Synthesis and 13C NMR study of the triangulo-clusters [Pt3(μ-CO)3 - (n)(μ-SO2)(n)(PR3)3] (n = 0-3)

Article abstract of DOI:10.1016/S0020-1693(99)00523-X

[Pt₃(μ-CO)₃(PPh₃)₃] was prepared in high yield by the reaction of [Pt₃(μ-CO)₃(PPh₃)₄] with H₂O₂. The carbonyl ligands in [Pt₃(μ-CO)₃(PR₃)₃] (PR₃ = PPh₃, PPh₂Bz, PCy₃, P(i)Pr₃) may be completely replaced by sulfur dioxide to give [Pt₃(μ-SO₂)₃(PR₃)₃] or partially by one SO₂ to give [Pt₃(μ-CO)₂(μ-SO₂)(PR₃)₃] (PR₃ = PCy₃ and P(i)Pr₃). On the other hand, the addition of one molar equivalent of carbon monoxide to [Pt₃(μ-SO₂)₃(PR₃)₃] (PR₃ = PCy₃; P(i)Pr₃) gave the complexes [Pt₃(μ-CO)(μ-SO₂)₂(PR₃)₃]. Complete simulation of the ¹³C NMR spectra could be achieved with the help of ¹⁹⁵Pt NMR data. (C) 2000 Elsevier Science S.A.

Full text of DOI:10.1016/S0020-1693(99)00523-X

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Inorganica Chimica Acta 303 (2000) 94–99
13
Synthesis and C NMR study of the triangulo-clusters
[Pt3(m-CO)3−n(m-SO2)n(PR3)3] (n=0–3)
Renzo Ros a, Augusto Tassan a, Ga´bor Laurenczy b, Raymond Roulet b,*
a Dipartimento di Processi Chimici dell’Ingegneria e Centro di Chimica Metallorganica del CNR, Via Marzolo 9, I–35131 Padua, Italy
b Institut de Chimie Mine´rale et Analytique de l’Uni6ersite´, BCH, CH–1015 Lausanne, Switzerland
Received 15 September 1999; accepted 4 November 1999
Abstract
[Pt3(m-CO)3(PPh3)3] was prepared in high yield by the reaction of [Pt3(m-CO)3(PPh3)4] with H2O2. The carbonyl ligands in
[Pt3(m-CO)3(PR3)3] (PR3=PPh3, PPh2Bz, PCy3, PiPr3) may be completely replaced by sulfur dioxide to give [Pt3(m-SO2)3(PR3)3]
or partially by one SO2 to give [Pt3(m-CO)2(m-SO2)(PR3)3] (PR3=PCy3 and PiPr3). On the other hand, the addition of one molar
equivalent of carbon monoxide to [Pt3(m-SO2)3(PR3)3] (PR3=PCy3; PiPr3) gave the complexes [Pt3(m-CO)(m-SO2)2(PR3)3].
Complete simulation of the 13C NMR spectra could be achieved with the help of 195Pt NMR data. © 2000 Elsevier Science S.A.
All rights reserved.
Keywords: Platinum complexes; Cluster complexes; Carbonyl complexes; Sulfur dioxide complexes
1. Introduction
2. Synthesis
A recurrent structural feature in cluster chemistry of
transition metals of Group 10 and 11 with d10-electron
configuration is the trimetallic unit M3 [1]. Since the
pioneering work by Chatt and Chini [2], a large number
of platinum triangulo-clusters of general formula
[Pt3(m–X)3L3] have been reported. All have 42 valence
electrons, tertiary phosphine as terminally bonded lig-
and L, and CO [2–11], SO2 [11–17] or RCN [18,19] as
bridging ligands X.
Till now these homonuclear platinum clusters have
been studied by 31P and 195Pt NMR, besides IR spec-
troscopy and X-ray analysis. To further investigate the
effect of s-donor and p-acceptor properties of CO and
SO2 bridging ligands in this type of compounds, we
now report their experimental and calculated 13C NMR
spectra.
The reaction of [Pt3(m-CO)3(PPh3)4] [2] with H2O2 at
45°C in acetone/dichloromethane solution results in the
quantitative oxidation of one free PPh3 [22] coming
from the dissociative equilibrium
[Pt3(m-CO)3(PPh3)4]?[Pt3(m-CO)3(PPh3)3] +PPh3
(1a)
Such an equilibrium has been confirmed by 31P NMR
studies on analogous triangulo clusters of platinum [23].
