Kinetics and mechanism of the cobalt(III) tetraammine complex-promoted hydrolysis of 4-nitrophenyl glycinate

Article abstract of DOI:10.1246/bcsj.75.1515

The kinetics of the hydrolysis of 4-nitrophenyl glycine ester (PNPG) catalysed by [Co(OH)(trien)(OH₂)]²⁺, [Co(OH)(tren)(OH₂)]²⁺ and [Co(OH)(en)₂(OH₂)]²⁺ complexes has been studied spectrophotometrically in weakly basic aqueous media (pH = 6.5 to 7.4). Kinetic experiments were carried out as a function of the pH, complex concentration and temperature. The rate of hydrolysis increases linearly with the complex concentration with a trend to wards rate saturation, suggesting the formation of associative species in a pre-equilibrium step. The pseudo-first order rate constant, k_obs, increases rapidly with a decrease in the hydrogen-ion concentration. The complexes promote the hydrolysis of 4-nitrophenyl glycinate significantly, and the acceleration rate is about 400-600. An attack of external OH^- on the chelated ester is suggested as a probable mechanism for the hydrolysis. The lower rate enhancements observed in the present study could probably be ascribed to a weaker cobalt(III) alkoxy carbonyl interaction in the chelate, owing to a decreased nucleophilicity of the carbonyl oxygen in the p-nitrophenyl ester. The activation parameters for all three complex-promoted reactions are found to be comparable, thus suggesting a common mechanism operative in all complex catalysed reactions.

Full text of DOI:10.1246/bcsj.75.1515

c. Jpn.
© 2002 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 75, 1515–1519 (2002) 1515
e Chemical
ciety of Ja-
n
e Chemical
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n
Kinetics and Mechanism of the Cobalt(Ⅲ) Tetraammine Complex-
Promoted Hydrolysis of 4-Nitrophenyl Glycinate
Hydrolysis of 4-Nitrophenyl Glycinate
A. A. Phulambrikar et al.
Alka Anant Phulambrikar and Chinmay Chatterjee*
02
15
19
SJA8
09-2673
260
Department of Chemistry, Indian Institute of Technology, Powai Mumbai 400 076, India
(Received August 6, 2001)
The kinetics of the hydrolysis of 4-nitrophenyl glycine ester (PNPG) catalysed by [Co(OH)(trien)(OH2)]2+
,
[Co(OH)(tren)(OH2)]2+ and [Co(OH)(en)2(OH2)]2+ complexes has been studied spectrophotometrically in weakly basic
aqueous media (pH = 6.5 to 7.4). Kinetic experiments were carried out as a function of the pH, complex concentration
and temperature. The rate of hydrolysis increases linearly with the complex concentration with a trend to wards rate sat-
uration, suggesting the formation of associative species in a pre-equilibrium step. The pseudo-first order rate constant,
kobs, increases rapidly with a decrease in the hydrogen-ion concentration. The complexes promote the hydrolysis of 4-ni-
trophenyl glycinate significantly, and the acceleration rate is about 400–600. An attack of external OH− on the chelated
ester is suggested as a probable mechanism for the hydrolysis. The lower rate enhancements observed in the present
study could probably be ascribed to a weaker cobalt(Ⅲ) alkoxy carbonyl interaction in the chelate, owing to a decreased
nucleophilicity of the carbonyl oxygen in the p-nitrophenyl ester. The activation parameters for all three complex-pro-
moted reactions are found to be comparable, thus suggesting a common mechanism operative in all complex catalysed
reactions.
01
02
.1
The metal ion-promoted hydrolysis of α-amino acid esters
has attracted considerable attention due to the relevance of
these systems to the reactions of a variety of metalloen-
zymes.1–4 Studies have dealt with catalysis by labile metal
ions, such as Cu(Ⅱ)5–11 and Ni(Ⅱ)12–14 as well as with non-la-
bile ones, like Pd(Ⅱ)15–17 and Co(Ⅲ).18–21 A major problem in
studying the hydrolysis of α-amino acid esters in the presence
of labile metal ions has been determining the actual binding
site of ligands and the identification of the catalytically active
species in the solution. The different pathways involved in the
hydrolysis reactions have been deduced from studies involving
kinetically robust complexes, predominantly of cobalt(Ⅲ).
