediates. In general, the rates of dissociative reaction mechanisms are largely independent of the nature of Y, increase with steric bulk around the metal ion of the overall coordination, show a positive AS- (as there are more species in the intermediate than in the reactants), and exhibit a positive sign for AV. ... 

The classic studies of octahedral substitution reactions involved Co(lll) ammine compounds, such as the example already shown previously in Equation (17.11). For this particular class of compounds, the experimental data yielded the following results ... 

Positive values for the entropy of activation and volume of activation suggest a dissociative mechanism. For example, AS =28J/Kmol and AV = 1.2 cm /mol for the octahedral substitution reaction of [Co(NH3)5H20] +. 

Octahedral substitution reactions also exhibit a kinetic chelate effect Coordination compounds containing a chelating ligand react more slowly than their counterparts containing two monodentate ligands with similar M-L bond strengths. For example, the rate constant for substitution of Ni[(bpy)] + is 3.3 X 10 s , almost 10 times slower than for [Ni(py)] +, which has a rate constant of 38.5. The pro-... 

Example 17-1. Some octahedral substitution reactions are believed to occur by an I, mechanism, where the transition state is seven-coordinate. Assuming a pentagonal bipyramidal transition state, calculate the LFAEs for d -d . Which electron configurations will lead to substitutionally inert metal ions, assuming an I, mechanism ... 

In general, the LFAEs are more positive for the associative mechanism than for the dissociative mechanism, adding further support to the theory that most octahedral substitution reactions are dissociative. The largest LFAEs for the associative mechanism largely mirror those for the dissociative mechanism d, LS d, and d , with the main exception being that LS d is also now included as Inert. 

For the octahedral substitution reaction [M(H20)j] + + HjO v hich metal is expected to yield the larger rate constant for substitution, M = Mn or Ni Explain. 

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