Structure - Reactivity correlations for the dissociative uncatalyzed isomerization of monoalkylbis(phosphine)platinum(II) solvento complexes

Article abstract of DOI:10.1021/ic970210l

Complexes of the type a's-[PtL₂Me₂] (1-14) (L = an extended series of phosphines of widely different steric and electronic properties) were synthesized, characterized, and used as precursors for the formation of cis-monoalkylplatinum(II) solvente species in methanol. The cleavage of the first platinum-alkyl bond by protonolysis is a fast process, but the subsequent cis to trans isomerization of the cationic solvento species [PtL₂- (Me)(MeOH)]⁺ is relatively slow and it can be monitored using ³¹P NMR or conventional spectrophotometry. A large collection of ¹H and ³¹P NMR data for cis-[PtL₂Me₂], cis-[PtL₂Me(MeOH)]⁺, and trans-[PtL₂Me(MeOH)]⁺ complexes showed interesting dependencies upon the size, the α-donor capacity, and the mutual position of the phosphines in the coordination sphere of the metal. The rate constants for isomerization of cis-[PtL₂Me(MeOH)]⁺ were resolved quantitatively into steric and electronic contributions of the phosphine ligands, by means of correlations with parameters which reflect their σ-donor ability (Χ values) and steric requirements (Tolman's cone angles, θ). The electronic and steric profiles obtained for these reactions are discussed within the framework of a mechanism which involves dissociative loss of the solvent molecule and interconversion of two geometrically distinct 3-coordinate T-shaped 14-electron intermediates. The factors controlling the stability of these coordinatively unsaturated species are discussed. The electronic and steric influences of phosphines as spectator ligands in a dissociative process are compared with those shown by these ligands when used as nucleophiles in associative substitution processes. The activation parameters ΔH^? and ΔS^? were measured using both conventional isothermal and non-isothermal spectrophotometric kinetics.

Full text of DOI:10.1021/ic970210l

4822
Inorg. Chem. 1997, 36, 4822-4830
Structure-Reactivity Correlations for the Dissociative Uncatalyzed Isomerization of
Monoalkylbis(phosphine)platinum(II) Solvento Complexes
Raffaello Romeo* and Giuseppe Alibrandi
Dipartimento di Chimica Inorganica, Chimica Analitica e Chimica Fisica and Istituto di Chimica e
Tecnologia dei Prodotti Naturali (ICTPN-CNR), Sezione di Messina, Universita` di Messina,
Salita Sperone 31, Vill. S. Agata, 98166 Messina, Italy
ReceiVed February 21, 1997X
Complexes of the type cis-[PtL2Me2] (1-14) (L ) an extended series of phosphines of widely different steric
and electronic properties) were synthesized, characterized, and used as precursors for the formation of
cis-monoalkylplatinum(II) solvento species in methanol. The cleavage of the first platinum-alkyl bond by
protonolysis is a fast process, but the subsequent cis to trans isomerization of the cationic solvento species [PtL2-
(Me)(MeOH)]+ is relatively slow and it can be monitored using 31P NMR or conventional spectrophotometry. A
large collection of 1H and 31P NMR data for cis-[PtL2Me2], cis-[PtL2Me(MeOH)]+, and trans-[PtL2Me(MeOH)]+
complexes showed interesting dependencies upon the size, the σ-donor capacity, and the mutual position of the
phosphines in the coordination sphere of the metal. The rate constants for isomerization of cis-[PtL2Me(MeOH)]+
were resolved quantitatively into steric and electronic contributions of the phosphine ligands, by means of
correlations with parameters which reflect their σ-donor ability (ø values) and steric requirements (Tolman’s
cone angles, θ). The electronic and steric profiles obtained for these reactions are discussed within the framework
of a mechanism which involves dissociative loss of the solvent molecule and interconversion of two geometrically
distinct 3-coordinate T-shaped 14-electron intermediates. The factors controlling the stability of these coordinatively
unsaturated species are discussed. The electronic and steric influences of phosphines as “spectator” ligands in a
dissociative process are compared with those shown by these ligands when used as nucleophiles in associative
substitution processes. The activation parameters ∆Hq and ∆Sq were measured using both conventional isothermal
and non-isothermal spectrophotometric kinetics.