These studies have almost exclusively involved the use of alkyl
esters (methyl or ethyl) which provide a poor leaving group. It
is normally necessary to monitor these reactions by the pH stat
technique, and is often not possible to greatly vary the metal-
to-ligand ratio. On the other hand, the use of p-nitrophenyl es-
ters which provide a good leaving group, present a number of
advantages. For instance, the reaction can be monitored spec-
trophotometrically. Also in view of the fact that a low ester
concentration is required, it is possible to study the kinetics of
hydrolysis reactions over a varying range of metal-to-ligand
ratios. The hydrolysis of p-nitrophenyl phosphate esters pro-
moted by the tetraammineaqua hydroxocobalt(Ⅲ) complex has
been the subject of several investigations.9–12 The results of
such studies reveal that the activity of the cobalt complex is
sensitive to a tetraammine ligand structure. In marked con-
trast, the metal ion or complex-promoted hydrolysis of p-nitro-
phenyl amino acid esters has scarcely been reported. The re-
ported investigations have dealt with the hydrolysis of these es-
ters promoted by aromatic aldehydes22 and Cu(Ⅱ) ions.23–24
The cobalt(Ⅲ)-promoted hydrolysis of p-nitrophenyl amino
acid esters has not yet been reported. The present paper de-
scribes the kinetics of the hydrolysis of the 4-nitrophenyl gly-
cinate (PNPG) catalysed by cis-hydroxoaquatetraammine co-
balt(Ⅲ) complexes. The tetraammine ligands were used to
block four coordination sites on Co(Ⅲ) and leave two sites for
interactions with the ester. In order to examine the effect of the
tetraammine ligand structure on the reactivity of complexes,
the hydroxoaqua the complexes of bis(didentate), en; linear
tetramine, trien, and tripodal tetramine, tren were employed for
the study.
Materials and Method
Preparation of 4-Nitrophenyl Glycinate (PNPG) Hydro-
bromide: The ester hydrobromide was prepared by reacting
N-benzyloxy-carbonyl ester with a saturated solution of HBr in
glacial acetic acid by a method of Bem-Ishai and Berger.25
Carbondioxide was evolved and the reaction was completed
within a few minutes. When the evolution of carbon dioxide
had ceased, dry ether was added to precipitate the ester hydro-
bromide, which was filtered off after keeping in a refrigerator
for 4 hours; it was then washed with ether and dried over sodi-
um hydroxide pellets. The crude 4-nitrophenyl glycinate ester
hydrobromide was recrystallised from absolute ethanol by the
addition of anhydrous ether. The purity of the sample was as-
certained by an elemental analysis. The melting point (213
°C) of the compound matched well with the reported value
(213 °C).24
The complexes trans-[CoCl2(en)2]Cl,26 α-cis-[CoCl2(trien)]-
Cl27 and [CoCl2(tren)]Cl28 were prepared following methods
described in the literature. An equimolar concentration (2.0 ×
10−3 mol dm−3) of [Co(OH)(tren)(OH2)]2+ and PNPG at pH
7.0 was heated at 40 °C for 3 hrs., whereupon the lowest ener-

1516 Bull. Chem. Soc. Jpn., 75, No. 7 (2002)
Hydrolysis of 4-Nitrophenyl Glycinate
gy d–d band shifted from 520 nm, characteristic of the
[Co(OH)(tren)(OH2)]2+ ion,29 to 500 nm, corresponding to the
absorption maximum of the red isomer of the [Co(gly)-
(tren)]2+ chelate,28 in which the tertiary amine nitrogen of tren
is trans to the amino group of glycine. A similar behaviour of
shifting of the absorption maximum to a shorter wavelength
was observed for other cobalt(Ⅲ) complexes used in the study.