Similarly, [Pt3(m-CO)3(PPh2Bz)3] (2a) [2] may be easily
obtained from the corresponding tetraphosphine
derivative.
Initial attempts of 13C-enrichment of [Pt3(m-CO)3-
(PR3)3] (PR3=PPh3 (1a), (PPh2Bz (2a), PiPr3 (4a)) by
exchange reaction under 1 atm of 13CO failed as the
trimeric nuclearity was not preserved, giving instead
tetranuclear and/or pentanuclear species [Pt4(m-
CO)5(PR3)4] and [Pt5(CO)(m-CO)5(PR3)4]. Chatt and
Chini had encountered similar reactivity of CO on
[Pt3(m-CO)3(PPh3)4] [2]. The enriched clusters [Pt3(m-
13CO)3(PR3)3] (1a*, 2a* and 4a*) may be obtained more
expensively by the appropriate reported methods [2,10]
using 99% 13C-enriched CO. Only the triplatinum clus-
ters containing bulky phosphines, such as PCy3, retain
We report also a simple and high yield synthesis of
[Pt3(m-CO)3(PPh3)3], a compound only scantily de-
scribed previously [20,21].
* Corresponding author. Tel.: +41-21-692 3861; fax: +41-21-692
3865.
0020-1693/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(99)00523-X

R. Ros et al. / Inorganica Chimica Acta 3030 (2000) 94–99
95
methods of Chatt and Chini [2] and Venanzi and
co-workers [10], respectively.
3.2. Preparation of [Pt3(CO)3(PPh3)3] (1a)
[Pt3(CO)3(PPh3)4] (10.18 g, 5.92 mmol) was stirred in
200 ml of acetone and 100 ml of dichloromethane. 15
ml of H2O2 (37%) were added at 45°C. The red solution
was stirred for 15 min at 45°C, then kept at 10°C and
quickly evaporated under reduced pressure to a volume
of ca. 130 ml, giving red microcrystals of 1a which were
filtered and recrystallized from dichloromethane/
ethanol. Yield 7.32 g (85%). M.p. 185–190°C dec. Anal.
Calc. for C57H45O3P3Pt3: C, 47.02; H, 3.11. Found: C,
46.83; H, 3.08%. IR, w(CO): Nujol, 1842m, 1784vs
Scheme 1. (i ) Excess of SO2; (ii ) 1 equiv. of SO2; (iii ) 1 equiv. of CO.
Bold numbers 1, 2, 3, 4 refer to PR3=PPh3, PPh2Bz, PCy3, PiPr3,
letters a, b, c, d refer to substitution degree of SO2 (n=0, 1, 2, 3),
respectively.
their nuclearity in solution under CO. This allows
12CO/13CO exchange already after some minutes, and
complete substitution giving [Pt3(m-13CO)3(PCy3)3] (3a*)
may be attained using a large excess of 13CO.
The reaction conditions to isolate the whole range of
CO/SO2 mixed derivatives [Pt3(m-CO)3−n(m-SO2)n-
(PR3)3] (n=0–3) are summarised in Scheme 1. Com-
pounds 3a–3d have already been reported in the
literature [5,16].
cm−1 31P and 195Pt NMR (CD2Cl2, 243 K): lP 55.0;
.
lPt −4525 ppm; 1J(PtꢀPt) 1728; 1J(PꢀPt) 4891;
3
2J(PꢀPt) 468; J(PꢀP) 54 Hz.
3.3. Preparation of [Pt3(CO)3(PPh2Bz)3] (2a)
PtCl2(1,5-cyclooctadiene) (748 mg, 2 mmol) was sus-
pended in an ethanol (95%, 60 ml) THF (40 ml) mix-
ture and then treated with diphenylbenzylphosphine
(1.1 g, 4.0 mmol). The resulting mixture was stirred for
3 h until the white PtCl2(PPh2Bz)2 complex was formed.
3. Experimental
3.1. General remarks
Table 1
Infrared data a for [Pt3(m-12CO)n(m-SO2)3−n(PR3)3] and [Pt3(m-
13CO)3−n(m-SO2)n(PR3)3] b
All reactions were performed under pure dry dinitro-
gen. Benzene, toluene and diethyl ether were refluxed
over sodium, treated with LiAlH4 and distilled and
stored over molecular sieves. All other solvents were of
reagent grade purity and were used without further
purification.
