Showing papers on "Aquation published in 1967"

The Photochemistry of Aqueous Hexacyanoferrate(II) Solutions. I. Photo-aquation Reaction at 3660 Å

Abstract: The photochemistry of hexacyanoferrate(II) ions in aqueous solutions was studied at 3660 A in the presence and in the absence of nitrous oxide as an electron scavenger. Only a photo-aquation reaction yielding aquopentacyanoferrate(II) and cyanide ions was observed to occur as the primary process. The formation of aquopentacyanoferrate(II) ions, determined spectrophotometrically after a reaction with sodium azide, was studied as a function of the irradiation time over a wide pH range (0.5 to 13). In neutral or alkaline solutions the amount of aquopentacyanoferrate(II) ions measured was small, because of an efficient thermal back reaction. The initial quantum yield depended on the pH; for example, the yield 0.52 at pH 0.65 and 0.89 at pH 4.0 for 0.01 M solutions at 25 °C. The yield decreased with an increase in the hexacyanoferrate(II) concentration.

Mechanism and steric course of octahedral aquation. Part IX. The stereoretentive aquation, anation, and base hydrolysis of the trans-dichloro(1,4,8,11-tetra-azacyclotetradecane)cobalt(III) cation and its solvento-derivatives

Abstract: The aquation equilibria and kinetics of trans-[Co cyclam Cl2]+(cyclam = 1,4,8,11-tetra-azacyclotetradecane) have been studied over a range of temperature. The anation of trans-[Co cyclam H2O Cl]2+ by thiocyanate and chloride ions is reported and SCN– is shown to be a much more effective reagent than Cl–. The base hydrolysis of trans-[Co cyclam Cl2]+ is a fast, two-stage reaction and has been studied in buffer solutions. In the pH region 6·2 to 7·4 the first stage, which has a first-order rate dependence on [OH–], is rate-determining but at higher pH the second stage, whose rate is independent of pH, becomes distinguishible and eventually rate-determining. This stage is identified as the aquation of trans-[Co cyclam OH Cl]+ and the assignment confirmed by studies on the independently prepared hydroxochloro-cation. All the reactions reported proceed with complete retention of configuration. The behaviour of the cyclam complexes is compared with that of the corresponding bisethylene-diamine species.

The solvolytic aquation and base hydrolysis of halogenopentaammine-rhodium(III) complexes

Abstract: The rates and Arrhenius parameters for the replacement of halogens in halogenopentaamminerhodium(III) complexes both by solvent water and by hydroxide ion have been determined and their relative mobilities of halogens compared with the corresponding observations in organic compounds. These reactions are expected to follow the pattern of SNAr reactions, viz., F > Cl ≈ Br > I, and, except for the fluoro complex, this is now confirmed experimentally. The kinetic results are discussed and an attempt is made to interpret them in terms of the electronegativity of the halogens, which facilitates bond formation with the incoming nucleophile. Solvation factors are also important in the control of reactivities. For these reactions, the rate order depends on opposing effects of activation energy and entropy, with the former predominating, and in this way they are in complete contrast to SN2 reactions of octahedral cobalt(III) complexes where the entropy factor alone controls the rate.

Preparation, aquation, and hydrogen exchange of the chloropentakis(methylamine)cobalt(III) ion

Abstract: The preparations and hydrogen exchange of the complex ions Co(MeNH2)5Cl2+ and Co(MeNH2)5OH23+ are described. The rate of acidic hydrolysis of the former into the latter has been determined. Evidence is presented suggesting that the chloro ligand has a strong labilising influence upon the trans-amine's hydrogen whereas the aquo ligand does not, and that the cis-amine's hydrogen is relatively little affected in either case.

Octahedral cobalt(III) complexes of the chloropentammine type. Part X. Effect of replacing chlorine by bromine on the reactivity of some cis-chloroaminebis(ethylenediamine)cobalt(III) cations

Abstract: Rate constants for aquation, and base hydrolysis of cis-[Co en2(amine)Br]2+ where amine may be isoproylamine, n-propylamine, allylamine, prop-2-ynylamine, ethylamine, methylamine, or hydroxylamine, have been determined and Arrhenius parameters calculated. The results are compared with data for the corresponding chloro-complexes. For solvolytic aquation, the effect of replacing chlorine by bromine arises from a combination of activation energy and entropy, acting in opposition, while for base hydrolysis, the latter appears to be the major factor controlling the difference in reactivity between the bromo-complexes and their chloro-analogues thus confirming the formation of an ion-pair between the complex cation and hydroxide ion as a prerequisite. The transition states for these substitutions invlove varying degrees of participation from the incoming group, depending on the nature of the amine. For the isopropylamine complex, steric congestion may provide a sufficient driving force for the departure of the outgoing halogen, but increasing nucleophilic interaction is required on proceeding along the amine series. In the absence of either added acid or alkali, the cis-bromohydroxylaminebis(ethylenediamine)cobalt(III) complex undergoes a spontaneous redox reaction.

The chromium(II)-catalysed dissociation of monohalogenochromium(III) complexes and the effect of non-bridging ligands on electron-transfer reactions

Abstract: The reactions of chromium(II) with monobromochromium(III) and monoiodochromium(III) which result in a net dissociation of the bromide and iodide have been studied in aqueous perchloric acid solutions, ionic strength µ= 2·0M, by spectrophotometric techniques. In each case the kinetics conform to a rate equation of the type Rate =k3′[Cr2+][CrX2+][H+]–1 corrections being necessary for the chromium(II)-independent aquation of CrX2+. A rate equation of the same form has previously been reported for the reaction of CrCl2+. In each case the reactants are believed to be Cr2+ and Cr(X)OH+, and the activated complex [CrOHCrX]3+. In the reaction with CrF2+ there is in addition a hydrogen-ion-independent path which is believed to correspond to the activated complex [CrOHCrHF]4+. At 25° the non-bridging anions have a large (ca. 104) effect on rates, I–(2·13 × 10–2 sec.–1) > Br–(1·74 × 10–3 sec.–1) > Cl–(3·2 × 10–4 sec.–1) > F–(ca. 1·6 × 10–6 sec.–1). This order can be accounted for by considering the relative ligand-field strengths of the halide ions and the ease with which Cr–X bonds can be stretched, but other factors may be important. The effect is of about the same order of magnitude as that of bridging halide ions in the reactions of Cr2+ with CrX2+ and of Cr2+ with Cr(NH3)5X2+. Other data for the reactions of Cr(NH3)OH2+ and Cr(H2O)OH2+ with Cr2+ are considered.

Adducts of co-ordination compounds. Part V. An intermediate in acid-catalysed aquation of a nitro-complex

Abstract: The compound with the composition [Co(NH3)4(NO2)2]NO3,HNO3, originally described by S. M. Jorgensen, is reformulated cis-[Co(NH3)4(HNO2)(NO2)](NO3)2, on the basis of spectroscopy. A new convenient method of preparation is described.