Kinetic Measurements: Kinetic measurements were car-
ried out on a Shimadzu UV-2100 spectrophotometer equipped
with a Julabo SC-12 constant-temperature water bath. The
temperature of the cuvette was maintained within 0.1 °C by
circulating thermostated water through the cell compartments
of the spectrophotometer. The pH measurements were made
on a Metrohm 654 digital pH meter (with 0.02 pH unit accu-
racy). The hydrolysis of 4-nitrophenyl glycinate was moni-
tored following an increase in the absorbance at the p-nitro-
phenolate absorption maximum (400 nm). The reactions were
carried out under pseudo-first order conditions, with a large ex-
cess of the complex over the ester. The pseudo-first order rate
constants for the reactions were calculated from the slope of
pH of the reaction solution did not change appreciably ( 0.1)
during the course of the reaction, due to a buffering effect of
the cobalt complex solution.18 The hydroxide-ion concentra-
tions were evaluated from the appropriate molar activity coeffi-
cients and the values of [pKw: 13.833 (30 °C); 13.68 (35 °C)
and 13.535 (40 °C)] reported in the literature.30 The hydrolysis
reactions have been studied as a function of the complex con-
centration, pH and temperature. The pseudo first-order rate
constants (kobs) under different experimental conditions are
summarized in Tables 1 to 3. The rate of hydrolysis increases
linearly with the complex concentration with a trend towards
rate saturation, as shown in Fig. 1. The rate of hydrolysis also
increases with an increase in the pH of the medium, as shown
in Fig. 2. The uncatalyzed rate constant (ko) at different pH
values takes the following values: at a pH of 6.5, ko = 0.80 ×
10−3/s; at a pH of 6.95, ko = 1.48 × 10−3/s, and at a pH of
7.35, ko = 2.00 × 10−3/s. The complex-promoted reactions
can be explained in terms of Eqs. 1 and 2, involving the rapid
formation of associated species with the coordination of gly-
cine ester in a pre-equilibrium process, followed by a slow
rate-determining base hydrolysis step:
plots of −ln(A −At) against time using a standard software
∞
program for a least-squres regression analysis. The A values
∞
were recorded after ten half lives. The reported rate constants
(kobs) are an average of three kinetic runs with an error estima-
tion of less than 5%. The ester stock solutions were prepared
in dry methanol and the complex solutions in freshly boiled
carbon dioxide-free double-distilled water. Because carbon di-
oxide is known to catalyse the hydrolysis of aryl esters of α-
amino acids,15 care was taken to prevent any CO2 contamina-
tion of the solutions.
HL+
H
+ + L
KM
M + L
[ML]
(1)
(2)
[ML] + OH− kMOH → Product
L + OH− or H2O → Product
ko
The complex is represented as M and the unprotonated ester
species as L. A plot of kobs versus the complex concentration
(Fig. 1) indicates that the reaction follows a two-term rate law
of the type
Results and Discussion
In typical kinetic measurements, a stock solution of cis-
[Co(OH)N4(OH2)]2+ (where N4= (en)2, trien or tren) was pre-
pared by adding 1.5 equivalents of NaOH solution to the
dichlorocomplex. After 20 minutes (1 hour for the tren com-
plex) the solution pH was adjusted to the desired value. Hy-
drolysis of the ester was initiated by adding 5.0 × 10−6 mol
dm−3 of a 0.015 mol dm3 ester stock solution to 2.50 cm3 of a
freshly prepared complex solution equilibriated at the experi-
mental temperature The ionic strength of the reaction mixtures
was maintained at 0.1 mol dm−3 with the help of NaCl. The
kMOH[OH-]kM[M]
(3)
kobs = ko
+
1 + KM[M]
where kobs. is the observed first-order rate constant at constant
pH, ko is the rate constant due to the background hydrolysis re-
action of L in the absence of the cobalt(Ⅲ) complex ion, KM is
the association constant and kMOH is the rate constant for a
Table 1. Pseudo First Order Rate Constants (kobs) for Hydrolysis of PNPG Ester Promoted by
[Co(OH) (trien) (H2O)]2+ at Different pH and Temperatures.a)
103 kobs/s−1
103[complex]/mol dm−3
pH = 6.95 0.05
pH = 6.50 0.05
pH = 7.35 0.05
303.3b)
1.65
2.40
3.10
4.25
5.20
6.10
6.75
7.20
—
308.3b)
2.35
3.50
4.55
5.55
6.65
7.75
8.60
9.45
10.10
313.3b)
3.20
4.65
5.87
6.98
8.50
9.82
10.91
11.78
—
308.3b)
1.27
1.70
2.18
2.70
3.20
—
308.3b)
3.42
4.85
6.26
7.57
9.07
—
2.5
5.0
7.5
10.0
12.5
15.0
17.5
20.0
22.5
—
—
—
—
—
—
a) [Ester] = 3 × 10−5 mol dm−3, I = 0.1 mol dm−3 b) T/k.

