Anation reactions of cobalt(iii) complexes

The kinetics of formation of
cis-[Co(tn)
 2
(OH
 2
)(OSO
 2
-O
)]
 +
 (tn = 1,3-diaminopropane), its
acid-catalysed aquation to the parent diaqua complex, anation of
trans-[Co(tn)
 2
(OH
 2
)(SO
 3
-
S)]
 +
 by N
 3

 -
,
NCS
 -
,
SO
 3

 2-
–HSO
 3

 -

, anation of
trans-[Co(tn)
 2
(OH)(SO
 3
-S
)] by SO
 3

 2-
 and acid-catalysed
aquation of
trans-[Co(tn)
 2
(SO
 3
-S)

 2
]
 -
 to the corresponding
(aqua)(sulfito-S) complex were investigated and the
results compared with analogous data for the corresponding
1,2-diaminoethane (en) complexes. Expanding the chelate-ring size from
five to six had virtually no effect on the rate of formation of the
sulfito-O complex, but retarded its acid-catalysed aquation.
The latter effect was attributed to a pK perturbation;
intramolecular hydrogen bonding between the co-ordinated H
 2
O
and sulfite in
cis-[Co(tn)
 2
(OH
 2
)(OSO
 2
-O
)]
 +
 hindered the protonation pre-equilibrium of the
sulfito-O complex involved in the acid-catalysed aquation. This
is further supported by the fact that there was no ring-size effect on
the acid-catalysed aquation of
trans-[Co(L–L)
 2
(OH
 2
)(OSO
 2
)
]
 +
 (L–L = tn or en). The strong
labilising action due to chelate-ring expansion is remarkably attenuated
by the trans effect of S-bonded sulfite as observed in the
anation of
trans-[Co(L–L)
 2
(OH
 2
/OH)(SO
 3

-S)]
 +/0
. However,
trans-[Co(tn)
 2
(OH
 2
)(SO
 3
-
S)]
 +
 was found to be prone to intramolecular
electron transfer between Co
 III
 and S
 IV
 under
thermal conditions unlike its en analogue, further reflecting the
ring-size effect. Flash photolysis of
trans-[Co(L–L)
 2
(OH
 2
)(SO
 3

-S)]
 +
 (L–L = en
or tn) generated the transient
trans-[Co(L–L)
 2
(OH
 2
)(OSO
 2

)]
 +
. The photochemical ligand isomerisation of both
complexes
(Co
 III
–SO
 3

 +
 →
 Co
 III
–OSO
 2

 +
) also
occurred at comparable rates
[k
 iso
 = (4.1 ± 
0.8) × 10
 4
 and
(3.2 ± 1.3) × 10
 4

s
 -1
 at 25 °C for the en and tn complexes
respectively]. Steady-state photolysis at 254 nm indicated that
trans-[Co(en)
 2
(OH
 2
)(SO
 3
-S
)]
 +
 underwent photoaquation and photoreduction.
Strikingly photoreduction could not be detected for this complex at
pH > 8.

Thermal and photochemical reactions of bis(diamine)(sulfito)cobalt( III ) complexes: effect of chelate-ring size

The kinetics of above titled reaction has been studied spectrophotometrically in various water–ethanol mixtures at different temperatures (45–60 °C). The following rate law has been established in the pH range 3.2 to 4.5.\fracd[Co(en)2(H2O)(LH)3+]dt=\frackaKE[Co(en)2(H2O)23+]T[LH]1+KE[LH]where LH represents the zwitterionic form of the amino acids viz. β-alanine and DL-valine. KE is the ion-pair equilibrium constant between substrate complex ion and zwitterionic ligand the values of which increase with the increase of organic component of medium, where reverse is true for ka, the interchange rate constant from outersphere complex to innersphere product. It was observed that at higher ligand concentration a limiting rate is attained, which has been explained due to completion of ion-pair formation. Increasing effect of pH and special catalytic effect of NO3− have been observed. Activation parameters (ΔH\eweq and ΔS\eweq) have been calculated and compared with isotopic water exchange reaction and other deaq...

