Anthony J. Poë

A common misconception about the Eyring equation

Linearization and direct fitting to the Eyring equation both give the entropy of activation with the same reliability as that of the enthalpy of activation.

Cone angles: Tolman's and Plato's

Abstract The successful application of Tolman cone angles for P-donor and other ligands in accounting quantitatively for steric effects in a wide variety of physicochemical processes is contrasted with the variability of cone angles obtained from crystallographic studies. It is maintained that the latter are not relevant in describing steric effects for reactions in solution. Problems with cone angles for ligands with conformational uncertainties are best dealt with by systematic measurement of deviations of data for those ligands from trends defined by ligands with less ambiguous cone angles. In fact a body of cone angles for all ligands could be obtained by adjusting cone angles to give perfect fits to individual steric profiles and then averaging the values obtained from a large number of such studies. In this way a set of cone angles could be obtained that are divorced from their origins in Tolmans models and justified solely by their successful quantitative application. Although it is tempting to relate these cone angles to some Platonically perfect set, consideration given here to even a small number of sets of data of this sort suggests that there may always be some ligands with uncertain cone angles. Thus, a combination of eight sets of data that include P(OMe) 3 as a ligand suggests that it should have its Tolman cone angle increased by ca. 10°. A much larger proposed increase is disputed. Another set of data suggests that its Tolman cone angle is appropriate when it is a nucleophile but that P(O i -Pr) 3 as a nucleophile should have its Tolman cone angle reduced by ca. 5°. Thus the deviant behaviour of conformationally ambiguous ligands may be systematic, and could depend on whether the P-donors are acting as ligands or nucleophiles. However, the concept of a Platonically perfect set of cone angles is probably justified for many ligands that are conformationally unambiguous and these ideal cone angles may be essentially identical with Tolmans values. In the course of these analyses a new set of π-acidity parameters was developed and values are tabulated.

Associative reactions of metal carbonyl clusters: systematic kinetic studies of some ruthenium and other clusters

Abstract Rate constants for associative reactions of metal carbonyl clusters with a range of P-donor nucleophiles of various basicities and sizes can always be fitted successfully to the equation log k2=∞+ β (pK′a+4)+γ(σ-σth))λ. The evolution of this equation, and the use of the electronic parameter pK′a and steric parameter σ assigned to particular P-donor nucleophiles, is outlined. λ is a switching function which is zero when B 5 σth (the steric threshold) and becomes unity when σ > σth. The data can be fitted to the equation by a least-squares programme developed initially by Professor W.P. Giering. The significance of the parameters a (defined as the standard reactivity of the carbonyl), β (the susceptibility of the carbonyl to the δ basicity of the nucleophiles), θth (the Tolman cone angle of the nucleophile where steric effects begin to be apparent, i.e. the steric threshold) and y (the sensitivity of the rates to the cone angles of the nucleophiles when θ > θth) are discussed in detail and their interrelationships considered. In particular the sharpness of the steric threshold is taken to imply that a substrate carbonyl undergoes an isomerization reaction when approached by the nucleophile. This produces a so-called transition state isomer (TSI) that contains a roughly conical opening into which all nucleophiles with θ ⩽ θth can fit without steric difficulty. When θ > θth the nucleophiles can attain the same metal-P-donor bond distance in the transition state if the TSI opens up further, so that y is a measure of the flexibility of the TSI. Alternatively the flexibility of the TSI may be so low that it cannot accommodate large nucleophiles unless they penetrate less deeply into the space in the TSI. The application of this systematic kinetic approach to the delineation of the dynamic character of metal carbonyl clusters is illustrated by examples, many of them unpublished, that involve Ru3(CO)11L clusters (where the effect of varying the P-donor substituent is probed), some Rh4 and Ir4 clusters, some high nuclearity Ru and Fe clusters, and a variety of osmium clusters. Not only are the clusters characterizable in this way but also the actual mechanistic paths followed, and the products formed, can undergo major changes which depend mainly on the size of the nucleophiles.

Kinetics and mechanism of the reaction of tetracobalt dodecacarbonyl with carbon monoxide under pressure

Abstract The kinetics of the reaction of tetracobalt dodecacarbonyl with carbon monoxide to form dicobalt octacarbonyl in n-hexane have been investigated over a wide range of temperature and CO pressure. The reaction is first order in [Co 4 (CO) 12 ]; the order in [CO] changes between one (at low pressures and high temperatures) and two (at high pressures and low temperatures). Activation parameters have been estimated and a mechanism involving initial reversible breaking of one CoCo bond, followed by irreversible breaking of a second, is proposed. The first step involves concerted addition of CO while the second can proceed with or without such addition.

Kinetics of substitution and oxidative elimination reactions of pentacarbonylruthenium(0)

Abstract The substitution reactions of pentacarbonylruthenium(0) with a number of phosphorus donor ligands have been shown to proceed by a simple carbon monoxide dissociative mechanism in cyclohexane at 30–50 °C. ΔH ≠ = 27.62 ± 0.40 kcal mol −1 and ΔS ≠ = 15.2 ± 1.3 cal K −1 mol −1 . The Ru(CO) 4 intermediate generated by CO dissociation is stable towards reaction with oxygen in the presence of triphenylphosphine but trimerises to form Ru 3 (CO) 12 in decalin at 125 °C under an atmosphere of 5%CO in a CON 2 mixture. Unlike Fe(CO) 4 , reaction of Ru(CO) 4 with PPh 3 does not lead to any direct formation of a bis phosphine product. The reaction of iodine with Ru(CO) 5 in cyclohexane to form cis -Ru(CO) 4 I 2 was studied in a stopped-flow apparatus and shown to proceed via essentially the same mechanism as the corresponding reaction of Fe(CO) 5 but at rates ca. 10 3 times faster. Reaction of I 2 with tricarbonylbis(triphenylphosphine)ruthenium(0) is not appreciably faster than that with Ru(CO) 5 . Ru(CO) 5 was prepared quantitatively in situ by photochemically induced reaction of dodecacarbonyltriruthenium with CO.