A. A. Phulambrikar et al.
Bull. Chem. Soc. Jpn., 75, No. 7 (2002) 1517
Table 2. Pseudo First Order Rate Constants (kobs) for Hydrolysis of PNPG Ester Promoted by
[Co(OH)(en)2 (OH2)]2+ at Different pH and Temperaturesa)
103 kobs/s−1
pH = 6.50 0.05
103[complex]/mol dm−3
pH = 6.95 0.05
pH = 7.35 0.05
303.3b)
1.58
2.10
2.58
3.07
3.70
4.25
4.65
5.05
5.45
5.85
308.3b)
2.20
2.95
3.70
4.44
5.04
5.65
6.36
6.75
7.33
7.72
313.3b)
2.85
3.90
4.55
5.31
6.20
6.86
7.85
8.60
9.41
10.10
308.3b)
1.25
1.65
2.05
2.40
2.75
—
308.3b)
3.01
3.96
3.96
5.80
6.75
7.55
—
2.5
5.0
7.5
10.0
12.5
15.0
17.5
20.0
22.5
25.0
—
—
—
—
—
—
—
a) [Ester] = 3 × 10−5 mol dm−3, I=0.1 mol dm−3 b) T/k.
Table 3. Pseudo First Order Rate Constants (kobs) for Hydrolysis of PNPG Ester Promoted by
[Co(OH)(tren)(OH2)]2+ at Different pH and Temperaturesa)
103 kobs/s−1
103[complex]/mol dm−3
pH = 6.95 + 0.05
pH = 6.50 + 0.05
pH = 7.35 + 0.05
303.3
1.65
308.3
2.35
3.05
3.90
4.75
5.50
6.30
7.10
7.80
8.20
8.85
313.3
3.08
308.3
1.25
1.70
2.15
2.55
3.00
—
308.3
3.10
4.30
5.30
6.25
7.05
—
2.5
5.0
7.5
10.0
12.5
15.0
17.5
20.0
22.5
25.0
2.30
3.10
3.75
4.40
5.10
5.60
6.15
6.40
6.90
3.90
5.15
5.92
7.13
8.06
8.50
9.25
9.84
—
—
—
—
—
—
—
—
—
a) [Ester]=3 × 10−5 mol dm−3, I=0.1 mol dm−3 b) T/k.
complex-promoted reaction. Rearranging Eq. 3 gives Eq. 4 :
the associated species, [ML]. The labilisation of the aqua for
substitution can be ascribed to the cis effect31 of the coordinat-
ed hydroxo group. A cis-libilisation effect of a coordinated hy-
droxide ion on a aqua ligand for substitution will not be
present in the dihydroxo species, because the latter can not
provide a good leaving group for the first act of substitution by
the ester, and thus the major reaction pathway would involve
predominant aqua hydroxo species. Thus, the increase in the
rate with increasing complex concentration at a fixed tempera-
ture and pH can be attributed to the higher concentration of the
ML species, which subsequently hydrolyses in a rate determin-
ing step. The trend towards rate saturation is observed only at
high ML ratios, indicating that the KM values are not very high.