IR spectra were taken on a Perkin–Elmer 983 and a
Perkin–Elmer FT–IR 2000 spectrometers, as Nujol
mulls between CsI discs or solutions in CaF2 cells.
13C, 31P{H} and 195Pt{H} NMR spectra were
recorded on Bruker 200, 360 and 400 MHz spectrome-
ters. The reported values result from simulations
(GNMR 4.0 program) of the experimental spectra tak-
ing into account the relative populations of all isoto-
pomers. lC in ppm relative to TMS, lP relative to 85%
H3PO4 (external), lPt relative to Na2PtCl6 in D2O. The
31P and 195Pt data reported below for 2a–4a and 3d in
CD2Cl2 at 293 K are in fair to good agreement with
those reported in the literature [5,16]. 2a: lP 56.0; lPt
) )
w(CO) (cm−1 w(SO2) (cm−1
(1a) [Pt3(m-12CO)3(PPh3)3]
(1a*) [Pt3(m-13CO)3(PPh3)3]
(2a) [Pt3(m-12CO)3(PPh2Bz)3]
(2a*) [Pt3(m-13CO)3(PPh2Bz)3]
(3a) [Pt3(m-12CO)3(PCy3)3]
(3a*) [Pt3(m-13CO)3(PCy3)3]
(4a) [Pt3(m-12CO)3(PiPr3)3]
(4a*) [Pt3(m-13CO)3(PiPr3)3]
1854w, 1793vs –
1812w, 1753vs –
1853vw, 1788vs –
1810vw, 1747vs –
1831w, 1763vs –
1789w, 1723vs –
1834w, 1769vs –
1789w, 1729vs –
(3b) [Pt3(m-12CO)2(m-SO2)(PCy3)3] 1851s, 1791vs 1213m, 1069s
(3b*) [Pt3(m-13CO)2(m-SO2)(PCy3)3] 1805s, 1745vs 1209m, 1070s
(4b) [Pt3(m-12CO)2(m-SO2)(PiPr3)3] 1854s, 1791vs 1206m, 1061s
(4b*) [Pt3(m-13CO)2(m-SO2)(PiPr3)3] 1803m, 1752vs 1207m, 1067s
(3c) [Pt3(m-12CO)(m-SO2)2(PCy3)3] 1857vs
(3c*) [Pt3(m-13CO)(m-SO2)2(PCy3)3] 1816vs
(4c) [Pt3(m-12CO)(m-SO2)2(PiPr3)3] 1861vs
(4c*) [Pt3(m-13CO)(m-SO2)2(PiPr3)3] 1818vs
1235m, 1089m,
1071s
1235m, 1088m,
1070s
1224m, 1082m,
1069s
1225m, 1079m,
1068s
1
1
2
−4447 ppm; J(PtꢀPt) 1644; J(PꢀPt) 4765; J(PꢀPt)
484; 3J(PꢀP) 62 Hz. 3a: lP 71.0; lPt −4421 ppm;
1J(PtꢀPt) 1571; J(PꢀPt) 4395; J(PꢀPt) 415; J(PꢀP) 57
Hz. 4a: lP 79.4; lPt −4435 ppm; 1J(PtꢀPt) 1622;
1J(PꢀPt) 4433; 2J(PꢀPt) 418; 3J(PꢀP) 57 Hz. 3d: lP 77.4;
lPt −4024 ppm; 1J(PtꢀPt) 722; 1J(PꢀPt) 3754; 2J(PꢀPt)
1
2
3
(1d) [Pt3(m-SO2)3(PPh3)3]
–
1271s, 1258m,
1084vs
(2d) [Pt3(m-SO2)3(PPh2Bz)3]
(3d) [Pt3(m-SO2)3(PCy3)3]
(4d) [Pt3(m-SO2)3(PiPr3)3]
–
–
–
1264s, 1088vs
1242s, 1075vs
1245s, 1077vs
3
307; J(PꢀP) 48 Hz.
a w(CO) in CH2Cl2; w(SO2) in Nujol.
b Asterisks indicate compounds with 13CO content \98%.
The starting compounds [Pt3(m-CO)3(PPh3)4] and
[Pt3(m-CO)3(PCy3)3] were prepared according to the

96
R. Ros et al. / Inorganica Chimica Acta 303 (2000) 94–99
Carbon monoxide was bubbled through the suspension.