Introduction
was drawn to the uncatalyzed cis to trans isomerization of
complexes of the type [Pt(PEt3)2(R)X] (PEt3 ) triethylphos-
phine, R ) alkyls or substituted aryl groups; X ) halide ions).3
The main features of the reaction mechanism involve dissocia-
tive loss of the coordinated halide group and interconversion
of two geometrically distinct 3-coordinate T-shaped cationic
intermediates. In protic solvents, whose role is crucial in
promoting the dissociation of the halide ion from the metal,4
the process is somewhat complicated by a possible solvolytic
pre-equilibrium between the halide species and a cis-[PtL2(R)S]+
solvento species. In a recent report Yamamoto5 has claimed
that “no cationic organoplatinum solvento complexes with
monodentate tertiary phosphines that have cis geometry have
been reported so far”. We discovered that these elusive
compounds can be formed “in situ” easily by protonolysis of
precursor dialkyls or mixed alkyl-aryl platinum complexes and
that the reason for their instability in solution is an easy
spontaneous conversion into the trans isomers.6 The process
is extremely simple and clean, it can be monitored by a variety
of spectroscopic techniques, and proceeds through the same
dissociative mechanism mentioned above. The trans solvento
products have been known for many years, and their reactivity
The course of many reactions of square planar complexes of
d8 transition metals, such as nucleophilic substitution, electron
transfer, oxidative addition, reductive elimination, thermal
decomposition, and interaction with molecules of biological
interest, is dictated by the geometry in the square planar
configuration.1 Despite the great chemical interest in under-
standing the way in which such species undergo geometrical
isomerization, very few mechanistic studies have been devoted
to date to this subject.2 During the last few years our interest
X Abstract published in AdVance ACS Abstracts, August 15, 1997.
(1) (a) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. C. Principles
and Applications of Organotransition Metal Chemistry; University
Science Books: Mill Valley, CA, 1987; Chapter 6. (b) Yamamoto,
A. Organotransition Metal Chemistry; Wiley: New York, 1986;
Chapter 6. (c) Crabtree, R. H. The Organometallic Chemistry of the
Transition Metals; Wiley: New York, 1994. (d) James, B. R. In
ComprehensiVe Organometallic Chemistry; Wilkinson, G., Stone, F.
G. A., Abel, E. W., Eds.; Pergamon Press: Oxford, U.K., 1982;
Chapter 51. (e) Parshall, G. W. Homogeneous Catalysis; Wiley-
Interscience: New York, 1980; Chapter 3. (f) Wilkins, R. G. Kinetics
and Mechanisms of Reactions of Transition Metal Complexes; VCH:
Weinheim, Germany, 1991. (g) Tobe, M. L. In ComprehensiVe
Coordination Chemistry; Wilkinson, G., Ed.; Pergamon Press: Oxford,
U.K., 1987; Vol. 1, pp 311-329. (h) Cross, R. J. In The Chemistry of
the Metal-Carbon Bond; Hartley, F. R., Patai, S., Eds.; John Wiley:
New York, 1985; Vol. 2, Chapter 8. (i) Whitesides, G. M. Pure Appl.
Chem. 1981, 53, 287. (l) Ryabov, A. D. Chem. ReV. 1990, 90, 403.
(m) Shilov, A. E. ActiVation of Saturated Hydrocarbons by Transition
Metal Complexes; D. Reidel: Hingham, MA, 1984. (n) Lippard, S. J.
Pure Appl. Chem. 1987, 59, 731. (o) Reedijk, J. Pure Appl. Chem.
1987, 59, 181. (p) Sherman, S. E.; Lippard, S. J. Chem. ReV. 1987,
87, 1153. (q) Nicolini, M., Ed. Platinum and Other Metal Coordination
Compounds in Cancer Chemiotherapy; Martinus Nijhoff Publishing:
Boston, MA, 1988.
(3) (a) Romeo, R.; Minniti, D.; Trozzi, M. Inorg. Chem. 1976, 15, 1134.
(b) Romeo, R.; Minniti, D.; Lanza, S. Inorg. Chem. 1979, 18, 2362.
(c) Romeo, R. Inorg. Chem. 1978, 17, 2040. (d) Faraone, G.; Ricevuto,
V.; Romeo, R.; Trozzi, M. J. Chem. Soc. A 1971, 1877. (e) Alibrandi,
G.; Monsu` Scolaro, L.; Romeo, R. Inorg. Chem. 1991, 30, 4007.
(4) (a) Romeo, R.; Minniti, D.; Lanza, S. Inorg. Chem. 1980, 19, 3663.
(b) Blandamer, M. J.; Burgess, J.; Romeo, R. Inorg. Chim. Acta 1982,
65, L179. (c) Blandamer, M. J.; Burgess, J.; Minniti, D.; Romeo, R.
