Ligand exchange and substitution at platinum(II) complexes: Evidence for a dissociative mechanism

Article abstract of DOI:10.1016/S0020-1693(02)01505-0

Square-planar complexes of the type cis-[Pt(Me)₂(Me ₂SO)(PR₃)] (1-6) where PR₃ represents a series of isosteric tertiary phosphanes [P(4-MeOC₆H₄) ₃, P(4-MeC₆H₄)₃, P(C ₆H₅)_3, P(4-FC₆H₄) ₃, P(4-ClC₆H₄)₃, P(4-CF ₃C₆H₄)₃] have been synthesised and fully characterised through elemental analysis, ¹H and ³¹P{¹H} NMR. The coupling constants ¹J _PtP with the isotopically abundant ¹⁹⁵Pt (33%, I=1/2) of 1-6, as those of the pyridine cis-[Pt(Me)₂(py)(PR₃)] derivatives (7-12), show linear dependencies on the basicity of the coordinated phosphane. The rates of dimethyl sulfoxide exchange for all the complexes have been measured at relatively low temperatures by ¹H NMR isotopic labelling experiments with deuterated chloroform as the solvent. Pyridine for dimethyl sulfoxide substitution has been studied at higher temperatures through conventional spectrophotometric techniques. The rates of both processes show no dependence on ligand concentration, for each complex the value of the rate of ligand substitution is in reasonable agreement with the value of the rate of ligand exchange at the same temperature, and the kinetics are characterised by largely positive entropies of activation. There is a compensation-effect between ΔH^? and ΔS^?, i.e., a greater ΔH^? is accompanied by a larger positive ΔS^?, indicating that all complexes react via the same mechanism. The basicity of the phosphane does not affect significantly the reaction rates. The general pattern of behaviour indicates that the rate determining step for substitution is the dissociation of the sulfoxide ligand and the formation of a three-coordinated [Pt(Me)₂(PR₃)] uncharged intermediate.

Full text of DOI:10.1016/S0020-1693(02)01505-0

Inorganica Chimica Acta 350 (2003) 143ꢀ151
/
www.elsevier.com/locate/ica
Ligand exchange and substitution at platinum(II) complexes:
evidence for a dissociative mechanism
Raffaello Romeo a,*, Luigi Monsu` Scolaro a,b, Maria Rosaria Plutino a,
Fabrizia Fabrizi de Biani c, Giovanni Bottari a, Andrea Romeo a,b
a Dipartimento di Chimica Inorganica, Chimica Analitica e Chimica Fisica, Universita` di Messina, ISMN-CNR, Sezione di Messina,
Salita Sperone 31, Vill. S. Agata, 98166 Messina, Italy
b Istituto Nazionale di Fisica della Materia, INFM, Unita` di Messina, Salita Sperone 31, Vill. S. Agata, 98166 Messina, Italy
c Dipartimento di Chimica, Universita` di Siena, Via Aldo Moro, 53100 Siena, Italy
Received 6 June 2002; accepted 25 July 2002
Dedicated in honor of Professor Pierre Braunstein
Abstract
Square-planar complexes of the type cis-[Pt(Me)2(Me2SO)(PR3)] (1ꢀ
phosphanes [P(4-MeOC6H4)3, P(4-MeC6H4)3, P(C6H5)3, P(4-FC6H4)3, P(4-ClC6H4)3, P(4-CF3C6H4)3] have been synthesised and
/6) where PR3 represents a series of isosteric tertiary
fully characterised through elemental analysis, 1H and 31P{1H} NMR. The coupling constants 1JPtP with the isotopically abundant
195Pt (33%, Iꢁ
basicity of the coordinated phosphane. The rates of dimethyl sulfoxide exchange for all the complexes have been measured at
/
1/2) of 1ꢀ
/
6, as those of the pyridine cis-[Pt(Me)2(py)(PR3)] derivatives (7ꢀ12), show linear dependencies on the
/
1
relatively low temperatures by H NMR isotopic labelling experiments with deuterated chloroform as the solvent. Pyridine for
dimethyl sulfoxide substitution has been studied at higher temperatures through conventional spectrophotometric techniques. The
rates of both processes show no dependence on ligand concentration, for each complex the value of the rate of ligand substitution is
in reasonable agreement with the value of the rate of ligand exchange at the same temperature, and the kinetics are characterised by
largely positive entropies of activation. There is a compensation-effect between DH% and DS%, i.e., a greater DH% is accompanied by a
larger positive DS%, indicating that all complexes react via the same mechanism. The basicity of the phosphane does not affect
significantly the reaction rates. The general pattern of behaviour indicates that the rate determining step for substitution is the
dissociation of the sulfoxide ligand and the formation of a three-coordinated [Pt(Me)2(PR3)] uncharged intermediate.
# 2003 Elsevier Science B.V. All rights reserved.
Keywords: Platinum(II) complexes; Kinetics; Dissociative mechanism; Phosphane effects
1. Introduction
refer to thermal decomposition of alkyl compounds [6ꢀ
9], insertion of olefins into the MꢀH bond [10,11],
reductive elimination [12], geometrical isomerisation
[13,14], electrophilic attack at the metalꢀcarbon bond
[15ꢀ18], exchange reactions [19], fluxional motions of
coordinated ligands [20ꢀ23]. On considering the ease
with which a square-planar complex adds a fifth ligand
to form five-coordinate species, the possibility of
dissociative processes for ligand exchange or substitu-
tion reactions has been for a long time rejected by the
scientific community [24].
/
/
The potential catalytic activity of square planar
complexes of d8 metals is related to the possibility of
dissociating a coordinated ligand. The 14-electron
species thus formed, in which the metal is coordinatively
and electronically unsaturated, is prone to accept and to
/
/
/
activate molecular fragments [1ꢀ4]. Recently, ligand
/
dissociation has been recognised as the initial step of
many fundamental processes involving square-planar
organometallic complexes [5]. The most recent studies
The first convincing evidence for the presence of
three-coordinate 14-electron species as key intermedi-
ates in nucleophilic substitution reactions comes from
* Corresponding author. Tel./fax: ꢂ
E-mail address: raf.romeo@chem.unime.it (R. Romeo).
/
39-090-39 3756.
