Aqueous solutions of unipositive cadmium; reactions of (CdI)2 (aq)†
2+
Olga A. Babich* and Edwin S. Gould*
Department of Chemistry, Kent State University, Kent, Ohio 44242, USA. E-mail: obabich@kent.edu
Received (in Irvine, CA, USA) 5th March 2001, Accepted 23rd April 2001
First published as an Advance Article on the web 15th May 2001
Aqueous solutions 1023 mol dm23 in (CdI)2, prepared by
treating O2-free solutions of Cd(ClO4)2 or Cd(O3SCF3)2 with
Cd powder at 65 °C, can be handled by conventional
methods; the comproportionation constant (Cd2+ + Cd "
Cd22+) is estimated as 0.018 (24 °C, I = 1.14 M) and the
formal oxidation potential as 20.45 V; this atypical state
2
22
readily reduces I3
, IrCl6 , pyridine complexes of
(NH3)5RuIII, and superoxo derivatives of (NH3)5CoIII.
The atypical oxidation state cadmium( ) has been prepared and
I
identified in the dimeric cation, (CdI)22+, in aluminium chloride
melts by Corbett et al.,2–4 and has been further studied in the
crystalline state.5 Moreover, a highly reactive species, thought
to be Cd+(aq), has been generated via pulse radiolysis of
aqueous Cd2+ solutions by several workers.6–8
Fig. 1 Variation of concentrations of unipositive cadmium (Cd2)2+ with
Cd(II) taken. Reactions with Cd metal powder were initiated at 65 °C and
were equilibrated at 24 °C. The slope of the regression line, 0.0177 ±
0.0003, is taken as the equilibrium quotient for the comproportionation
reaction: Cd2+ + Cd " Cd22+, corresponding to a DE° value 20.10 V.
However, we find no reports describing aqueous solutions of
unipositive cadmium manipulable by conventional methods. By
avoiding halide and other nucleophilic ligands which favor
disproportionation of Cd(
), we have generated 1024–1023
I
molar solutions of this state, have estimated its redox potential,
and have examined several of its reactions.
20.35 V for Cd(
thermodynamically comparable to U(III) (E° 20.52 V).
The sensitivity of Cd( ) to both strong acids and bases limits
I
,0). Dimeric Cd( ) is then a reductant
I
All preparations and reactions involving Cd( ) were carried
I
out under argon. Typically, cadmium carbonate (0.97 g, G. F.
Smith 99.995%)9 was dissolved in a 5% molar excess of
concentrated HClO4 or triflic acid (HTf), diluted to 15 ml,
heated to 60–65 °C, treated with 1.90 g of Cd powder (Aldrich
325 mesh) with stirring for 5–10 min, then cooled to 24 °C.
Stirring was maintained for 20–30 min. After centrifugation, the
I
the number of redox reactions that can be studied. Rate
constants for four such reactions are summarized in Table 1.
Conversions are first order in both redox partners. Solutions of
2+
Cd2 do not react perceptibly with PtCl622, vitamin B12a
(aquacob(III)alamin), quinoxaline, or the N-methylphenazon-
ium cation, and its reaction with Cr(VI) in 2-ethyl-2-hydroxy-
butanoate buffer (pH 3.6) is inconveniently slow.
Cd(
KI3 (352 nm). At equilibrium (24 °C), 1.7–1.8% of the Cd(II
taken is converted to Cd( ). After separation from Cd metal, it
I) content in the supernatant was estimated by reaction with
)
I
Reactions with the le2 oxidants, IrCl622, Ru(III) and the
[(NH3)5CoIII]2–superoxo cation almost certainly involve an
1
decays slowly (t ⁄ = 25 h at 24 °C).
2
Attempted analogous preparations of ZnI (from Zn metal and
ZnTf2) and MgI (from Mg metal and MgTf2) yielded no soluble
reductant.
odd-electron species related to monomeric Cd( ). Formation of
I
2+
this from the dimer in a preequilibrium homolysis (Cd2 " 2
Cd+) would be reflected in a half-order dependence on
[reductant], contrary to our kinetic picture. Generation of this
transient must then require an act of electron transfer to the
Concentrations of the reducing ion are very nearly propor-
tional to [CdII] taken (Fig. 1), thus being consistent with the
formulation Cd22+, rather than monomeric Cd+. The compro-
portionation constant [eqn. (1)] corresponds to a DE° value of
oxidant. Since it is likely that this transfer precedes breakage of
3+
the Cd–Cd bond, we have designated this intermediate as Cd2
.
We have further chosen this as a reasonable candidate for the
necessary follow-up step, although generation and reaction of
monomeric Cd+ itself cannot be excluded. Kinetic curves
obtained with each of these le2 reagents show no irregularity
indicative of accumulation or loss of this odd-electron species
on the time scale of the principal reaction, pointing to a two-step
process, eqns. (2) and (3).
Cd2+ + Cd " Cd22+; K = 0.0177 ± 0.0003 (24 °C,
I = 1.14 M)
(1)
20.10 V, which, in combination with the standard potential for
Cd(II,0) (20.403 V),10 yields potentials 20.45 V for Cd(II
, ) and
I
† Electron Transfer, part 146. For part 145, see ref. 1.
Cd22+ + IrIV ? Cd23+ + IrIII (slow, rate-determining) (2)
Table 1 Reductions with aqueous cadmium(
I
), 24 °Ca
Oxidant
Product
I/M
l/nm
k/dm3 mol21 s21
2b
I3
I2
0.075
0.030
0.060
0.28
352
520
295
489
(1.00 ± 0.04) 3 105
68 ± 3
[(4-AcPy)(NH3)5Ru]3+
[(NH3)5Co(O2)Co(NH3)5]5+
IrCl6
[(4-AcPy)(NH3)5Ru]2+
[(NH3)5Co(O2)Co(NH3)5]4+ c,d
IrCl6
(3.8 ± 0.1) 3 102
22c
32
(1.41 ± 0.04) 3 103
a [Cd22+] = 2.5 3 1026–2.6 3 1024 M; [Cd2+] = 1.5 3 1024–1.5 3 1022 M; [oxidant] = 5.5 3 1026–4.2 3 1024 M. b Solution buffered with 0.025 M
N-(2-acetamido)-2-aminoethanesulfonic acid (ACES); pH 6.8. c Reaction with Cd22+ in excess. d Reduction of (CoIII)2–superoxo to (CoIII)2–peroxo cation;
pH 5.6.
998
Chem. Commun., 2001, 998–999
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b102041m