For all of these complexes used during the present study, it is
observed that at a fixed complex concentration and tempera-
ture, the rate increases with the pH. In the pH range of 6.5 to
7.4, although the concentration of the hydroxoaqua form of the
complex increases marginally, that of the ester changes appre-
ciably. The pKa of the 4-nitrophenyl glycinate hydrobromide
(HL) is reported to be 7.1. Thus, the concentration of the un-
protonated form of the ester L species is 18, 42 and 64% at pH
values of 6.45, 6.95 and 7.35 respectively. Therefore, as the
concentration of L increases, the forward reaction, as shown in
1/(kobs − ko) = 1/kMOH [OH−] KM[M] + 1/kMOH [OH−]. (4)
The double reciprocal plot of 1/(kobs − ko) versus 1/[M] is
found to be linear, indicating the validity of Eq. 3. The values
of KM and kMOH calculated from the intercept by the slope (=
KM), and substituting the values of [OH−] in the slope of the
plots using a linear regression analysis, are tabulated in Table
4. The complex-promoted reactions were studied at three dif-
ferent temperatures (30, 35, and 40 °C) at pH 6.95. The mag-
nitude of the rate constants (kMOH) along with the activation pa-
≠
rameters,
equation,
≠ and
(
, evaluated using the transition state
∆S
∆H
≠
≠
)
,
are
k = RT / Nh exp −∆H / RT + ∆S / R
(
)
summarised in Table 4. The complex ion-catalysed hydrolysis
of p-nitrophenyl glycinate has been studied in the pH range of
6.5 to 7.4, which is close to the physiological pH range. The
pKa1 and pKa2 values of tetraamminediaqua cobalt(Ⅲ) com-
plexes are about 5.6 and 8.1, respectively,18 and hence in the
experimental pH range the hydroxo aqua form of the complex
is the predominant species. The first step in the hydrolysis re-
action involves a substitution of the aqua group of the complex
by the amine group of the ester, resulting in the formation of

1518 Bull. Chem. Soc. Jpn., 75, No. 7 (2002)
Hydrolysis of 4-Nitrophenyl Glycinate
Table 4. Values of KM, ko and kMOH at Different Temperatures for Hydrolysis of PNPG Ester (pH = 6.95 0.05 [PNPG] = 3.0 ×
10−5 mol dm−3, I=0.1 mol dm−3
)
[Co(OH)(trien)(H2O)]2+
[Co(OH)(trien)(H2O)]2+
[Co(OH)(trien)(H2O)]2+
107[OH−]/
mol dm−3
T/K
KM
103 ko/s−1
10−4 kMOH
/
mol dm−3 s−1
9.89
KM
103 ko/s−1
10−4kMOH
/
mol dm−3 s−1
8.74
KM
103 ko/s−1
10−4 kMOH
/
mol dm−3 s−1
8.17
303.3
308.3
313.3
1.91
2.72
3.80
27
20
14
1.0 0.1
1.5 0.2
2.0 0.2
23
17
11
1.0 0.1
1.5 0.2
2.0 0.2
36
31
23
1.0 0.1
1.5 0.2
2.0 0.2
10.22
9.19
8.35
11.97
10.53
9.39
≠ kJ mol−1
51.7 3.2
31.7 6.5
22.7 3.0
−75.6 5.0
51.7 3.2
31.7 6.5
20.3 3.5
−84.3 4.5
51.7 3.2
31.7 6.5
18.2 3.0
−92.0 5.0
∆H
∆S ≠ JK−1 mol−1
Fig. 1. Variation of Rate Constants (kobs) with Complex Con-
centration for PNPG Ester at pH = 6.95 0.05 and temp
= 35.1 0.1 °C.
Eq. 1, is favoured. This results in a higher a concentration of
ML, leading to an increase in the rate.
The N-bound monodentate ester species thus formed can
undergo base hydrolysis via any one of the three possible path-
ways (Pathways 1 to 3), as shown in Scheme 1.
Fig. 2. Variation of Rate Constants (kobs) with Complex Con-
centration [Co (OH) (tren) (OH2)]2+ for PNPG Ester at pH
(temp = 35.1 0.1 °C.)
It has been demonstrated that the observed rate accelera-
tions can be correlated with the mechanism operative in the
system.2 The rate accelerations observed for a reaction pro-
ceeding via an external hydroxide ion attack on the N-bound
monodentate ester (Pathways 1) are very low (< 100), while
for reactions involving chelate ester species (Pathways 2) they
are of the order of 103–105. The intramolecular pathway (Path-
ways 3) leads to a very large rate enhancement, ~109–1011.
The second-order rate constant (kMOH), obtained for the metal-
promoted pathway for trien, tren and (en)2 complexes, are
found to be of the order of 105 (Table 4). The value of kOH for
the base hydrolysis of L is reported to be 2.3 × 102 dm3 mol−1
therefore, seems to be the most probable mechanism for the
hydrolysis of 4-nitrophenyl glycinate.