0020-1693/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0020-1693(02)01505-0

144
R. Romeo et al. / Inorganica Chimica Acta 350 (2003) 143ꢀ151
/
kinetic studies of complexes having a set of donor atoms
of the type cis-[Pt(C,C)(S,S)] (C a strong s donor
carbon group; S, thioether or sulfoxide) [25]. Factors
which concur in promoting dissociation are (i) high
electron density at the metal which prevents the
approach of the axially incoming nucleophile; (ii) bond
weakening at the leaving group due to the trans
influence of strong s-donors (in the present cases, the
cooling the filtrate to 0 8C. Their purity was checked by
the measurement of the 31P{1H} NMR spectra and the
crystals were stored under oxygen-free N2. All the other
reagents were of the highest commercial grade available
and were used as received, or were purified by distilla-
tion or recrystallisation when necessary. Dimethyl
sulfoxide was purified by liquid chromatography on
˚
alumina under argon and stored over 4 A molecular
sieves. Other solvents used were purified and dried by
standard techniques. Chloroform for use in kinetic runs
organic carbanions which form strong Ptꢀ
/
C s-bonds);
and (iii) the stabilisation by the remaining set of three in-
plane ligands of a three-coordinated 14-electron inter-
mediate without changing the singlet ground state.
These favourable factors do not apply in the presence
of ligands which exhibit major p-acceptor capabilities.
In the complex cis-[PtPh2(CO)(SEt2)], the relief of the
excess of electron density at the metal by the carbon
monoxide and the presence of a low-lying LUMO, p-
was degassed by means of a series of ‘freezingꢀpumping’
/
cycles and eventually stored in Schlenk tubes. Deuter-
ated chloroform and dimethyl sulfoxide were used as
received from Aldrich Chemical Co. Microanalyses were
performed by Redox Analytical Laboratories, Milan.
2.1.1. cis-(Dimethyl)(dimethylsulfoxide)(tris(4-
methoxyphenyl)phosphane)platinum(II), cis-
[PtMe2(Me2SO)(P(4-MeOC6H4)3)] (1)
A solution of P(4-MeOC6H4)3 (105.7 mg, 0.3 mmol)
in CH2Cl2 (10 ml) was added very slowly dropwise to a
solution of cis-[PtMe2(Me2SO)2] (114.4 mg, 0.3 mmol)
in CH2Cl2 (30 ml). The reaction mixture was left aside
for 1 h, then concentrated under vacuum and the
complex (145 mg, 74% yield) separated out as solid
delocalised over the Ptꢀ
/
Cꢀ/O atoms, facilitates a
changeover of mechanism from dissociative to associa-
tive [26]. Thus, the cis ligands play also a crucial role for
the electronic properties of the metal centre and, in
principle, a fine tuning of the reaction mechanism is
possible playing with the s-donor and p-acceptor
capabilities of the ligands. Also, it has become evident
that not necessarily the substrate must carry one or
more Ptꢀ
/
C bonds to dissociate. In fact, an appropriate
upon adding petroleum ether (v/vꢁ1/1). Anal. Calc. for
/
set of strong s-donor groups can favour sufficient
accumulation of electron density at the metal and
stabilise the 14-e intermediate. This is the case for the
complex cis-[Pt(SiMePh2)2(PMe2Ph)2], which shows a
partial tetrahedral distortion [27].
C25H33O4PSPt: C, 45.80; H, 5.07. Found: C, 46.1; H,
1
4.98%. IR: n(Sꢁ
/
O) 1100 cmꢃ1. H NMR (CDCl3): d
4
7.53 (m, JPH
3
9.9 Hz, JHH
ꢁ
/
ꢁ8.6 Hz, 6H, H2,6), 6.90
/
(d, 3JHH
ꢁ8.6 Hz, 6H, H3,5), 3.81 (s, 9H, OCH3), 2.76 (s,
/
3JPtH
2JPtH
2JPtH
ꢁ
/
12.8 Hz, 6H, Sꢀ
68.2 Hz, 3H, Ptꢀ
79.0 Hz, 3H, Ptꢀ
CH3).31P{1H} NMR (CDCl3):
1923 Hz).
All the other phosphane complexes (2ꢀ
/
CH3), 0.57 (m, JPH
ꢁ
ꢁ
/
7.5 Hz,
9.3 Hz,
3
3
We report here on ligand exchange and substitution
kinetics of the complexes cis-[Pt(Me)2(Me2SO)(PR3)],
where PR3 represents a series of isosteric tertiary
phosphanes. The goal was to check whether the s-
donor capability of the phosphanes, contrary to carbon
monoxide, is able to promote a dissociative mechanism.
An additional objective was to check at what extent the
basicity of the phosphanes controls the reaction rates.
We can anticipate that the presence of a phosphane does
not induce a changeover of the dissociative mechanism
but rather causes an acceleration of the dissociation rate
with respect to that of cis-[Pt(Me)2(Me2SO)2] [28].
ꢁ
/
/
CH3), 0.47 (m, JPH
/
ꢁ
/
/
d 24.9 (1JPtP
ꢁ
/
/6) were pre-
pared following a synthetic procedure similar to that
described above.
2.1.2. cis-(Dimethyl)(dimethylsulfoxide)(tris(4-
methylphenyl)phosphane)platinum(II), cis-
[PtMe2(Me2SO)(P(4-MeC6H4)3)] (2)
Anal. Calc. for C25H33OPSPt: C, 49.42; H, 5.47.
1
Found: C, 49.74; H, 5.54%. IR: n(Sꢁ
/
O) 1093 cmꢃ1. H
4
NMR (CDCl3): d 7.50 (m, JPH
3
9.9 Hz, JHH
ꢁ
/
ꢁ7.4
/
Hz, 6H, H2,6), 7.18 (d, 3JHH
ꢁ
/
7.4 Hz, 6H, H3,5), 2.76 (s,
CH3), 2.36 (s, 9H, CH3), 0.57
70.4 Hz, 3H, PtꢀCH3), 0.45
CH3).
1915 Hz).
2. Experimental
3JPtH
ꢁ
/
12.8 Hz, 6H, Sꢀ
/
3
(m, JPH
2
6.7 Hz, JPtH
ꢁ
ꢁ
/
ꢁ
/
/
3
(m, JPH
2
2.1. Materials
/
9.3 Hz, JPtH
ꢁ
/
80.1 Hz, 3H, Ptꢀ
/
31P{1H} NMR (CDCl3): d 26.5 (1JPtP
ꢁ
/
K2PtCl4 (Strem Chemical Co.) was purified from
metallic Pt and K2PtCl6 by dissolving it in water and
filtering. trans-[Pt(CH3)2(Me2SO)2] was prepared ac-
cording to a published method [29], and was crystallised
several times from diethyl ether. The phosphorus ligands
were purchased from Strem and were crystallised from
ethanol by dissolving in the hot solvent, filtering, and
2.1.3. cis-
(Dimethyl)(dimethylsulfoxide)(triphenylphosphane)-
platinum(II), cis-[PtMe2(Me2SO)(P(C6H5)3)] (3)
Anal. Calc. for C22H27OPSPt: C, 46.72; H, 4.81.
