The Croup Replacement Factors (G.R.F.) of halogens in nucleophilic substitutions of halogenopentamminechromium(III) complexes both by solvent water and by hydroxide ion have been compared with the corresponding data available for the analogous cobalt(III) cations. Although reactions in the latter system are typical SN2 processes with concurrent bond formation and fission, the mechanism of substitutions in halogenopentamminechromium(III) complexes depends on the nature of the halogen, being bimolecular for the fluoro and unimolecular for the other halogeno cations. The kinetic results are discussed partly along lines similar to those employed for the interpretation of organic nucleophilic substitutions.

Although chloropentaamminecobalt(n~) chloride can be obtained from cobalt(11) chloride, ammonia, ammonium chloride, and hydrogen peroxide by the method of Willard and Hall,l attempts to prepare the corresponding pentakismethylamine complex lead to the formation of a blue precipitate. We have now successfully prepared the complex [ C O C~( C H~N H~)~] C~~ by the action of buffered aqueous methylamine on solid trans-dichlorotetrakispyridinecobalt(~rr) chloride hexahydrate. The chloride so obtained can be converted into the nitrate by the action of ammonium nitrate.However, attempts to extend the method to higher homologous alkylamines were not successful.Both salts are purple-red in colour, and are more soluble in water than the corresponding chloropentaamminecobalt(~n) compounds. In basic and neutral aqueous solutions, the complex cation decomposes completely to yield the brown insoluble cobalt oxide. If, however, it is dissolved in dilute nitric acid, the coordinated chloride is slowly replaced by water. The product is presumably the aquopentakismethylaminecobalt(~~~) cation, but attempts to prepare salts of this aquo ion have so far proved unsuccessful.The visible absorption spectrum of a solution of chloropentakismethylaminecobalt(1rr) nitrate in dilute nitric acid (Amax 555 mp, emax 63.2 1. mole-1 cm-1; Amax 382 mp, emax 65 .O 1. mole-1 cm-1) was virtually identical with that for the corresponding chloride salt, indicating that the conversion of the chloride into nitrate by the action of ammonium nitrate leaves the complex cation intact. Compared with the spectrum of the corresponding pentaammine complex, the first absorption peak (d-d band) for the present cation occurs a t a longer wavelength (and with a higher extinction coefficient), reflecting the relative strengths of the field produced by the five ammonia and the five methylamine ligands.The hydrolysis of chloropentakismethylaminecobalt(n~) nitrate was studied in dilute nitric acid, and the first-order rate constants obtained (see Experimental)

The relative facility of replacement of halogens in inorganic nucleophilic substitution is compared with the patterns observed in organic compounds, which are strongly indicative of mechanism. I n accordance with our views on this, substitutions of halogenopentamminecobalt (111) complexes with hydroxide ion in water are expected to follow the pattern of saturated aliphatic SK2 reactions, uiz., I; < C1 < Br or I, and this is now confirmed experimentally. The kinetic results arc discussed and a n attempt is made t o interpret them by analogy with aliphatic SN reactions in ternis of electron affinities, bond dissociation, and snl\-ation energies, a n d thc corresponding changes in entropy.INTERESTING axid useful analogies may be drawii between nucleophilic substitution in organic and inorganic compounds, particularly when the relative ease of replacement of fluorine and other halogens is used to indicate the mechanism.Bunnett and his co-workers l y 2 and Miller and his co-workers 3-6 have used this relationship to support the concept that aromatic Ss2 reactions proceed via a relatively stable intermediate complex. The latter commented 4 7 6 that any more facile replacement of fluorine results from its greater electronegativity which facilitates rate-determining bond formation between the reagent and carbon at the reactive centre, and not from bond strength factors. Factors leading to either high or low mobility of fluorine were discussed, with supporting theoretical computations.6 In SN2 reactions this relative mobility (relative to chlorine = 1 which has been called the group replacement factor or G.R.F.') indicates whether significant bond breaking occurs in the rate-determining step, or not. I n the former case, the G.R.F. of fluorine is consistently less than unity but, as discussed below, is not simply dependent on the bond strength. In the latter case, the G.R.F. is greater than unity due to the electronegativity effect.*yG Interpretation (see below) is in terms of a one-step (saturated aliphatic type), or twostep (aromatic type) reaction proceeding via a relatively stable intermediate complex.

All four spin‐allowed transitions in the visible spectra of the complexes [Co(RNH2)5Cl]2+, R = H, Me, Et, n‐Pr, and n‐Bu, have been resolved and the relevant crystal field and RACAH parameters determined with a method previously described. The results appear to be consistent within the series and agree well with those reported in the literature for structurally related systems.

The second-order rate constants for the base hydrolysis of chloro-and fluoro-pentamminecobalt(l1i) and the corresponding chromium(ll1) cations in mixed solvents have been determined over a range of temperatures and the Arrhenius parameters have been calculated. The reaction media used were mixtures of methanol (or ethanol) with water in the mole ratio 1 :4.5. The higher rates in these solvents compared with those in water are apparently compatible with both the conjugate-base and the ion-pair mechanisms of octahedral base hydrolysis. Since, for the corresponding hexammine type of cations in alkaline solutions, spectroscopic evidence points to the presence of the ion-pair, and not the conjugate-base, the kinetic results are discussed in terms of the ion-pair mechanism.