Inorg. Chim. Acta 1985, 96, 129.
(5) Yamamoto, A. J. Organomet. Chem. 1995, 500, 337.
(6) Alibrandi, G.; Minniti, D.; Monsu` Scolaro, L.; Romeo, R. Inorg. Chem.
1988, 27, 318.
(2) Anderson, G. K.; Cross, R. J. Chem. Soc. ReV. 1980, 9, 185.
S0020-1669(97)00210-3 CCC: $14.00 © 1997 American Chemical Society

Isomerization of Platinum(II) Solvento Complexes
Inorganic Chemistry, Vol. 36, No. 21, 1997 4823
were distilled prior to use. The remaining phosphines were used as
received from Strem. All the other chemicals were the highest grade
commercially available and were used as received or purified by
distillation or crystallization when needed.
Instrumentation. 1H and 31P NMR spectra were obtained on a
Bruker AMX R-300 spectrometer equipped with a broad-band probe
operating at 300.13 and 121.49 MHz, respectively. 1H chemical shifts
are measured relative to the residual solvent peak and are reported in
δ units downfield from Me4Si. 31P chemical shifts, in parts per million,
are relative to external phosphoric acid. The temperature within the
probe was checked using the methanol or ethylene glycol method.12
Microanalysis were performed by Redox Analytical Laboratories, Milan,
Italy.
Synthesis of Complexes. All the complexes were synthesized under
a dry oxygen-free nitrogen atmosphere using standard Schlenk-tube
techniques. The reaction products were handled under nitrogen in the
refrigerator. Elemental analyses were consistent with the theoretical
formulas. 1H and 31P NMR spectra were taken in a 3:1 (v:v) CD2Cl2/
CD3OD mixture at 253 K.
cis-[Pt(Me2SO)2(CH3)2] was prepared according to a published
method13 and was crystallized several times from a 1:1 dichloromethane/
diethyl ether mixture.
has been investigated in detail in reactions of olefin insertion
or of â-hydrogen elimination.7
There are many interesting features in these processes: (i)
Dissociative pathways in platinum(II) chemistry are rare.8 (ii)
We are far from a complete understanding of the factors that
promote the formation and stabilization of 3-coordinate 14-
electron species, as well as of the efficiency with which they
can interconvert, as in uncatalyzed isomerization, undergo
intramolecular processes, as in some â-hydride elimination
reactions,9 or be intercepted in solution by the solvent, nucleo-
philes, or other chemical species. (iii) These coordinatively
unsaturated species offer favorable low-energy reaction routes
to catalytic processes as an alternative to 4- and 5-coordinate
species. (iv) Our understanding of substituent effects of
phosphine ligands is still small, despite their crucial role in
coordination and organometallic chemistry.10
In this study, we were interested in searching for a correlation
between the lability of bis(phosphine) monoalkyl solvento
complexes of cis geometry and the nature of the coordinated
phosphines. Thus, a series of known and new complexes of
the type cis-[PtL2Me2] (L ) an extended series of phosphines
of widely different steric and electronic properties) were
synthesized and used as precursors for the formation of cis-
[PtL2(R)(MeOH)]+ in methanol. The rates of the ensuing
uncatalyzed isomerization were measured at various tempera-
tures. The results are discussed within the framework of a
dissociative mechanism. The application of QALE (quantitative
analysis of ligand effects)11 to the rate data provided a means
of ascertaining the relative importance of electronic and steric
properties of the “spectator” ligands in governing the lability
of the substrates. A clear structure-reactivity correlation was
obtained. The way in which the size and the electron-releasing
ability of the substituents on the phosphorus atoms influence
the spectroscopic properties of the dialkyl and solvento com-
plexes complexes is also discussed.
Dialkyl Substrates. Some of the complexes cis-[PtL2(CH3)2] are
well-known and were prepared with a variety of synthetic procedures,
L (reference number): PEt3 (14); PPh3 (14); PMe3 (15); PMe2Ph (16);
PMePh2 (16); P(4-MeC6H4)3 (17). The known compounds and the new
ones (L ) P(Prn)3, P(Pri)3, P(4-ClC6H4)3, P(3-ClC6H4)3, P(4-MeOC6H4)3,
P(2-MeOC6H4)3, P(3-MeC6H4)3) were prepared by using essentially the
following general procedure. A weighed amount of cis-[Pt(Me2SO)2-
(Me)2] was reacted in degassed dichloromethane with the stoichiometric
amount of phosphine. In the case of the least reactive phosphines, the
reaction mixture was set aside overnight. After evaporation of most
of the solvent, the complex separated out as oil or solid on adding
light petroleum (bp 60-80 °C) and cooling. The residue was
crystallized from a suitable solvent. The identity of known complexes
was checked by their NMR spectra. The identity and purity of the
new compounds were established by elemental analysis and by 1H and
31P NMR.
