Isomerization, solvolysis, ion association, and solvation of cis- and trans-dichlorobis(ethylenediamine)cobalt(III) cations in protic and in dipolar aprotic solvents
Ion association constants
at 30� have been determined for the cis-[Co en, Cl2]+Cl-
ion pair in NN-dimethylformamide (DMF), NN-dimethylacetamide
(DMA), and at 20.0�, 25.0�, and 30.0� in dimethyl sulphoxide
(DMSO), by a spectrophotometric method. Association constants for the cis-[Co en2
Cl2]+Br- and the trans- [Co en2
Cl2]+Cl- ion pairs
have also been determined in DMF at 30�.
The anation of the
cis-chlorodimethylformamidebisethylenediaminecobalt(III) ion (cis- [CoCl(DMF)
en2]2+) by bromide ion in N,N-dimethylformamide (DMF) has
been studied. The reaction has an SNIIP1 mechanism, and
when due allowance is made for ion association its activation parameters
closely parallel those of other SNIIP anation reactions in DMF.
The solvolysis and isomerization of the
dichlorobisethylenediaminecobalt(III) ions (cis- and trans-[CoCl2 en2]+)
and the bromochlorobisethylenediaminecobalt(III) ions (cis- and trans- CoBrCl
en2]+) in DMF have been examined over a range of anion
concentration. In all cases solvolysis reactions are important, although direct
isomerization appears to become an important path for the reaction of the trans
complexes at high anion concentrations. Solvolysis of the cis-[CoCl2
en2]+ ion involves an SN2 mechanism.1 The
cis-[CoBrClen2]+, trans-[CoCl2 en2]+,
and trans-[CoBrCl en2]+ ions all react by an SN1
mechanism.
The equilibrium system
(SOL)+Br-+cis-[CoBr2
em]+ =
cis-[CoBr(SOL) en2]2++2Br-
=trans[CoBr2en2]2++Br-+SOL
where SOL represents either
of the solvents NN-dimethylformamide (DMF) or NN-dimethylacetamide
(DMA), has been studied. The compounds cis-
[CoBr(DMF) en2](ClO4)2,
and cis-[CoBr(DMA) en2]XO3,ClO4,
have been isolated and thus it has been possible to study these equilibria
using as starting materials both the cis-
and trans-dibromo complexes (cis- and trans-[CoBr2 en2]+) and the solvento
complexes (cis-[CoBr(DXA) en2]2+ and cis-[CoBr(DMF) en2]2+).
The mechanism of the bromide entry reactions
cis-[CoBr(SOL) en2]2+
+Br- = cis- and trans-[CoBr2
en2]++SOL
is bimolecular in DMA while
in DMF the reaction is dissociative: the rate showing bromide concentration
dependence only in the concentration range where ion association is not
complete. In both systems the isomerization proceeds mainly through the solvento complex;
in dimethylacetamide there is evidence for a
seven-coordinated intermediate. The isomerization equilibria
are found to be bromide concentration dependent, cis-[CoBr2 en2]+
being favoured by high bromide concentrations. This is consistent with the
lower stability of the trans-[CoBr2 en2]-
ion pair with bromide ion.
The equilibria
represented by the following equations:
(See diagram in paper)
have been studied in
anhydrous dimethyl sulphoxide at various chloride ion
concentrations. The mechanisms of the reactions are interpreted as dissociative
and this assignment is consistent with the radiochloride
exchange data. The kinetic and equilibria results are
dependent on ion association which is separately treated by conductance
measurements in the case of cis-[CrCl2 en2]+Cl-.
Activation energies are recorded.
The solvolysis reactions
(see diagram in article)
have been studied in
anhydrous dimethyl sulphoxide (DMSO) over a range of bromide concentrations and
at various mole fractions of DMSO in NN-dimethyl-acetamide (DMA). The
solvolysis of both isomers results in nearly complete removal of first bromide
in pure DMSO while in DMA almost insignificant solvolysis occurs. The influence
of ion association on the measured rates is discussed. Activation energies are
presented for all reactions.
The kinetics and mechanism
of the isomerization of cis- and trans- dichloro- bisethylenediaminechromium(III)
ions, cis- and trans-[CrCl2 en2]+, have been investigated in
anhydrous NN-dimethylformamide (DNF). The complex cis- chlorodimethylformamidebisethylenediaminechromium-
(III), cis-[CrCl(DMF) en2]2+,
is found to be a necessary intermediate and to remain a significant part of the
system at equilibrium.
The equilibria are found to depend on the
concentration of free chloride ion as has been found in the comparable cobalt
system.��� Activation parameters have been determined for
isomerization reactions using both the cis- and trans-(CrCl2
en2)+ ions as starting
materials and for the chloride entry reaction into the cis-[CrCl(DMF)
en2]2+ ion which has been
isolated as cis-[CrCl(DMF) en2](ClO4)2,R2O.