1
Found: C, 46.46; H, 4.98%. IR: n(Sꢁ
/
O) 1095 cmꢃ1. H
NMR (CDCl3): d 7.66ꢀ7.60 (m, 6H, H2,6), 7.40ꢀ
/
/7.37

R. Romeo et al. / Inorganica Chimica Acta 350 (2003) 143ꢀ
/151
145
3
(m, 9H, H4ꢂ
CH3), 0.57 (m, JPH
/
H3,5), 2.77 (s, JPtH
ꢁ
/
13.2 Hz, 6H, Sꢀ
6.7 Hz, JPtH 70.7 Hz, 3H,
78.7 Hz, 3H,
CH3). 31P{1H} NMR (CDCl3): d 28.7 (1JPtP
1904
/
2.1.8. cis-(Dimethyl)(pyridine)(tris(4-
methylphenyl)phosphane)platinum(II), cis-
[PtMe2(py)(P(4-MeC6H4)3)] (8)
3
2
ꢁ
ꢁ
/
ꢁ
ꢁ
/
3
2
Ptꢀ
/
CH3), 0.46 (m, JPH
/
9.1 Hz, JPtH
/
3
Ptꢀ
/
ꢁ
/
1H NMR (CDCl3): d 8.31 (m, JPtH
ꢁ
/
22.0 Hz,
3
Hz).
3JHH
1H, H4 ꢀ
3JHH
7.1 Hz, 6H, H3,5), 6.88 (m, JHH
2H, H3,5
py), 2.31 (s, 9H, CH3), 0.60 (s, 3JPH
2JPtH
84.1 Hz, 3H, Ptꢀ
ꢁ
/
6.6 Hz, 2H, H2,6
ꢀ
/
py), 7.41 (m, JHH
ꢁ7.8 Hz,
/
/
py), 7.32 (m, 3JHH
ꢁ7.1 Hz, 6H, H2,6), 7.02 (m,
/
3
ꢁ
/
ꢁ
/
6.6, 7.8 Hz,
8.2 Hz,
7.7 Hz,
CH3). 31P{1H} NMR (CDCl3):
1957 Hz).
2.1.4. cis-(Dimethyl)(dimethylsulfoxide)(tris(4-
fluorophenyl)phosphane)platinum(II), cis-
[PtMe2(Me2SO)(P(4-FC6H4)3)] (4)
ꢀ
/
ꢁ
ꢁ
/
3
ꢁ
/
/
CH3), 0.52 (s, JPH
/
2JPtH
ꢁ
/
68.2 Hz, 3H, Ptꢀ
/
Anal. Calc. for C22H24F3OPSPt: C, 42.65; H, 3.90.
1
d 27.3 (1JPtP
ꢁ
/
Found: C, 43.89; H, 3.93%. IR: n(Sꢁ
/
O) 1093 cmꢃ1. H
NMR (CDCl3): d 7.58 (m, 6H, H2,6), 7.11 (m, 6H, H3,5),
3
2.83 (s, JPtH
3
ꢁ
ꢁ
ꢁ
/
13.9 Hz, 6H, Sꢀ
69.9 Hz, 3H, Ptꢀ
78.7 Hz, 3H, Ptꢀ
1898 Hz).
/CH3), 0.52 (m, JPH
ꢁ
/
2.1.9. cis-
(Dimethyl)(pyridine)(triphenylphosphane)platinum(II),
cis-[PtMe2(py)(P(C6H5)3)] (9)
2
6.6 Hz, JPtH
3
/
/
CH3), 0.43 (m, JPH
ꢁ
/
2
9.8 Hz, JPtH
(CDCl3): d 27.3 (1JPtP
/
/
CH3). 31P{1H} NMR
3
ꢁ
/
1H NMR (CDCl3): d 8.31 (m, JPtH
ꢁ20.9 Hz,
/
3JHH
pyꢂH2,6), 7.20 (m, 9H, H3,5
7.3 Hz, 2H, H3,5 py), 0.60 (s, JPH
83.8 Hz, 3H, Ptꢀ
69.4 Hz, 3H, Ptꢀ
CH3). 31P{1H} NMR (CDCl3): d 29.3
(1JPtP
1941 Hz).
ꢁ
/
5.4 Hz, 2H, H2,6
ꢀ
/
py), 7.48ꢀ
/
7.37 (m, 7H, H4 ꢀ
5.4,
7.7 Hz, JPtH
/
/
ꢂ
/
H4), 6.88 (m, 3JHH
ꢁ
/
2.1.5. cis-(Dimethyl)(dimethylsulfoxide)(tris(4-
chlorophenyl)phosphane)platinum(II), cis-
[PtMe2(Me2SO)(P(4-ClC6H4)3)] (5)
3
2
ꢀ
/
ꢁ
/
ꢁ
/
3
2
/
CH3), 0.55 (s, JPH
ꢁ
/
7.9 Hz, JPtH
ꢁ
/
/
Anal. Calc. for C22H24Cl3OPSPt: C, 39.50; H, 3.62.