cis-[Pt(PMe3)2(Me)2] (1): crystallized from light petroleum. 1H
2
3
NMR: δ 1.44 (d, JPH ) 8.1 Hz, JPtH ) 19.8 Hz, 18H, PCH3), 0.37
Experimental Section
(m, JPtH ) 65.5 Hz, 6H, PtCH3). 31PNMR: δ -23.1 (1JPtP ) 1761
2
Materials. Solvents used in the synthetic procedures were distilled
under nitrogen from appropriate drying agents (diethyl ether from
sodium benzophenone; dichloromethane from barium oxide; methanol
from magnesium methoxide; dimethyl sulfoxide, at a low pressure, from
CaH2, after preliminary filtration through an alumina column) and then
stored in dried, N2-filled flasks over activated 4 Å molecular sieves.
Methanol for use in kinetic runs was obtained by purification of
spectrophotometric grade methanol (Aldrich). Deuterated solvents for
NMR measurements were used as received from Aldrich Chemical Co.
Solid phosphines were recrystallyzed from EtOH, by dissolving in
the hot solvent, filtering, and cooling the filtrate to 0 °C. The crystals
were stored under N2. The phosphines PEt3 and PMePh2 (Aldrich)
Hz).
cis-[Pt(PMe2Ph)2(Me)2] (2): crystallized from methanol. 1H
NMR: δ 7.5-7.3 (m, 10H, PPh); 1.47 (d, 2JPH ) 7.3 Hz, 3JPtH ) 21.3
Hz, 12H, PCH3), 0.44 (m, 2JPtH ) 66.9 Hz, 6H, PtCH3). 31P NMR: δ
-10.0 (1JPtP ) 1794 Hz).
cis-[Pt(PEt3)2(Me)2] (3): crystallized first from light petroleum and
then from methanol. 1H NMR: δ 1.75 (m, 12H, PCH2-), 1.05 (m,
2
18H, PCCH3), 0.30 (m, JPtH ) 64.7 Hz, 6H, PtCH3). 31P NMR: δ
9.3 (1JPtP ) 1843 Hz).
cis-[Pt(P(Prn)3)2(Me)2] (4): oil, crystallized several times from
acetone/light petroleum and finally from acetone. 1H NMR: δ 1.75
(m, 12H, PCH2-), 1.45 (m, 12H, PCCH2-), 1.01 (t, 3JHH ) 7.3, 18H,
2
PCCCH3), 0.31 (m, JPtH ) 64.7 Hz, 6H, PtCH3). 31P NMR: δ -0.1
(7) Romeo, R.; Alibrandi, G.; Monsu` Scolaro, L. Inorg. Chem. 1993, 32,
4688 and references therein.
(8) Romeo, R. Comments Inorg. Chem. 1990, 11, 21.
(1JPtP ) 1828 Hz).
cis-[Pt(PMePh2)2(Me)2] (5): crystallized from acetone/light petro-
leum. 1H NMR: δ 7.41-7.27 (m, 20 H, PPh), 1.63 (d, 2JPH ) 6.6 Hz,
(9) (a) Alibrandi, G.; Monsu` Scolaro, L.; Minniti, D.; Romeo, R. Inorg.
Chem. 1990, 29, 3467. (b) Alibrandi, G.; Cusumano, M.; Minniti, D.;
Monsu` Scolaro, L.; Romeo, R. Inorg. Chem. 1989, 28, 342.
(10) (a) McAuliffe, C. A.; Lavason, W. Phosphine, Arsine and Stibine
Complexes of Transition Elements; Elsevier: New York, 1979. (b)
Pignolet, L. H. Homogeneous Catalysis with Metal Phosphine
Complexes; Plenum Press: New York, 1983. (c) McAuliffe, C. A.
Phosphorus, Arsenic, Antimony and Bismuth Ligands. In Compre-
hensiVe Coordination Chemistry; Wilkinson, G., Ed.; Pergamon
Press: New York, 1987, Vol. 2, Chapter 14, pp 989-1066.
(11) (a) Lorsbach, B. A.; Bennett, D. M.; Prock, A.; Giering, W. P.