On addition of an excess of KOH (ca. 10 mmol) in H2O
(20 ml), the mixture slowly turned red and absorbed
carbon monoxide. After 2 h of stirring, the solvent was
partially removed under reduced pressure and the red–
brown solid was removed, washed with water and
ethanol and then dried in vacuo. This solid, which
appeared to be a mixture of 2a and [Pt3(CO)3(PPh2Bz)4]
(IR spectrum), was dissolved in dichloromethane and
filtered through a silica gel pad (30 cm) and eluted with
diethyl ether. The red filtrate was evaporated to dryness
in vacuo and the residue washed with methanol.
Dichloromethane (20 ml) and acetone (40 ml) were
added and the red solution was warmed at 40°C and
treated with 1.5 ml of H2O2 (37%). The solution dark-
ened and immediately was cooled at 0°C and quickly
evaporated under reduced pressure to a volume of ca.
25 ml, giving red microcrystals of 2a which were re-
crystallized from dichloromethane/methanol. Yield
0.95 g (63%). M.p. 193–198°C, dec. Anal. Calc. for
C60H51O3P3Pt3: C, 48.10; H, 3.43. Found: C, 48.32; H,
periodically bubbled through the solution as long as its
IR spectrum showed the disappearance of any w(CO)
absorption (ca. 24 h). The volume of the suspension
was reduced in vacuo to 25 ml, hexane was added and
the mixture was cooled at 0°C. The pale orange micro-
crystals were filtered off, washed with cold benzene,
hexane and dried in vacuo. Yield 0.49 g (91%). Anal.
Calc. for C54H45O6P3Pt3S3: C, 41.46; H, 2.90. Found: C,
41.04; H, 2.93%. 31P and 195Pt NMR (CDCl3, 293 K):
1
1
lP 62.4; lPt −4209 ppm; J(PtꢀPt) 594; J(PꢀPt) 4090;
3
2J(PꢀPt) 429; J(PꢀP) 53 Hz.
3.5. Preparation of [Pt3(v-SO2)3(PPh2Bz)3] (2d)
As for 1d starting with [Pt3(m-CO)3(PPh2Bz)3] (0.225
g, 0.15 mmol) in 60 ml of benzene and gaseous SO2
until disappearance of any w(CO) absorption (ca. 20 h).
Yield 0.208 g (86%) as pale orange microcrystals. Anal.
Calc. for C57H51O6P3Pt3S3: C, 42.62; H, 3.20. Found: C,
42.78; H, 3.25%. 31P and 195Pt NMR (CDCl3, 293 K):
1
1
lP 64.3; lPt −4090 ppm; J(PtꢀPt) 644; J(PꢀPt) 3891;
3
3.50%. IR, w(CO): Nujol, 1783s cm−1
.
2J(PꢀPt) 402; J(PꢀP) 53 Hz.
3.4. Preparation of [Pt3(v-SO2)3(PPh3)3] (1d)
3.6. Preparation of [Pt3(v-SO2)3(PiPr3)3] (4d)
[Pt3(m-CO)3(PPh3)3] (0.50 g, 0.343 mmol) was dis-
solved in benzene (120 ml, N2 atm) and SO2 was
SO2 was bubbled periodically through a stirred
dichloromethane solution (50 ml) of [Pt3(m-CO)3-
Table 2
Molar fractions of 195Pt and 13CO isotopomers of [Pt3(CO)3L3] for three selected (1, 50 and 98%) enrichment in 13CO, and spin systems used in
calculations (italics)

R. Ros et al. / Inorganica Chimica Acta 3030 (2000) 94–99
97
Table 3
31P and 195Pt NMR data of clusters [Pt3(m-CO)3−n(m-SO2)n(PiPr3)3] (n=0–3) in CD2Cl2 at 293 K; l in ppm; J in Hz
P2
P3
Pt1
Pt2
Pt3
Pt3(CO)3(PiPr3)3 (4a)
lP=79.4
P1
P2
P3
Pt1
Pt2
57
57
57
4433
418
418
418
4433
418
418
418
4433
1622
1622
lPt=−4435
1622
Pt3(CO)2(SO2)(PiPr3)3 (4b)
lP1=96.8
lP2=lP3=75.6
P1
P2
P3
Pt1
Pt2
51
48
49
51
61
4383
366
366
433
4081
299
433
299
4081
1827
196
lPt1=−3978
lPt2=lPt3=−4584
1827
Pt3(CO)(SO2)2(PiPr3)3 (4c)
lP1=lP2=92.2
P1
P2
P3
Pt1
Pt2
53
53
3992
371
320
371
3992
320
338
338
3901
650
650
lP3=72.3
lPt1=lPt2=−4061
lPt3=−4714
1770
Pt3(SO2)3(PiPr3)3 (4d)
lP=91.3
P1
P2
P3
Pt1
Pt2
49
49
3748
326
326
326
3748
326
705
326
326
3748
705
705
lPt=−4054
(PiPr3)3] (0.30 g, 0.261 mmol). After 2.5 days a pale
orange solution was obtained where the IR spectrum
showed the disappearance of any w(CO) absorption.