1
O) 1083 cmꢃ1. H
ꢁ
/
Found: C, 39.46; H, 3.73%. IR: n(Sꢁ
/
3
NMR (CDCl3): d 7.50 (m, JPH
3
9.8 Hz, JHH
8.4 Hz, 6H, H3,5), 2.84
ꢁ
/
ꢁ8.4
/
3
Hz, 6H, H2,6), 7.36 (m, JHH
ꢁ
/
2.1.10. cis-(Dimethyl)(pyridine)(tris(4-
fluorophenyl)phosphane)platinum(II), cis-
[PtMe2(py)(P(4-FC6H5)3] (10)
3
3
(s, JPtH
ꢁ
ꢁ
ꢁ
/
14.2 Hz, 6H, Sꢀ
69.5 Hz, 3H, Ptꢀ
77.9 Hz, 3H, Ptꢀ
(CDCl3): d 28.1 (1JPtP
1881 Hz).
/
CH3), 0.52 (m, JPH
ꢁ
ꢁ
/
6.9
9.4
2
3
Hz, JPtH
Hz, JPtH
/
/
CH3), 0.43 (m, JPH
/
2
3
/
/
CH3). 31P{1H} NMR
1H NMR (CDCl3): d 8.31 (m, JPtH
ꢁ
/
21.7 Hz,
3
ꢁ
/
3JHH
ꢁ
/
5.9 Hz, 2H, H2,6
ꢀ
/
py), 7.48 (m, JHH
ꢁ
/
7.5 Hz,
py), 7.41 (m, 6H, H3,5), 6.97 (m, 6H, H2,6), 6.88
1H, H4 ꢀ
/
3
3
py), 0.56 (m, JPH
(m, Javꢁ
2JPtH
84.2 Hz, 3H, Ptꢀ
2JPtH
69.5 Hz, 3H, Ptꢀ
d 27.2 (1JPtP
1940 Hz).
/7 Hz, 2H, H3,5
ꢀ
/
ꢁ
ꢁ
/
8.0 Hz,
8.0 Hz,
2.1.6. cis-(Dimethyl)(dimethylsulfoxide)(tris(4-
trifluoromethylphenyl)phosphane)platinum(II), cis-
[PtMe2(Me2SO)(P(4-CF3C6H4)3] (6)
3
CH3), 0.55 (s, JPH
ꢁ
ꢁ
/
/
/
/
/
CH3). 31P{1H} NMR (CDCl3):
ꢁ
/
Anal. Calc. for C25H24F9OPSPt: C, 39.02; H, 3.14.
1
Found: C, 38.43; H, 3.23%. IR: n(Sꢁ
NMR (CDCl3): d 7.80ꢀ7.60 (m, 12H, H2,6
(s, 3JPtH CH3), 0.56 (m, 3JPH
14.3 Hz, 6H, Sꢀ
2JPtH
70.4 Hz, 3H, PtꢀCH3), 0.45 (m, JPH
2JPtH CH3). 31P{1H} NMR (CDCl3):
78.1 Hz, 3H, Ptꢀ
d 30.4 (1JPtP
1847 Hz).
/
O) 1060 cmꢃ1. H
/
ꢂ
/
H3,5), 2.88
6.6 Hz,
9.3 Hz,
2.1.11. cis-(Dimethyl)(pyridine)(tris(4-
chlorophenyl)phosphane)platinum(II), cis-
[PtMe2(py)(P(4-ClC6H4)3)] (11)
ꢁ
/
/
ꢁ
ꢁ
/
3
ꢁ
/
/
/
3
ꢁ
/
/
1H NMR (CDCl3): d 8.29 (m, JPtH
ꢁ
/
21.2 Hz,
7.8 Hz,
3
ꢁ
/
3JHH
1H, H4 ꢀ
3JHH
5H, H3,5
3H, Ptꢀ
ꢁ
/
6.4 Hz, 2H, H2,6
ꢀ
/
py), 7.50 (m, JHH
ꢁ
/
/
py), 7.34 (m, 3JHH
ꢁ8.5 Hz, 6H, H2,6), 7.23 (m,
/
3
ꢁ
/
8.5 Hz, 6H, H3,5), 6.97 (m, JHH
ꢁ
/
6.4, 7.8 Hz,
7.8 Hz, JPtH 69.9 Hz,
83.2 Hz,
2.1.7. cis-(Dimethyl)(pyridine)(tris(4-
methoxyphenyl)phosphane)platinum(II), cis-
[PtMe2(py)(P(4-MeOC6H4)3)] (7)
Prepared ‘in situ’ by adding a slight excess of pyridine
to 0.5 ml of a chloroform-d solution of complex 1 in a
3
py), 0.57 (m, JPH
2
ꢀ
/
ꢁ
/
ꢁ
ꢁ
/
3
2
/
CH3), 0.55 (m, JPH
ꢁ
/
7.9 Hz, JPtH
CH3). 31P{1H} NMR (CDCl3): d 27.7 (1JPtP
/
3H, Ptꢀ
/
ꢁ
/
1916 Hz).
1
3
NMR tube. H NMR (CDCl3): d 8.33 (d, JPtH
ꢁ
ꢁ
/
20.9
7.6
9.1 Hz, 6H, H2,6),
6.0, 7.6 Hz, 2H, H3,5 py), 6.74 (m,
9.1 Hz, 6H, H3,5), 3.80 (s, 9H, OCH3), 0.60 (s,
3
3
Hz, JHH
ꢁ
/
6.0 Hz, 2H, H2,6
ꢀ
/
py), 7.43 (m, JHH
/
2.1.12. cis-(Dimethyl)(pyridine)(tris(4-
trifluoromethylphenyl)phosphane)platinum(II), cis-
3
Hz, 1H, H4 ꢀ
/
py), 7.35 (m, JHH
6.90 (m, JHH
3JHH
3JPH
3JPH
ꢁ
/
3
ꢁ
/
ꢀ
/
[PtMe2(py)(P(4-CF3C6H4)3] (6)
3
ꢁ
/
1H NMR (CDCl3): d 8.30 (m, JPtH
ꢁ20.3 Hz,
/
2
ꢁ/8.2 Hz, JPtH
ꢁ
/
84.1 Hz, 3H, Ptꢀ
68.2 Hz, 3H, Ptꢀ
CH3). 31P{1H}
1970 Hz).
All the other pyridine derivatives (8ꢀ
pared following the procedure described above.
/
CH3), 0.52 (s,
3JHH
H4ꢀpyꢂ
py), 0.64 (s, JPH
ꢁ
/
6.0 Hz, 2H, H2,6
ꢀpy), 7.7ꢀ
/
/
7.4 (m, 13H, H3,5
6.0, 7.7 Hz, 2H, H3,5
8.3 Hz, JPtH
70.4 Hz, 3H, Ptꢀ
83.6 Hz, 3H, Ptꢀ
CH3). 31P{1H} NMR (CDCl3): d 29.5 (1JPtP
1884 Hz).
ꢂ
/
2
3
ꢁ
/
7.7 Hz, JPtH
ꢁ
/
/
/
/
H2,6), 7.29 (m, JHH
ꢁ
/
ꢀ
/
NMR (CDCl3): d 25.7 (1JPtP
ꢁ
/
ꢁ
/
ꢁ
/
/
3
2
3
2
/
12) were pre-
CH3), 0.59 (s, JPH
ꢁ/8.3 Hz, JPtH
ꢁ
/
/
ꢁ
/

146
R. Romeo et al. / Inorganica Chimica Acta 350 (2003) 143ꢀ151
/
2.2. Instruments
must be carried out with extreme care to avoid local
excess of reagent and the easy formation of disubstituted
cis-[Pt(Me)2(PR3)2] products.
