Rate laws square-planar complexes

Mechanistic interpretation of activation volumes on square-planar complexes is complicated by the geometry. The sterically less crowded complexes may have loosely bound solvent molecules occupying the axial sites above and below the plane. Replacing them in the formation of a five-coordinate transition state or intermediate may result by compensation in relatively small volume effects. It is therefore difficult to distinguish between Ia and A mechanisms from the value of the activation volume. Nevertheless, the AV values are negative and together with the second-order rate laws observed, point to an a-activation for those solvent exchange reactions. 

One of the earliest studies of the kinetics of substitution reactions of square planar complexes is that of the Cl exchange of [AuClJ- (22). A two-term rate law was found for the exchange rate and it was suggested that this may prove to be general behavior for square complexes. 

The Rate Law for Nucleophilic Substitution in a Square Planar Complex... 

Rate studies of H2 addition to square-planar complexes have been conducted.58 Measurements at room temperature in several aromatic solvents yield a bimolecular rate law ... 

A kinetic study by Harrod and Smith of oxidative addition to a square planar cationic iridium complex also supports the three-center mechanism (258). The rate law is first order in the iridium complex and first order in hydrosilane. Determination of the activation parameters indicated a moderate activation enthalpy (AH — 5-6 kcal/mol) and a large negative activation entropy (AS — -47 e.u.) No variations were observed on changing the solvent. Harrod and Smith concluded that oxidative addition proceeds via a concerted three-center transition state in which little bond-making or bond-breaking had occurred. The activation enthalpy was attributed to a deformation of the square planar complex on its approach to the transition state. 

Little systematic work exists on the role of the solvent in substitution reactions of other square planar complexes. The two-term rate law is found to characterise all the reactions so far studied. 

Palladium(ii).—Exchange of acac with Pd(acac)2 follows the two-term rate law normal for square-planar complexes ... 

The rate behavior of associative ligand substitutions of square-planar complexes reflects the two pathways that often occur in parallel with and without solvent assistance. Thus, the corresponding rate law provided in Equation 5.8 contains two terms, one of which tjrpically corresponds to the rate constant for associative substitution with solvent assistance and one of which typically corresponds to the rate constant for associative substitution without solvent assistance. Often a plot of versus [Y] appears like the plot... 

Platiniim(u).—General. The rate law common to almost all substitution reactions of square-planar complexes is... 

Effects of Non-reacting Ligands. The two important effects of non-reacting ligands on the reactivity of platinum(n), as of other square-planar complexes are the trans effect and steric effects of bulky substituents. The latter lead to the so-called pseudo-octahedral behaviour wherein associative attack by the incoming nucleophile is prevented by the bulk of the nonreacting ligand. This results in a purely dissociative mechanism and a rate law... 

Entering Groups. The kinetics of the reactions of [PdClJ " with 2,2 -bipyridyl (cf. [PtClJ + bipy, above) and with ethylenediamine have been investigated. Whereas the reaction of [PdClJ " with 2,2 -bipyridyl follows a simple second-order rate law, reactions of [Pd(tu)J + with a range of amino-acids follow the usual two-term rate law [equation (4)] for substitution in square-planar complexes. There appears to be some correlation between reaction rates and pA values of the amino-acids. The reactions of cis-EPdCp-YCeHiNC)-(L)Xg] with / -substituted anilines are discussed in the chapter on reactions of co-ordinated ligands. 

Rates of substitution at square-planar complexes are usually sensitive to variation of the nature of the solvent. Both the ki and values for the normal rate law for substitution at such complexes. 

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... 

The general rate law for substitution in square-planar Pt(II) complexes is valid for the reaction... 

Homoleptic solvent exchange on square-planar Pd and Pt complexes show second order rate laws and negative values for both the entropies and the... 

---