Thiocyanato complexes, reaction rates

As expected, the complexes FeLsCl -m (L = DMSO m<2) do not form Fe P at measurable rates. However, NCS catalyzes the reaction sufficiently to permit rate studies. Lower thiocyanato-complexes of Fe(III) are stable over periods of several months in DMSO/EtOH, unlike their behaviour in water as carefully studied by Dainton (62). Since, in solutions containing Fe(NCS)2+, the concentrations of Fe(II) are negligible compared to Fe(III), insertion via Fe(II) can be ruled out as a significant contribution to the rates observed. The data in Fig. 24 fit a rate law of the type (56), kz r O.3 M- s-i. 

Rates and activation enthalpies for base hydrolysis of the Second-order rate constants at 25 °C are 88.5 and 1081 mol s respectively, and the activation enthalpies are 9.6 1.9 and 10.1 2.5 kcal mol. The predominant reaction of the 5 -thiocyanato-complex rra j-[Co(en)2(NH3)(SCN)] in basic solution is base hydrolysis, with less than 10% isomerization to the iV-thiocyanato-isomers. Base hydrolysis of [Co(NH3)5(SCN)] + is accompanied by much more isomerization. 5-Thiocyanato is approximately as good a leaving group as chloride from cobalt(m) in basic solution. The activation enthalpy for base hydrolysis of [Co(NH3)g(S203)]+ is 30.4 kcal mol the activation entropy is 25.7 cal deg mol". In dilute alkali, base hydrolysis of [Co(en)2(ox)]+ gives 100% m-[Co(en)2(OH2)2] +, but in 4M alkali, the product distribution is... 

Iron. The rate law for the reaction of the S-bonded thiocyanato-complex [Fe(CN)5(SCN)] with hydroxide ion suggests two parallel paths. The first is a D or 5 Nl(lim) path, with an [Fe(CN)6] intermediate discriminating between hydroxide ion and thiocyanate ion. The second path is thought to involve the intermediacy either of [Fe(CN)5(OH)(SCN)] or of [Fe(CN)6(NCS)] . An S Nlcb process is, of course, precluded here. The reactions of optically... 

However, if reaction went by way of aquation to an aquo complex and then anation of the aquo complex to the thiocyanato, and we knew this rate constant, then the optical density would do the following since the aquo complex has a lower extinction coefficient than the nitrato, the density would drop first and then eventually rise, and of course the two curves would come together eventually. 

Another interesting point is the relative rates of the reactions of the azido and thiocyanatopentaammines. The relative rates of these two reactants with iron (I I) ion are similar to those with chromium (I I), that is, the azide is four to five powers of ten more rapid than is the thiocyanato. I am suggesting that this might be a criterion for inner sphere activated complex as opposed to an outer sphere complex. With trisdipyridylchromium(II) ion, which must react via an outer sphere process, the azido and thiocyanato rates are relatively comparable, and the same also for vanadium (I I) ion which also probably procedes via an outer sphere activated complex. 

Ligand Replacement Unidentate by Unidentate.—dissociative mechanism is proposed for the replacement of nitrito- or of thiocyanato-ligands in trans-[Co(dmgH)2(N02)2] and in rm/w-[Co(dmgH)2(NCS)2] respectively by thiourea. Thiocyanate is replaced much more rapidly than nitrite from these cobalt(m) centres. Substitution at bisdimethylglyoximatocobalt(m) complexes can be catalysed by cobalt(n) complexes. This has been demonstrated by the elucidation of the rate law for the cobalt(n)-catalysed reaction of trans-[Co(dmgH)2(PPhs)(NOa)] with pyridine ... 

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