1H and 31P{1H} NMR spectra were obtained in
CDCl3 solution on a Bruker AMX R-300 spectrometer
equipped with a broad-band probe operating at 300.13
The reaction products were isolated as pure solid
compounds and fully characterised by elemental analy-
sis and spectrometric techniques. Selected 1H and
1
and 121.49 MHz, respectively. H chemical shifts were
1
31P{1H} NMR data are collected in Table 1. The H
measured relative to the residual solvent peak and are
reported in d units downfield from Me4Si. 31P{1H}
chemical shifts, in parts per million, are given relative to
external phosphoric acid. The temperature within the
probe was checked using the methanol or ethylene glycol
method [30]. Slow spectrophotometric kinetics were
carried out with a rapid-scanning Hewlettꢀ
Model 8452 A spectrophotometer or, at a selected
wavelength, with a PerkinꢀElmer Lambda 3 spectro-
NMR spectra contain at low fields the expected
resonances of the phosphane phenyls, an average
resonance at dꢁ
coupling constant JPtH
/
2.819
/
0.05 due to Me2SO with a
13.590.5 Hz with the iso-
topically abundant 195Pt (33%, Iꢁ
1/2), and two multi-
plets for the coordinated methyl groups with average
values at dꢁ0.559 7091 Hz) and at dꢁ
0.02 (2JPtH
0.459 78.890.8 Hz), respectively. The two
0.02 (2JPtH
3
ꢁ
/
/
/
/
Packard
/
/
ꢁ
/
/
/
/
/
ꢁ
/
/
photometer, interfaced with a microcomputer for data
collection and equipped with a cell compartment
methyl groups can be unambiguously identified on the
basis of the different trans influence of PR3 and Me2SO,
and the magnitude of values observed for the methyl
thermostated by a Perkinꢀ
programmer (PTP). The temperature accuracy was
0.05 8C. Stopped-flow kinetic runs were recorded on
/
Elmer Peltier temperature
2JPtH coupling constants. The lowest value of 2JPtH
Hz (at dꢁ0.55) belongs to the methyl group trans to the
strong s-donor PR3, while, for the methyl group trans
ꢁ70
/
9
/
/
the HP8452A spectrophotometer equipped with a Hi-
Tech SFA-20 Rapid Kinetic Accessory. Rate constants
were evaluated using the SCIENTIST software package
[31].
2
to Me2SO, the coupling constant increases to JPtH
78.8 Hz (at dꢁ0.45). Additional methyl signals of the
para-substituents groups are observed for compounds 1
and 2. The 31P resonance has an average value at dꢁ
2892 but the coupling constants show a significant
ꢁ
/
/
/
/
3. Results
dependence on the nature of the coordinated phos-
phanes.
3.1. 1H NMR characterisation
The reactions of 1ꢀ
/
6 with pyridine to yield the
The synthesis of the phosphane compounds 1ꢀ
according to reaction (1)
/
6,
compounds 7ꢀ12 were carried out ‘in situ’ in an NMR
/
tube. On changing pyridine for Me2SO the average
value of the chemical shift of the trans methyl increases
cis[Pt(Me)2(Me2SO)2]ꢂPR3
0 cis[Pt(Me)2(Me2SO)(PR3)]ꢂMe2SO
to dꢁ
/
0.599
/
0.02 (2JPtH
ꢁ
/
83.890.4 Hz) while that of
/
(1)
Table 1
Selected H and 31P{1H} NMR data for complexes of the type cis-[PtMe2(Me2SO)(PR3)] (1ꢀ
1
/
6), and for the relative pyridine cis-[PtMe2(py)(PR3)]
d(31P)
derivatives (7ꢀ
12) a
/
R
n
d(1H)
CH3ꢀ
b
/
Pt
CH3ꢀ
/
Pt c
Others
4-MeOC6H4
4-MeC6H4
C6H5
1
0.47(79)
0.45(80)
0.46(79)
0.43(79)
0.43(78)
0.45(78)
0.60(84)
0.60(84)
0.60(84)
0.56(84)
0.55(83)
0.59(84)
0.57(68)
0.57(70)
0.57(71)
0.52(70)
0.52(70)
0.56(70)
0.52(68)
0.52(68)
0.55(69)
0.55(70)
0.57(70)
0.64(70)
2.76(13, CH3ꢀ
2.76(13, CH3ꢀ
2.77(13, CH3ꢀ
2.83(14, CH3ꢀ
2.84(14, CH3ꢀ
2.88(14, CH3ꢀ
/
S)
S)
S)
S)
S)
S)
24.9(1923)
26.5(1915)
28.7(1904)
27.3(1898)
28.1(1881)
30.4(1847)
25.7(1970)
27.3(1957)
29.3(1941)
27.2(1940)
27.7(1916)
29.5(1884)
2
/
3
/
4-FC6H4
4-ClC6H4
4-CF3C6H4
4-MeOC6H4
4-MeC6H4
C6H5
4
/
5
/
6
/
7
8.33(21, H2,6
8.31(22, H2,6
8.31(21, H2,6
8.31(22, H2,6
8.29(21, H2,6
8.30(20, H2,6
ꢀ
/
py)
py)
py)
py)
py)
py)
8
ꢀ
/
9
ꢀ
/
4-FC6H4
4-ClC6H4
4-CF3C6H4
10
11
12
ꢀ
/
ꢀ
/
ꢀ
/
a
Recorded in CDCl3 as solvent at 298.2 K. Chemical shifts (d) in ppm relative to TMS; 195Pt coupling constants, given in parentheses together
with the assignment, are in Hz.
b
CH3 trans to Me2SO or pyridine.