The volume of the solution was reduced in vacuo to 15
ml, hexane was added and the mixture was cooled to
0°C. The precipitate was filtered, washed with hexane
and dried in vacuo. Yield 0.239 g (73%) as orange
microcrystals. Anal. Calc. for C27H63O6P3Pt3S3: C,
25.78; H 5.05. Found: C, 25.61; H, 5.00%. 31P and 195Pt
NMR (CDCl3, 293 K): lP 91.3; lPt −4054 ppm;
dichloromethane (70 ml) at room temperature and 1
equiv. of CO (2.59 ml measured at 25°C and 1 atm) was
added with a gas syringe. After stirring for 1.5 h, the
pale orange solution was concentrated in vacuo and
n-heptane added. The yellow–orange precipitate was
recrystallized from dichloromethane/n-heptane. Yield
0.11 g (85%). Anal. Calc. for C28H63O5P3Pt3S2: C,
27.52; H, 5.20. Found: C, 27.71; H, 5.28%. 31P and
195Pt NMR data: see Table 3.
1J(PtꢀPt) 705; J(PꢀPt) 3748; J(PꢀPt) 326; J(PꢀP) 49
3.9. Preparation of 13CO enriched clusters
1
2
3
Hz.
The 13C isotopically labelled derivatives were pre-
pared similarly as above, using 13CO enriched to 99%,
or a suitable 12CO/13CO mixture for selected enrich-
ments. The IR data are collected in Table 1.
3.7. Preparation of [Pt3(v-CO)2(v-SO2)(PiPr3)3] (4b)
[Pt3(m-CO)3(PiPr3)3 (0.135 g, 0.117 mmol) was dis-
solved in dichloromethane (50 ml) at −20°C and 1
equiv. of SO2 (2.87 ml at 25°C and 1 atm) was added
with a gas syringe. After stirring for 1 h, the orange
mixture was concentrated in vacuo and n-heptane
added. The precipitate obtained was recrystallized from
dichloromethane/n-heptane. Yield 0.098 g (70%) as or-
ange microcrystals. Anal. Calc. for C29H63O4P3Pt3S: C,
29.37; H, 5.35. Found: C, 29.60; H, 5.44%. 31P and
195Pt NMR data: see Table 3.
4. Spectroscopic characterisation of the compounds
The calculation of the 13C NMR data of the clusters
[Pt3(*CO)3L3] was based on the spectra corresponding
to selected enrichments in 13CO. Up to 20 isotopomers
may contribute to a given spectrum. These arise from
the number of 195Pt nuclei (0 to 3), the number of 13C
nuclei (0 to 3), and their relative positions. Table 2
shows the relative populations of the various isoto-
pomers used in the calculations for three selected en-
richments: 1, 50, and 98% 13CO. The values of
3.8. Preparation of [Pt3(v-CO)(v-SO2)2(PiPr3)3] (4c)
Complex 4d (0.133 g, 0.106 mmol) was dissolved in

98
R. Ros et al. / Inorganica Chimica Acta 303 (2000) 94–99
Fig. 1, concerning [Pt3(*CO)3(PiPr3)3] with their selected
enrichments, and a summary of all 13C NMR data is
given in Table 4, (the J(CꢀP) coupling constants are all
smaller than 2–3 Hz).
Table 1 shows as expected a decrease of w(CO) values
of 39–46 cm−1 upon substituting 12CO by 13CO.
The IR frequencies for the series 4a, 4b and 4c move
to higher w(CO) and those of 4b, 4c and 4d move to
higher w(SO2) as the proportion of SO2 in the cluster
increases, since SO2 is a better p-acceptor than CO. The
1J(PtꢀC) values of 4a*, 4b* and 4c* (Table 4) also
increase as the proportion of SO2 in the cluster in-
creases. Consistently, the weighted average of the three
Scheme 2.