Organometallics 1995, 14, 869. (b) Lorsbach, B. A.; Prock, A.;
Giering, W. P. Organometallics 1995, 14, 1694. (c) Chen, L.; Poe¨,
A. J. Coord. Chem. ReV. 1995, 143, 265. (d) Hudson, H. E.; Poe¨, A.
J. Organometallics 1995, 14, 3238. (e) Farrar, D. H.; Poe¨, A. J.; Zhang,
Y. J. Am. Chem. Soc. 1994, 116, 6252. (f) Dahlinger, K.; Falcone, F.;
Poe¨, A. J. Inorg. Chem. 1986, 25, 2654. (g) Poe¨, A. J. Pure Appl.
Chem. 1988, 60, 1209.
3JPtH ) 20.6 Hz, 6H, PCH3), 0.29 (m, 2JPtH ) 68.4 Hz, 6H, PtCH3). 31
NMR: δ 7.6 (1JPtP ) 1851 Hz).
P
cis-[Pt(PPh3)2(Me)2] (6): crystallized from benzene. 1H NMR: δ
2
7.4-7.1 (m, 30 H, PPh), 0.29 (m, JPtH ) 69.9 Hz, 6H, PtCH3). 31P
NMR: δ 28.0 (1JPtP ) 1910 Hz).
cis-[Pt(P(4-MeC6H4)3)2(Me)2] (7): crystallized from dichloromethane/
(12) (a) Van Geet, A. L. Anal. Chem. 1968, 40, 2227. (b) Van Geet, A. L.
Anal. Chem. 1970, 42, 679.
(13) Eaborn, C.; Kundu, K.; Pidcock, A. J. J. Chem. Soc., Dalton Trans.
1981, 933.
(14) Chatt, J.; Shaw, B. L. J. Chem. Soc. 1959, 705.
(15) Chatt, J.; Shaw, B. L. J. Chem. Soc. 1959, 4020.
(16) Ruddick, J. D.; Shaw, B. L. J. Chem. Soc. (A) 1969, 2801.
(17) Clark, H. C.; Manzer, L. E. J. Organomet. Chem. 1973, 59, 411.

4824 Inorganic Chemistry, Vol. 36, No. 21, 1997
Romeo and Alibrandi
and the Eulero method21 for solving the differential equation
light petroleum. 1H NMR: δ 7.3-7.0 (m, 24H, PPh), 2.33 (s, 18H,
CCH3), 0.24 (m, 2JPtH ) 67.8 Hz, 6H, PtCH3). 31P NMR: δ 26.0 (1JPtP
) 1924 Hz).
d(Dt - D∞)
dt
-
)
cis-[Pt(P(4-ClC6H4)3)2(Me)2] (8): crystallized from dichloromethane/
methanol. 1H NMR: δ 7.3-7.2 (m, 24H, PPh), 0.35 (m, 2JPtH ) 69.8
Hz, 6H, PtCH3). 31P NMR: δ 26.7 (1JPtP ) 1887 Hz).
cis-[Pt(P(4-MeOC6H4)3)2(Me)2] (9): crystallized from dichlo-
romethane/ methanol. 1H NMR: δ 7.3-7.0 (m, 24H, PPh), 3.79 (s,
∆Sq
∆Hq
R(T0 + Rt)
k(T0 + Rt)
exp
exp -
(Dt - D∞) (1)
[
]
[
]
h
R
where R is the temperature gradient and D∞, ∆Hq, and ∆Sq are the
parameters to be optimized. For a detailed description of the VTK
method see ref 18 and the Supporting Information, including Figure
SI1.
2
18H, O-CH3), 0.27 (m, JPtH ) 68.4 Hz, 6H, PtCH3). 31P NMR: δ
24.7 (1JPtP )1930 Hz).
cis-[Pt(P(Pri)3)2(Me)2] (10): crystallized from light petroleum/
acetone. 1H NMR: δ 2.43 (m, 6H, PCH-), 1.22 (m, 36H, PCCH3),
2
0.34 (m, JPtH ) 66.0 Hz, 6H, PtCH3). 31P NMR: δ 29.8 (1JPtP
)
Results
1866 Hz).
cis-[Pt(P(3-MeC6H4)3)2(Me)2] (11): crystallized from dichlo-
Organometallic complexes of the type cis-[PtL2(Me)2] (L )
PEt3, PMe3, PPh3) can be prepared starting from cis-[PtL2Cl2]
and methylating agents14-16 or using other synthetic procedures
such as the reaction of [Pt(COD)(Me)2] with phosphines (PPh3
and P(4-MeC6H4)3),17,22 the reaction of [Pt2Me4(µ-SMe2)2] with
phosphines (PPh3, PEt3, PMe3, PMe2Ph, PMePh2),23 and,
eventually, the reaction of cis-[Pt(Me2SO)2(Me)2] with phos-
phines (PPh3).13 In our hands, this latter method proved to be
very useful, and therefore, an extended series of phosphine
complexes was synthesized, according to eq 2
romethane/ methanol. 1H NMR: δ 7.3-7.1 (m, 24H, PPh), 2.15 (s,
2
18H, CCH3), 0.27 (m, JPtH ) 69.1 Hz, 6H, PtCH3). 31P NMR: δ
28.1 (1JPtP ) 1923 Hz).