CH3 trans to phosphane ligand.
c

R. Romeo et al. / Inorganica Chimica Acta 350 (2003) 143ꢀ
/151
147
the methyl trans to PR3 remains substantially un-
2
0.04, JPtH
Ffꢁ
/
c1ꢂ
/
c2 exp(ꢃkexcht), with c1, c2 and kexch as the
/
changed (dꢁ
/
0.569
/
ꢁ
/
69.09
/
0.9 Hz).
parameters to be optimised. A similar analysis was
performed on the molar fraction of the nondeuterated
bound dimethyl sulfoxide. From the McKay equation,
3.2. Kinetics
Rexch
ꢁ
/
kexchab(aꢂ
b)ꢃ1 (where Rexch is the rate of the
exchange process, a the concentration of complex, and b
/
the concentration of free sulfoxide) the pseudo-first-
order rate constants, kobs
their values are collected in Table S1 in Section 6. The
exchange rate constants were independent of the added
dimethyl sulfoxide concentration.
3.2.1. Isotopic exchange
The rates of sulfoxide exchange on cis-[PtMe2(Me2-
SO)(PR3)] (1ꢀ6) were obtained by isotopic labelling, at a
low temperature range (253ꢀ283 K). A known volume
of dimethyl sulfoxide-d6 was added with a microsyringe
into a 5 mm NMR tube containing a prethermostated
solution of complex in chloroform-d. At least a seven-
fold excess of (CD3)2SO over complex was ensured in
any run. The rate of exchange (Fig. 1, upper plot) was
monitored through the increase in intensity of the signal
at d 2.63 ppm relative to the free Me2SO and the
matching decrease of the signal at 2.76 ppm for the
ꢁRexch/a, were calculated and
/
/
/
3.2.2. Spectrophotometric kinetics
The substitution reactions of Me2SO from cis-
[PtMe2(Me2SO)(PR3)] (1ꢀ/6) by pyridine (py), according
to Eq. (2):
cis[PtMe2(Me2SO)(PR3)]ꢂpy
0 cis[PtMe2(py)(PR3)]ꢂMe2SO
bound Me2SO. The molar fraction of free Me2SO, Ffꢁ
/
(2)
[Me2SO]f/([Me2SO]fꢂ[Me2SO]b), was obtained by inte-
/
gration of both signals. The exchange of the label
follows a first-order rate law (Fig. 1, lower plot) and
the first-order rate constant, kexch (sꢃ1), was obtained
from the nonlinear regression analysis of the equation
were carried out in the temperature range of 308ꢀ
/
280 K.
Slow kinetics were carried out in a double-set silica cell
located in the thermostated cell compartment of the
spectrophotometer and monitored by repetitive scan-
ning of the spectrum at suitable times, using chloroform
as the solvent. The reactions, carried out in the presence
of at least a tenfold excess of pyridine. There was no
indication of the presence of side reactions or of
significant amounts of any intermediate species and
the conversion of the substrate into the product is seen
as a single step. All reactions obeyed a first-order
rate law until well over 90% conversion. Fastest
reactions required the use of stopped-flow spectro-
photometry under pseudo-first-order conditions. Rate
constants kobs (sꢃ1) were obtained from non-linear
least-squares fits of the experimental data to Atꢁ
/
Aꢀꢂ(A0ꢃAꢀ)exp(ꢃkobst), with A0, Aꢀ, and kobs as
/
/
/
the parameters to be optimised (A0, absorbance after
mixing of reagents; Aꢀ, absorbance at completion of
reaction). The pseudo-first-order rate constants kobs for
reaction (2) are reported in Table S1 inSection 6 for a
range of temperature and py concentrations. The
exchange rate constants were independent of the added
pyridine concentration. The variable-temperature rate
constants, taken together with those for isotopic label-
ling (Section 3.2.1) were fitted to the Eyring equation
and the derived activation parameters are reported in
Table 2.
Fig. 1. 300-MHz 1H NMR spectra of a 3.67 mm cis-[PtMe2(Me2-
SO)P(4-MeC6H4)3] and 250 mm (CD3)2SO solution in deuterated
chloroform at 273 K as a function of time. Upper plot: coordinated
4. Discussion
Organometallic complexes of the type cis-
[PtMe2(Me2SO)(PR3)] (1ꢀ
reaction of cis-[Pt(Me)2(Me2SO)2] and phosphanes in an
3
Me2SO (dꢁ/2.76 ppm, JPtH
ꢁ/
12.8 Hz); free Me2SO (dꢁ2.63 ppm).
/
/
6) can be prepared from the
Lower plot: Exponential curves for the change with time of the molar
fraction of coordinated and free dimethylsulfoxide.

148
R. Romeo et al. / Inorganica Chimica Acta 350 (2003) 143ꢀ151
/
Table 2
Ligand properties of phosphorus(III) compounds, spectroscopic
properties, rate constants and activation parameters for ligand
electronic and aryl contributions of the phosphane
ligands, by use of multiparameter equations. In the
present case, since the phosphanes are isosteric and
contain the same number of aryl rings, the values of u
and Ear are constant for all the phosphanes, and
therefore, the only variable of interest is x, the s
donicity of the ligand. The electronic profiles seen in
Fig. 2 were constructed by linear regression analysis of
the simple equation:
exchange and substitution reactions of the complexes (1ꢀ
/6)
a
b
c
d
n
x
1JPtP
1JPtP
k1
DH% e
DS% f
DG% g
1
2
3
4
5
6
10.5
11.5
1923
1915
1970
1957
1941
1940
1916
1884
0.0519 91.09
0.0588 89.59
0.0616 87.09
0.0415 85.09
0.0327 84.49
0.0275 89.09
/
0.3
0.7
0.8
0.7
0.9
0.3
369
329
249
149
109
249
/
3
80.3
80.0
79.8
80.8
81.4
81.8
/
/2
13.25 1904
/
/3
15.7
16.8
20.5
1898
1881
1847
/
/2
/
/3
/
/1
1JPtP ꢁvꢂa(x)
(3)
a
b
c
Values in cmꢃ1 taken from ref. [32].
1JPtP coupling constants in Hz relative to compounds 1ꢀ
1JPtP coupling constants in Hz relative to compounds 7ꢀ
12.
Dissociation rate constants (sꢃ1) for dimethylsulfoxide exchange
/
6.
and show that the value of the Ptꢀ
for 1ꢀ6 and 7ꢀ
better electron donors (smaller x). The values of
a (ꢃ7.290.8 Hz cm for 1ꢀ6, and ꢃ8.190.9 Hz cm
for 7ꢀ12), which represent the sensitivity of JPtP to the
/P coupling constant
/
d
/
/12 is enhanced as the phosphanes become
and substitution at 298 K.
e
Enthalpies of activation (kJ molꢃ1) from variable-temperature
/
/
/
/
/
1
kinetics.
f
/
Entropies of activation (J Kꢃ1 molꢃ1).