Positional isomerism of triphenylphosphine-nonacarbonylmanganeserhenium

Abstract The positional isomer ax -(OC) 5 MnRe(CO) 4 PPh 3 has been isolated and characterized by IR and 31 P NMR spectroscopy. Spectroscopic and kinetic evidence for the existence of the less stable isomer ax -PPh 3 (OC) 4 MnRe(CO) 5 in solution has also been obtained. The CO stretching frequencies have been assigned on the basis of IR and Raman spectroscopic measurements.

Dissociative kinetics of ‘lightly bonded ligands’ L from the clusters Rh6(CO)15L (L = DMSO, NCMe, cyclooctene, THF and EtOH)

Abstract The kinetics of displacement of some ‘lightly bonded ligands’, L , ( L = dimethylsulfoxide (DMSO), NCMe, cyclooctene, tetrahydrofuran and EtOH) from Rh 6 (CO) 15 L in CHCl 3 or CH 2 Cl 2 by P(OPh) 3 have been investigated. Mass law retardation shows that the reactions are stoichiometrically dissociative and proceed via a {Rh 6 (CO) 15 } intermediate that could be stabilized by the entry of solvent into the vacant coordination site or by extra CO bridging. The limiting rate constants, k 1 , increase by ca. 10 5 along the series listed above ( t 1/2 values decrease from ≈100 s to 1 ms at 25°C). A plot of log k 1 versus log k rel ( k rel = relative equilibrium constants for the reactions) is linear with a gradient of 0.87 ± 0.05 which indicates high degrees of Rh- L bond breaking in the transition states. The low values of Δ S ‡ for L = DMSO and NCMe (−4.5 ± 4.4 and 17 ± 12 J K −1 mol −1 , respectively) suggest that the slower reactions may require some stabilization of the developing {Rh6(CO) 15 } moiety by transition state solvation or extra CO bridging. This stabilization could amount to as much as 30 kJ mol −1 for L = DMSO. Relative rates of reactions of P-donors with the intermediate {Rh 6 (CO) 15 } suggest that these reactions are I d in character. Reactions with Bu 4 NBr show that Br − is much more nucleophilic towards {Rh 6 (CO) 15 } than the neutral nucleophiles and that the nucleophilicity of the ion pair Bu 4 N + Br − is coincidentally similar to that of the P-donors because pronounced Rh-Br − bond making is offset by the attraction due to the cation.

Synthesis and fragmentation kinetics of tetrakis(triphenylphosphine)octacarbonyltetracobalt

Abstract Reaction of Co 4 (CO) 12 with triphenylphosphine is shown to proceed at room temperature in n-heptane or 1,2-dichloroethane (DCE) to form Co 4 (CO) 8 (PPh 3 ) 4 which has been isolated and characterized by analysis and IR spectroscopy. The complex is rather unstable to air and light, especially in solution. Its IR spectra show that Co 4 (CO) 8 (PPh 3 ) 4 probably exists as an equilibrium mixture of forms containing symmetrically bridging and semibridging CO groups, the form with semibridging carbonyls being favored by steric effects and by DCE as a solvent as compared to n-heptane. The kinetics of fragmentation in DCE to form Co 2 (CO) 6 (PPh 3 ) 2 have been studied and shown to proceed by three paths that involve, in order of decreasing ease, rate-determining PPh 3 dissociation, CO dissociation, and spontaneous activation without loss or gain of ligands in forming the transition state. The activation parameters for spontaneous fragmentation suggest that the degree of weakening of the bonding within the Co 4 cluster is quite high in the transition state and that the Co 4 cluster is weaker than the Ru 3 cluster in Ru 3 (CO) 9 (PPh 3 ) 3 . The complexes Co 4 (CO) 8 {P(OMe) 3 } 4 , Co 4 (CO) 8 (dppm) 2 (dppm = Ph 2 PCH 2 PPh 2 ), and probably Co 4 (CO) 8 (P-n-Bu 3 ) 4 are shown to be much more stable to fragmentation that Co 4 (CO) 8 (PPh 3 ) 4 and the instability of the latter is ascribed to the steric effect of its large substituents.

Associative substitution reactions of ·Re(CO)5

Studies of competition between substitution and chlorine abstraction reactions of photochemically generated ·Re(CO)5 radicals show that substitution proceeds via a second-order associative path and not via CO dissociation.

Reaction mechanisms of metal–metal bonded carbonyls. Part VIII. Substitution reactions of cyclo-tris(tetracarbonylruthenium)(3Ru–Ru)

Reactions of cyclo-tris(tetracarbonylruthenium)(3Ru–Ru), (I), with some phosphorus- and arsenic-donor ligands (L), mainly in decalin, have been studied over a range of temperature. Observed pseudo-first-order rate constants obey the rate equation kobs=k1+k2[L] and activation parameters have been obtained. Values of log k2 obey a moderately good linear free-energy relation when plotted against corresponding values of Δ(h.n.p.)(the relative half-neutralization potentials for titration of the nucleophiles with perchloric acid in nitromethane), but those for triphenyl- and tricyclohexyl-phosphine lie well below the line. The use of such relations, and of deviations from them, in indicating the relative contributions of bond making in reactions of a wide variety of metal carbonyl complexes is described and compared with other methods. Reactions of complex (I) appear to involve an exceptionally high degree of bond making. The relative strengths of carbonyl and phosphorus-donor ligands as nucleophiles in attacking vacant co-ordination sites in several complexes are also considered.