cis-[Pt(P(3-ClC6H4)3)2(Me)2] (12): crystallized from dichloromethane/
methanol. 1H NMR: δ 7.4-7.1 (m, 24H, PPh), 0.39 (m, 2JPtH ) 70.6
Hz, 6H, PtCH3). 31P NMR: δ 29.5 (1JPtP ) 1876 Hz).
cis-[Pt(PCy3)2(Me)2] (13): crystallized from dichloromethane/ ac-
etone. 1H NMR: δ 2.3-1.1 (m, 66H, Cy), 0.31 (m, 2JPtH ) 64.4 Hz,
6H, PtCH3). 31P NMR: δ 20.1 (1JPtP ) 1828 Hz).
cis-[Pt(P(2-MeOC6H4)3)2(Me)2] (14): crystallized from chloroform/
methanol. 1H NMR: δ 7.5-6.6 (m, 24H, PPh), 3.3 (s, 18H, O-CH3),
2
-0.30 (m, JPtH ) 70.6 Hz, 6H, PtCH3). 31P NMR: δ 9.5 (1JPtP
)
cis-[Pt(Me2SO)2(Me)2] + 2L f
cis-[PtL2(Me)2] + 2Me2SO (2)
1839 Hz).
Kinetics. The kinetics of isomerization of cis-[PtL2(Me)(MeOH)]+
(L ) alkyl, aryl, and mixed alkyl-aryl phosphines) complexes were
followed spectrophotometrically by the traditional method, at constant
temperature (CTK) and by a non-isothermal method (VTK).18,19
(a) CTK Method. The kinetics were followed by repetitive scanning
of the spectrum at suitable times in the range 320-220 nm or at a
fixed wavelength, where the difference of absorbance with the trans
product was largest. The reactions were carried out in a silica cell, in
the thermostated cell compartment of a Cary 219 or of a rapid-scanning
Hewlett-Packard Model 8452 A spectrophotometer, with a temperature
accuracy of (0.05 °C. The reactions were started by adding with a
syringe a prethermostated solution of cis-[PtL2(Me)2] in methanol to a
thermostated methanol solution of H+BF4-. In all cases the concentra-
tion of acid was sufficient to produce a fast cleavage of the first Pt-
CH3 bond. All the reactions obeyed a first-order rate law until well
over 90% of the reaction and the rate constants ki (s-1 ) were obtained
from a nonlinear least-squares fit of the experimental data to Dt ) D∞
+ (D0 - D∞) exp(-kit) with D0, D∞, and ki as the parameters to be
optimized (D0 ) absorbance after mixing of reagents, D∞ ) absorbance
at completion of reaction). Activation parameters were derived from
a linear least-square analysis of ln(ki/T) Vs T-1 data.
The removal of both molecules of sulfoxide from the starting
material by the phosphines (L) is fast and easy, the reactions
go to completion, and the desired products were obtained in
high yield and purity. Complications arise only with the most
sterically demanding phosphines. Thus, for P(2-MeC6H4)3 and
P(2-MeOC6H4)3, the reaction mixture showed the contemporary
presence of mono- and bis-substituted phosphine products, and
this latter product was obtained in a satisfactory pure solid form
only in the case of P(2-MeOPh)3. Attempts to synthesize the
compound with P(But)3 failed. The new compounds, therefore,
are those containing P(Prn)3 (4), P(4-ClC6H4)3 (8), P(4-
MeOC6H4)3 (9), P(Pri)3 (10), P(3-MeC6H4)3 (11), P(3-ClC6H4)3
(12), PCy3 (13), and P(2-MeOC6H4)3 (14). Traces of dimethyl
sulfoxide incorporated in the products were eliminated by
1
crystallization. The complexes were characterized by H and
31P NMR in a 3:1 CD2Cl2/CD3OD mixture, cooled at 253 K,
and selected resonances are listed in Table 1.