Gibbs free energies of activation (kJ molꢃ1) at 298 K.
inductive effects brought about by substituents on
phosphorous, are comparable to those of other com-
g
plexes having a Cꢀ
/
Ptꢀ
/
P bond axis (aꢁ
/
ꢃ
/
6.899
/
0.2 Hz
1:1 ratio. If the synthetic procedure is carried out with
the necessary precautions to avoid the fast and easy
removal of both molecules of sulfoxide to yield bis-
substituted phosphane products [13], the reactions go to
completion and the desired products are obtained in
high yield and purity.
cm for cis-[PtMe2(PR3)2] and aꢁ
/
ꢃ
/
8.29
/0.9 Hz cm for
1
cis-[PtPh2(CO)(PR3)]). Coupling constants JPtP of
closely related planar platinum(II) complexes can be
regarded as a good diagnostic probe of the extent of the
covalence of the Ptꢀ
correspond to longer Ptꢀ
/
P bond [38]. Smaller values of 1JPtP
P bond distances and to higher
/
trans influences of groups in trans position to the bond
[39]. Steric effects play also a significant role [40]. Steric
repulsion prevents maximum overlap of phosphorus and
platinum bonding orbitals and changes the s character
1
4.1. Stereoelectronic analysis of JPtP coupling constants
1
The values of JPtP coupling constants of the cis-
[PtMe2(Me2SO)(PR3)] (1ꢀ
(7ꢀ12) complexes in Table 2 are in the range typical of a
phosphorus atom trans to carbon in platinum(II)
compounds [13]. They encompass a range of 76ꢀ86 Hz
/
6) and cis-[PtMe2(py)(PR3)]
present in the metalꢀ
/
phosphorus bond. In the QALE
/
analysis this is characterised by the onset of a steric
1
threshold in the plot of JPtP versus the cone angle and
/
by a sharp change in the direction of steric effects. For
the complexes cis-[PtMe2(PR3)2] [13], the steric thresh-
1
old observed in the steric profile of the JPtP coupling
constants, corresponds to a value of Tolman’s cone
and appear to be dependent on the nature of the
coordinated phosphane. Usually the equations and the
protocol to perform quantitative (QALE) analysis of a
physicochemical property, as suggested by Giering et al.
[32], imply the knowledge of electronic parameters such
as x (an infrared parameter) [33] which measures the s
donicity of the ligand (the electron-donor ability de-
creases as x increases), or Ear (the aryl-effect parameter)
[34] which depends on the number of pendent aryl
groups on the phosphane and of u Tolman’s cone angle
[35], that measures the steric requirements of the ligand.
Since the response of the analysed property to x is
assumed to be linear over the entire range of ligands,
while the response to the steric parameter u is not linear,
the possibility of the onset of a steric threshold (ust)
must be taken into consideration. In previous studies we
1
have performed a complete QALE analysis on the JPtP
values of the complexes trans-[Pt(PR3)2(Me)(MeOH)]ꢂ
[13], cis-[PtPh2(CO)(PR3)] [36], [Pt(Nꢀ
/
Nꢀ
/
C)(PR3)]ꢂ-
[37],
Fig. 2. Electronic profiles, showing the dependence of the coupling
1
constants JPtP upon the electronic parameter x for cis-[PtMe2(Me2-
(NꢀNꢀCHꢁ6-(1-methylbenzyl)-2,2?-bipyridine)
/
/
/
and cis-[PtMe2(PR3)2] [13]. The values of the coupling
constants (1JPtP) were resolved quantitatively into steric,
SO)(PR3)] (plot A) and for cis-[PtMe2(py)(PR3)] (plot B). Numbers
refer to the phosphane ligands as listed in Table 2.

R. Romeo et al. / Inorganica Chimica Acta 350 (2003) 143ꢀ
/151
149
angle (1498) above which the PAꢀ
/
Ptꢀ
/
PB angle and the
the identity of the rate of dissociation with the rate of
ligand exchange; and (iii) the sign and the magnitude of
the entropy of activation always positive. The solvents
used were insufficiently coordinating to give an associa-
tively activated contribution. The pattern of behaviour
is strictly similar to that found in our previous studies on
cis-[PtMe2(Me2SO)2] [28] and other dialkyl and diaryl
systems [25], where the easy dissociation of thioethers or
sulfoxides was shown to be the rate determining step of
the substitution. The same dissociative mechanism
(Scheme 1) applies which involves, (i) dissociative loss
of Me2SO from the substrate (k1 path) to yield a three-
coordinated 14-electron intermediate; (ii) competition
for it between the reentry of Me2SO (via kꢃ1), and the
attack of a ligand (either *Me2SO or py, via k2) to yield
the observed products. The rate law is given by the
equation:
PtꢀP bond separation of the two phosphane ligands
/
suddenly increase, as a result of a strong steric conges-
tion at the square coordination plane. Accordingly, the
relative stability of the dimethyl-bisphosphane platinu-
m(II) complexes was found by Nolan et al. [41] to be
strongly influenced by the size of the phosphane, with
larger cone angles resulting in less thermodinamically
stable complexes.
4.2. Analysis of the kinetic data
The rates of both reactions on cis-[PtMe2(Me2-
SO)(PR3)] (1ꢀ6) (sulfoxide exchange and pyridine for
/
sulfoxide substitution (eq.2)) are not affected by the
concentration of the added ligand. The temperature
dependency of the substitution reactions (CHCl3 as the
solvent) was determined at a higher range than that of
the isotopic labelling (CDCl3 as the solvent). From the
overall data set it is possible to construct a unique
Eyring plot, as that shown in Fig. 3, which encompasses
a rather wide range of temperatures. From similar plots
values of the activation enthalpies DH% and of the
activation entropies DS% were derived. Interestingly,
dissociation rate constants (sꢃ1) for dimethylsulfoxide
exchange and substitution have identical values at the
same temperature. The values of the rate constants k1 at
various temperatures and ligand concentrations are
collected in Table S1 in Section 6. Values of the unique
rate constants k1 at 298 K and of the activation
parameters DH%, DS%, and DG% are reported in Table 2.
The experimental kinetic evidence points unequivo-
cally to a dissociative mode of activation for the
kobs ꢁk1[Nu]=f(kꢃ1=k2)[Me2SO]ꢂ[Nu]g
that for k2ꢁkꢃ1 reduces to kobs k1.