Kinetics of substitution of aqua-ligands from cis-diaquobis(ethylenediamine)-cobalt(III) complex by β-alanine and DL-valine in various EtOH-water mixtures

The kinetics of dissociation of malonate from the malonatobistrimethylenediaminecobalt(III) complex have been investigated in aqueous perchloric acid media (0.1–1.0 mol dm−3 over a temperature range of 50–70°C at an ionic strength of 2.2 mol dm−3 adjusted with sodium perchlorate. The variation of observed pseudo-first-order rates on [acid] suggests the operation of two concurrent paths with kobs = ko + kH + [H+]. where ko and kH+ are the rate constants for uncatalysed and acid-catalysed pathways respectively. Under identical conditions the malonato complex was found to dissociate 103 times faster rate than that of corresponding oxalato complex. The influence of basicity and chelate ring size of the carboxylate and diamine groups on the aquation rate has been discussed. Analysis of rate data corresponding to kH+ suggests a dissociative mechanism involving the protonated complex.

Kinetics of dissociation of the malonatobistrimethylenediamine cobalt (III) complex in acidic media

Activity in this area remains high, with a considerable volume of interesting work appearing. Review articles of interest include a general discussion of the chemistry of cobalt(III) complexes covering the 1981 literature,(1) and an excellent review by Tobe(2) on the base hydrolysis of metal complexes [predominantly Co(III) and Cr(III) complexes] which will be of great value in defining future research activities. Dixon and Sargeson(3) have also reviewed many aspects of the reactions of coordinated ligands on cobalt(III).

Substitution Reactions of Inert Metal Complexes—Coordination Numbers 6 and Above: Cobalt

The kinetics of anation of cis-diaquobis(1,3-diaminopropane) cobalt(III) by oxalic acid and bioxalate anion have been studied at 35°, 40° and 45°C, I=1.0 M(NaClO4). The observed rate law for the formation of Co(tmd)2C2O+ 4, is valid in the range of [Ox]T=0.03–0.50 M, pH=0.3–2.03. The anation rate constants, k1 and k2, for cis-{Co(tmd)2(OH2)3+ 2, H2C2O4) and cis-{Co(tmd)2(OH2)3+ 2, HC2O4-] ion pairs respectively are found to be virtually the same (k1=10.7 × 10−4 sec−1 and k2=11.0 × 10−4 sec−1 at 40°C). Temperature independent oxalic acid and bioxalate ion-pair association constants (Q1 and Q2) are 1.7(±0.3)M−1 and 11.5(±0.7)M−1, respectively. The activation enthalpy and entropy for k1 and k2 paths are 122.6 kJ mol−1 89.5 JK−1 mol−1 and 103.8 kJ mol−1, 29.3 JK−1 mol−1, respectively. The anation of cis-{Co(tmd)2(OH2)3+ 2, X} ion pairs (X=H2C2O4 or HC2O− 4) is believed to involve Id mechanism. The reaction is catalysed by NO− 3.

Kinetics of anation of cis-diaquo-bis-(1,3-diaminopropane) cobalt(iii) by oxalate

The anation of β-cis-[Co(trien)(OH2)2]3+ ion by oxalic acid, bioxalate anion and oxalate dianion has been studied at 30–45°C and I = 1.0 M (KNO3). The common anation rate constant, k 0, of {β-cis-[Co(trien)(OH2)2]3+, H2C2O4} and {β-cis-[Co(trien)(OH2)2]3+, HC2O4 −} ion-pairs is found to be 6.7 × 10−5 sec−1 at 40°C. The observed anation rate constant levels off to a limiting value (7.0 (·0.8) X 10−3, sec−1) at 40°C, pH = 4.00 (·0.05) and [C2O4 2-] ± 0.15 M, which is characteristic of the anation rate constant (k 1) of the oxalate dianion ion-pair. Activation enthalpy and entropy for the k 0 and k 1, paths are 24.5 K.cal mole−1, 0.8 cal. deg−1 mole−1 and 28.9 K.cal mole−1, 24.0 cal. deg−1 mole−1, respectively. The temperature independent ion-pair equilibrium constant for H2C2O4 ion-pair was found to be 6.8 (·1.2) M−1. The anation of {β-cis-[Co(trien)(OH2)2]3+-X} ion pairs (X = H2C2O4, HC2O4 − or C2C4 2-) is believed to involve an ‘Id’ mechanism.

Nityananda Ray

Papers

Kinetics of anation of cis-diaquo-bis-(1,3-diaminopropane) cobalt(iii) by oxalate

KINETICS AND MECHANISM OF ANATION OF β-cis-(DIAQUO)-(TRIEN)COBALT(III) BY OXALATE