Spectral Characteristics of the Complexes. The methyl
part of the 1H NMR spectrum of all the cis-[PtL2(Me)2]
complexes is similar to that described in detail for L ) PPh3,24
PEt3,25 PMe2Ph,16 and PMe3.26 The 31P NMR spectrum shows
a single resonance with low values of 1JPtP coupling constants,
typical of phosphorus atoms trans to carbon in platinum
complexes.25,47
(b) VTK Method. Kinetic runs were carried out using a Perkin-
Elmer Lambda 3 spectrophotometer, connected to a 486/DX IBM
microcomputer and equipped with a cell compartment thermostated
by a Perkin-Elmer PTP (Peltier temperature programmer), which allows
a controlled change of the temperature with time with an accuracy of
(0.05 °C. The temperature was checked by a platinum resistor inserted
into the spectrophotometric cell and connected to the computer. The
reactions were started as described above for the CTK method, the
only difference being that the starting temperature (T0) was made
sufficiently low to slow down the initial rate of reaction. This procedure
makes negligible the dead-time before reaching the linear part of the
temperature program of the PTP device. The reactions were followed
at a fixed wavelength, and the absorbance-time data were automatically
acquired by using the Perkin-Elmer PECSS program. The processing
of the stored data was performed by use of the MicroMath SCIENTIST
program20 with a Powell modified Marquadt algorithm21 for the fitting
A solution of a weighed amount of cis-[PtL2(Me)2] in a 3:1
CD2Cl2/CD3OD mixture was frozen in the NMR tube and then
added with the stoichiometric amount of an ethereal solution
of HBF4. The temperature was slowly increased to 253 K, and
1
the H and 31P NMR spectra of the ensuing cis-monoalkyl
(22) Kistner, C. R.; Hutchinson, J. H.; Doyle, J. R.; Storlie, J. C. Inorg.
Chem. 1963, 2, 1255.
(23) (a) Scott, J. D.; Puddephatt, R. J. Organometallics 1983, 2, 1643. (b)
Puddephatt, R. J.; Rashidi, M.; Fakhroeian, Z. J. Organomet. Chem.
1990, 406, 261. (c) Yang, D. S.; Bancroft, G. M.; Dignard-Bailey, L.;
Puddephatt, R. J.; Tse, J. S. Inorg. Chem. 1990, 29, 2487.
(24) Greaves, E. O.; Bruce, R.; Maitlis, P. M. J. Chem. Soc., Chem.
Commun. 1967, 860.
(25) Allen, F. H.; Pidcock, A. J. Chem. Soc. A 1968, 2700.
(26) Goodfellow, R. J.; Hardy, M. J.; Taylor, B. F. J. Chem. Soc., Dalton
Trans. 1973, 2450.
(18) (a) Alibrandi, G. Inorg. Chim. Acta 1994, 221, 31. (b) Alibrandi, G.;
Micali, N.; Trusso, S.; Villari, A. J. Pharm. Sci. 1996, 85, 1105.
(19) Zhang, S.; Brown, T. L. Inorg. Chim. Acta 1995, 240, 427.
(20) MicroMath Scientific Software, Salt Lake City, UT 84121.
(21) Press, W. H.; Flannery, B. P.; Teukolsky, S. A.; Vetterling, W. T.
Numerical Recipes; Cambridge University Press: Cambridge, U.K.,
1986.

Isomerization of Platinum(II) Solvento Complexes
Inorganic Chemistry, Vol. 36, No. 21, 1997 4825
1
Table 1. Selected H and 31P NMR Data for Dialkyl Phosphine Complexes of Platinum(II) and for the Cis and Trans Monalkyl Solvento
Species Obtained upon Protonolysis and Subsequent Isomerizationa
complexes
cis-[PtL2(Me)2]
cis-[PtL2Me(MeOH)]+
trans-[PtL2Me(MeOH)]+
no.
