Carol P. J. Vuik

The mechanism of anation of trans-bromoaquotetracyanoplatinate(IV) by bromide

Abstract Studies by previous workers of the reaction trans -Pt(CN) 4 BrOH − 2 + Br − → trans -Pt (CN) 4 Br 2− 2 have been extended to higher bromide ion concentrations and the rate equation k obs = {p[Br − ] 2 |(1 + q[Br − ]/} + {r[Br − ] 3 |(1 + s [Br − ])} is obeyed. Even at the lower values of [Br − ] used by previous workers the contribution to k obs of the second term in the rate equation rises to almost 50%. Its effect is to diminish the influence of the denominator ( 1 + q[Br − ] ) in the first term so that kobs, appeered to be proportional to [Br − ] 2 . A mechanism involving reversible first order (1st term) or bromide-assited (2nd term) reduction of an ion-pair, Br⋯BrPt(CN) 4 OH 2− 2 , is consistent with the rate equation, i.e. the reductive elimination oxidative addition (REOA) mechanism is followed. There are, however, some restrictions on the nature of the reduced intermediates.

Base hydrolysis of trans-bis(ethylenediamine) dihalogenorhodium(III) complexes

Base hydrolysis of the complexes trans-[Rh(en)2X2]+(X = Cl, Br, and I) obeys the rate equation kobs=K1+k2[OH–] at an ionic strenth of 1·0 mol dm–3 where k2/k1≈ 1 dm3 mol–1. Activation parameters for the[OH–]-indepedent path are essentially the same as those for aquation. The [OH–]-dependent path leads to virtually complete trans→cis isomerisation when X = Cl or Br. and to ca. 50% when X = 1. The values of ΔH2‡ and ΔS2‡ lie at the upper end of an isokinetic plot that includes data for the trans-[Rh(en)2(OH)X]+ complexes. An excellent ‘pseudo-isokinetic’ plot of ΔH2‡–ΔH1‡ against ΔS2‡– DeltaS1‡ has also been obtained. The isokinetic plot lies above the corresponding plot for complexes with co-ordinated NH groups trans to the leaving group, the respective isokinetic temperatures being ca. 100 and 190 °C. It seems probable that the reactions that lead to extensive streochemical rearrangement occur by an SN1 CB mechanism but that reactions of complexes further down the isokinetic plot may well have a mechanism closer to the SN1 IP end of an SN1 CB–SN1 IP mechanistic spectrum.

Kinetics of axial ligand exchange in ruthenium(II) tetraphenylporphyrin complexes

Variable-temperature 1H n.m.r. lineshape analysis has been used to determine the kinetics of axial ligand exchange in 1-methylimidazole and 4-t-butylpyridine complexes of benzyl isocyanide(tetraphenylporphyrinato)ruthenium(II). The results are compared with those for other ruthenium(II) porphyrins and for ruthenium(II) complexes of simple amine ligands, and discussed in terms of π bonding and anti-symbiotic effects. The cis effect of tetraphenylporphyrin is much less in ruthenium(II) complexes than in their iron(II) analogues. This is considered as definite evidence for the formation of a high-spin intermediate in the latter case.

Kinetics of ligand exchange in iron(II) complexes of 2,3,9,10-tetramethyl-1,4,8,11-tetra-azacyclotetradeca-1,3,8,10-tetraene

Variable-temperature 1H n.m.r. lineshape analysis has been used to determine the kinetics of axial ligand exchange in some iron(II) complexes of 2,3,9,10-tetramethyl-1,4,8,11-tetra-azacyclotetradeca-1,3,8,10-tetraene (L1). Exchange rates on FeL1 follow the order 2-methylimidazole 1-methylimidazole ≃ imidazole. Rates of 1-methyl-imidazole exchange show cis effects of the order tetraphenylporphinate L1 > tetrabenzo[b,f,j,n][1,5,9,13]tetra-azacyclohexadecine ∼ bis[dimethylglyoximate(1–)]∼ phthalocyaninate(2–), which are interpreted in terms of σ and π bonding and the hole size of the macrocycle.