1J(PtꢀPt) were also needed for the calculations and are
reported in Section 3, or in the literature for 2a–4a [5],
and 3b–3d [16]. The results of the calculations of the
31P and the 195Pt NMR data of the clusters 4a–4c are
given in Table 3, with the numbering scheme shown in
Scheme 2.
The values of the molar fractions of the various
isotopomers and of the 1J(PtꢀPt) coupling constants
were introduced as input data in the GNMR 4.0 pro-
gram to simulate the 13C NMR spectra of the triangulo-
clusters. An example of such a simulation is shown in
1
1J(PtꢀPt) values, and the three J(PtꢀP) values (Table
3) decrease on going from 4a (1622 and 4433 Hz,
respectively), 4b (1512, 4196), 4c (1025, 3960) to 4d
(705, 3748).
The effect of the identity of the phosphine ligands on
the NMR parameters is less clear cut, since both their
basicity and cone angle vary on modifying the sub-
stituents at phosphorous. Clusters with PCy3 must be
Fig. 1. Observed (left) and simulated (right) 13C NMR spectra of [Pt3(*CO)3(PiPr3)3] with the selected enrichments: 1, 50 and 98% 13CO.

R. Ros et al. / Inorganica Chimica Acta 303 (2000) 94–99
99
Table 4
13C NMR data for [Pt3(m-13CO)3−n(m-SO2)n(PR3)3] a (n=0–2) in CD2Cl2
PR3
lC
1J(PtꢀC)
2J(PtꢀC)
2J(CꢀC)
1J(PtꢀPt)
b
(1a*)
PPh3
PPh2Bz
PCy3
PiPr3
PCy3
PiPr3
PCy3
PiPr3
245.5
251.6
259.2
256.7
238.8
237.8
220.2
219.3
723
718
692
701
752
758
796
783
44.1
45.3
55.4
50.1
59.7
59.4
75.0
76.7
22.6
22.7
22.9
23.9
22.8
23.3
–
1728
1644
1571
1622
(2a*) c
(3a*) d
(4a*) e
(3b*) e
(4b*) e
(3c*) e
(4c*) e
–
a lC in ppm; J in Hz.
b At 233 K.
c At 223 K.
d At 243 K.
e At 293 K.
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left out of the comparisons. Indeed, [Pt3(m-SO2)3(PCy3)]
for example has unusually large PtꢀPt bond distances
owing to the preponderance of steric repulsions be-
tween the cyclohexyl groups and the bridging ligands
[1b]. For the clusters bearing three m-COs, the w(CO)
values (Table 1) and 1J(PtꢀC) values (Table 4) do
indeed decrease along the sequence 1a*, 2a* and 4a*,
which corresponds to increasing basicity of the phos-
phine ligand. However, the corresponding values of
1J(PtꢀPt) and 1J(PtꢀP) (Section 3) do also decrease
along the same sequence, when they should increase
instead if increasing basicity were the sole factor.
In conclusion, the only correlation found to hold in
these triangulo-clusters is between the variations of the
1
1
1
w(CO), J(PtꢀC), J(PtꢀPt) and J(PtꢀP) values and the
variation of the p-acceptor capability of the bridging
ligands.
The reactions of these clusters with alkenes and
alkynes are currently being examined.
[14] M.F. Hallam, D.M.P. Mingos, J. Organomet. Chem. 315 (1986)
C35.
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Williams, J. Chem. Soc., Dalton Trans. (1988) 1461.
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Jeffrey, D.M.P. Mingos, J. Chem. Soc., Dalton Trans. (1990)
3335.
[17] R.A. Burrow, D.H. Farrar, J.J. Irwin, Inorg. Chim. Acta 181
(1991) 65.
Acknowledgements
[18] C.E. Briant, D.I. Gilmour, D.M.P. Mingos, R.W.M. Wardle, J.
Chem. Soc., Dalton Trans. (1985) 1693.
[19] J.L. Haggitt, D.M.P. Mingos, J. Organomet. Chem. 462 (1993)
365.
The Swiss National Science Foundation and
MURST are thanked for financial support.
[20] W. Zhai, Z. Zhao, Youji Huaxve, (1986) 134 (Chem. Abstr. 107
(1986) 115724j); A. Jiang, Q. Cong, Jiegon Huaxve, 4 (1985) 96
(Chem. Abstr. 105 (1985) 124739d).
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