Concerning the base hydrolysis of β-cis[CoCl(trien)-
(glyOEt)]2+, an 18O tracer study has indicated that only 16% of
β-cis[CoCl(trien)(gly)]+ is formed via the intramolecular step,
and that primarily the reaction proceeds via chelate forma-
tion.32 The hydrolysis reaction in β-cis-[Co(OH) (trien) (gly
gly OR)]2+ is believed to occur via the chelated intermediate
species.33 The formation of the chelate demands the substitu-
tion of bound hydroxide. This was thought to be likely, as in a
study of β-cis[Co(OH)(trien)(NH3)]2+, where it was observed
that the exchange between coordinated hydroxide and the sol-
vent was very fast at 25 °C and a pH of 7–8.34 The chelate es-
ter species in [Co(OH)(trien) (gly gly OR)]2+ could thus be
formed via a competition between the solvent water and carbo-
nyl oxygen during the hydroxide exchange. A similar mecha-
nism leading to the formation of the chelate ester species is
s−1 24
A comparison of the values of kMOH/kOH gives ~400–
.
600. Thus, the mechanism involving the monodentate ester
species (Pathways 1) and the intramolecular pathway (Path-
ways 3) can be ruled out based on the observed rate accelera-
tions. An attack of external OH− on the chelated ester species,

A. A. Phulambrikar et al.
Bull. Chem. Soc. Jpn., 75, No. 7 (2002) 1519
17, 1088 (1978).
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7
8
R. W. Hay and P. Banerjee, J. Chem. Soc., Dalton Trans.,
1980, 2452, and references therein.
(Pathway 1)
(Pathway 2)
9
H. Chakrabarty and M. L. Rahman, Trans. Met. Chem., 18,
545 (1993).
10 H. Chakrabarty, N. Paul, and M. L. Rahman, Transition.
Met. Chem. London, 19, 524 (1994).
11 R. W. Hay and R. B. Nolan, J. Chem. Soc., Dalton, Trans.,
1974, 2452.
12 M. M. Shonkry, W. M. Hosny, and M. M. Khalil, Transi-
tion. Met. Chem., 20, 252 (1995).
13 D. E. Newlin, M.A. Pellack, and R. Nakon, J. Am. Chem.
Soc., 99, 1078 (1977).
14 N. Ahmad, M. A. Haque, and M. M. Ali, Ind. J. Chem.
Sect. A, 36, 228 (1997).
15 R. W. Hay and A. K. Basak, J. Chem. Soc., Dalton Trans.,
1982, 1819, and references therein.
16 R. W. Hay and M. P. Pujari, Inorg. Chim. Acta, 123, 175
(1986).
17 H. Chakrabarty and M. L. Rahman, Transition. Met. Chem.
London, 19, 481 (1990).
(Pathway 3)
18 R. A. Kenley, R. H. Fleming, R. M. Laine, D. S. Tse, and J.
S. Winterle, Inorg. Chem., 23, 1870 (1984).
19 J. Chin and X. Zou, J. Am.Chem. Soc., 110, 223 (1988).
20 J. Chin, M. Banaszczyk, V. Jubian and X. Zou, J. Am.
Chem. Soc., 111, 186 (1989).
21 G. H. Rawji and R. M. Milburn, Inorg. Chim. Acta, 150,
227 (1988).
22 R. W. Hay and L. Main, Aust. J. Chem., 21, 155 (1968).
23 R. W. Hay and A. K. Basak, Inorg. Chim. Acta, 123, 237
(1986).
24 R. W. Hay and A. K. Basak, J. Chem. Soc., Dalton Trans.,
1986, 39.
25 D. Ben-Ishai and A. Berger, J. Org. Chem., 17, 1564
(1952).
26 M. Krishnamurthy, J. Inorg. Nucl. Chem., 34, 3915 (1972).
27 A. M. Sargeson and G. H. Searle, Inorg. Chem., 6, 787
(1967).
Scheme 1.
likely to be operative in the cis-[Co(OH) (trien) (NH2 CH2
COOC6H4NO2)]2+complex.
The activation parameters, (
∆H
≠ and
≠ (Table 4) for the
∆S
trien, tren and (en)2 complex-promoted reactions, have been
found to be similar. This suggest that a common mechanism,
involving an attack of external OH− on the chelate ester spe-
cies, is operative in all three complex-promoted reactions. The
rate enhancements (~400–600) observed in the present study
were slightly lower in comparison to those (103–105) observed
for typical bidentate coordination esters. This could probably
be ascribed to the weaker alkoxy carbonyl interaction in the
chelate, owing to the decreased nucleophilicity of the carbonyl
oxygen in the p-nitrophenyl amino acid ester.
28 E. Kimura, S. Young, and J. P. Collman, Inorg. Chem., 9,
1183 (1970).
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