The circumstance that complexes 1ꢀ
(4)
/
ꢁ
/
/
6 react with a
dissociative mechanism, like to the analogous di-alkyl
[28] or di-aryl [25] bis-sulfoxideplatinum(II) complexes,
suggests that PR3 for Me2SO substitution does not
imply a changeover of mechanism as found on changing
CO for Me2SO. This contrasting behaviour stems from
the enormous difference of s-donor and p-acceptor
abilities of PR3 and CO ligands. The complexes cis-
[PtMe2(Me2SO)2] [28] [k1ꢁ
DH%ꢁ 1 kJ molꢃ1, DS%ꢁ
869
DG%ꢁ
[PtPh2(Me2SO)2] [42] [k1ꢁ
DH%ꢁ 1 kJ molꢃ1, DS%ꢁ
649
DG%ꢁ
/
(1.189
/
0.1)ꢄ
/
10ꢃ2 sꢃ1
,
,
/
/
/
ꢃ
/
39
/
2 J Kꢃ1 molꢃ1
/
84 kJ molꢃ1 in benzene at 303 K] and cis-
/
(1.409
/
0.2)ꢄ
/
10ꢃ2 sꢃ1
,
,
/
/
/
ꢃ
/
669
/
8 J Kꢃ1 molꢃ1
/
84 kJ molꢃ1 in benzene at 303 K] show compar-
compounds examined (1ꢀ6). The assessment of a
/
able reactivity and free energy of activation, while cis-
[PtMe2(Me2SO)(PPh3)] [k1ꢁ(6.16)ꢄ
10ꢃ2 sꢃ1, DH%ꢁ
87.09 249
0.8 kJ molꢃ1, DS%ꢁ 3 J Kꢃ1 molꢃ1, DG%ꢁ
dissociative mechanism can be made essentially on the
basis of: (i) the independence of the dissociation rate on
the nature and concentration of the entering group; (ii)
/
/
/
/
/
/
/
79.8 kJ molꢃ1 in CHCl3 at 298 K] appears to be
somewhat more inclined to dissociation. It must be
concluded that it is sufficient to transmit a reasonable
excess of electronic density into the metal by ‘non
innocent’ ligands to induce dissociation. Overflow of
electron density at the metal has the effect of (i)
destabilising the ground state; (ii) decreasing the elec-
trophilic ability of the substrate; (iii) preventing the
Fig. 3. Eyring plot constructed by use of the rate data of both isotopic
labelling experiments (empty squares) and pyridine for Me2SO
substitution reactions (filled squares) on the complex cis-[PtMe2(Me2-
SO)P(4-ClC6H4)3].
Scheme 1.

150
R. Romeo et al. / Inorganica Chimica Acta 350 (2003) 143ꢀ151
/
approach of the lone pair of a nucleophile to the metal;
(iv) favouring the complete loss of the leaving group
with its lone pair and the formation of a three-
coordinate intermediate in which the total concentration
of charge density around the metal is largely decreased.
Strong s-donation from the h1-carbanions and from the
coordinated phosphane confer sufficient thermody-
namic stability to the T-shaped intermediates formed
6. Supplementary material
Table S1 giving primary kinetic data (pseudo-first-
order rate constants, kobsd (sꢃ1), for isotopic labelling
experiments and for nucleophilic substitution on com-
pounds 1ꢀ6) as a function of temperature and ligand
/
concentration. This material is available from the
authors on request.
from dissociation of 1ꢀ
/
6.
The stereoelectronic analysis of the coupling con-
1
stants JPtP for 1ꢀ
/6 has shown that there is an efficient
Acknowledgements
flow of charge density along the Pꢀ
/
PtꢀC bond axis on
/
changing the basicity (x values) of the coordinated
phosphane. This electronic flow hardly affects the
individual rates of the various complexes (the average
The authors wish to thank the Ministero dell’Univer-
sita` e della Ricerca Scientifica e Tecnologica (MURST),
Programmi di Ricerca Scientifica di Rilevante Interesse
value of the free activation energy being DG%avꢁ
/
80.79
0.8 kJ molꢃ1), even if linear correlations with x are
observed for the activation enthalpy [DH%ꢁ
(10291)ꢃ
(1.09 (809
0.1)x] and for the activation entropy [DS%ꢁ
1)ꢃ(4.1790.07)x], considering rogue point the data for
/
Nazionale, Cofinanziamento 2000ꢀ2002, the Universita`
/
degli Studi di Messina and CNR for funding this work.
/
/
/
/
/
/
/
/
References
complex 6. Thus, on increasing the phosphane basicity
both DH% and DS% increase and DG% remains substan-
tially constant. This result for a cis-effect is not
surprising once we have seen that the change of the
nature of the coordinated cis-group (from Me2SO to
PR3) has had so little effect on the reactivity. The
correlations observed for DH% and DS% with x imply a
linear correlation between DH% and DS% [isokinetic plot,
[1] J.P. Collman, L.S. Hegedus, J.R. Norton, R.C. Finke, Principle
and Applications of Organotransition Metal Chemistry, Univer-
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[2] R.H. Crabtree, The Organometallic Chemistry of the Transition
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[3] G.W. Parshall, Homogeneous Catalysis, Wiley-Interscience, New
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[4] A. Yamamoto, Organotransition Metal Chemistry, Wiley, New
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DH%ꢁ
/
(81.79
/
0.8)ꢂ
/
(0.259
0.03)DS%]. The application
/
of the Linert method [43] to check whether we are really
dealing with an isokinetic plot and with an isokinetic
temperature, or simply with a ‘compensation effect’ has
revealed that this latter is the case. The compensation
suggests, as expected, that all complexes react with the
same mechanism.
[5] R. Romeo, Comm. Inorg. Chem. 11 (1990) 21.
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/
Carbon Bond, vol. 2, Wiley, New York, 1985, Chapter 8, and
Refs. [8,9].
[8] G.M. Whitesides, Pure Appl. Chem. 53 (1981) 287.
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A complex having a set of donor atoms of the type
cis-[Pt(C,C)(S)(P)] (Cꢁ
strong s-donor h1-carbanions;
/
S, sulfoxide; P, tertiary phosphane) behaves like cis-
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ligand exchange and substitution is well established.
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acceptor (carbon monoxide) the change from Me2SO to
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