phosphine (L)
δ(1H)(PtCH3)
δ(31P)
δ(1H)(PtCH3)
δ(31PA)b
δ(31PB)c
δ(1H)(PtCH3)
δ(31P)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
PMe3
PPhMe2
0.37 (65.5)
0.44 (66.9)
0.30 (64.7)
0.31 (64.7)
0.29 (68.4)
0.29 (69.9)
0.24 (67.8)
0.35 (69.8)
0.27 (68.4)
0.34 (66.0)
0.27 (69.1)
0.39 (70.6)
0.31 (64.4)
-0.30 (70.6)
-23.1 (1761)
-10.0 (1794)
9.3 (1843)
-0.1 (1828)
7.6 (1851)
28.0 (1910)
26.0 (1924)
26.7 (1887)
24.7 (1930)
29.8 (1866)
28.1 (1923)
29.5 (1876)
20.1 (1828)
9.5 (1839)
0.56 (41.2)
0.66 (37.5)
0.52 (43.4)
0.59 (45.0)
0.57 (41.3)
0.62 (41.2)
0.55 (43.5)
0.67 (48.1)
0.58 (48.6)
-4.0 (1780)
7.4 (1820)
-29.6 (4367)
-16.7 (4500)
8.9 (4427)
-2.7 (4424)
-2.2 (4655)
15.3 (4823)
12.3 (4807)
13.3 (4854)
10.1 (4798)
0.58 (89.0)
0.51 (85.3)
0.48 (86.8)
0.48 (89.0)
0.40 (86.7)
0.30 (82.4)
0.25 (86.8)
0.26 (80.7)
0.29 (80.1)
0.57 (88.2)
0.25 (79.4)
0.25 (86.8)
0.25
-6.4 (2789)
5.4 (2919)
PEt3
25.5 (1827)
16.7 (1801)
20.3 (1846)
35.8 (1903)
34.3 (1924)
35.5 (1897)
33.5 (1945)
25.9 (2817)
15.2 (2793)
18.4 (3021)
32.7 (3139)
30.9 (3108)
31.3 (3178)
29.3 (3085)
38.6 (2841)
32.8 (3128)
33.4 (3214)
27.9 (2807)
24.1 (3274)
P(Prn)3
PPh2Me
PPh3
P(4-MeC6H4)3
P(4-ClC6H4)3
P(4-MeOC6H4)3
P(Pri)3
P(3-MeC6H4))3
P(3-ClC6H4)3
PCy3
0.58 (41.0)
0.72 (47.1)
35.2 (1890)
37.4 (1860)
14.3 (4836)
15.1 (4876)
P(2-MeOC6H4)3
0.46 (96.0)
2
1
a Resonances in ppm from TMS and H3PO4 at 253 K; solvent ) 3:1 CD2Cl2/ CD3OD (v/v); JPtH in Hz for PtCH3 and JPtP in Hz are given in
parentheses. b PA, phosphorus atom trans to the methyl group. c PB, phosphorus atom trans to MeOH.
solvento species were recorded. As a typical example, in the
case of cis-[Pt(PMe3)2(Me)(MeOH)]+, the methylplatinum
resonance appears at δ 0.56, as four lines of equal intensity
due to the coupling with two nonequivalent 31P atoms, with
195Pt satellites (2JPtH ) 41.2 Hz). The two nonequivalent
phosphine ligands give two 31P resonances, δ(PA) ) -4.0 (1JPtP
) 1780 Hz) and δ(PB) ) -29.6 (1JPtP ) 4367 Hz). The low
coupling constant 1JPtPA is indicative of a phosphorus atom trans
collected in Table 2, together with the values of the rate constant
ki/s-1 for the isomerization at 298.16 K. The values of ∆Hq
and ∆Sq obtained by the VTK method are collected in Table 2,
where they can be compared with those obtained with the CTK
method.
Discussion
Easy displacement of dimethyl sulfoxide by phosphines from
the complex cis-[PtMe2(DMSO)2], according to eq 2, led to the
synthesis of a wide series of complexes of the type cis-[PtL2-
Me2]. All attempts to obtain the compound with P(But)3 failed,
as a consequence of the strong steric congestion of the bulky
phosphines occupying cis-positions. The compound with P(2-
MeC6H4)3 was detected in solution, but all efforts to isolate it
were unsuccessful. The synthesis of the compound containing
P(2-MeOC6H4)3 came as a surprise because, on the basis of its
encumbrance, we would have expected severe steric restriction
to the formation of this molecule. Unfortunately, we were
unable to grow suitable crystals for a crystallographic study,
but it seems safe to conclude that a remarkable intermeshing
and distortion of the two bulky phosphines must take place to
minimize steric congestion at the metal center.27-29
1
to carbon, while the value JPtPB ) 4367 Hz is consistent with
the presence of a very weak trans donor ligand such as methanol.
2
The low value of the JPP coupling constant (12 Hz) is typical
of cis phosphines. Essentially the same pattern is observed when
a moderate excess of acid is used and, therefore, cis configu-
ration is retained at 253 K, as a result of Pt-CH3 bond breaking.
On increase of the temperature to 300 K, the isomerization can
be monitored through the decrease in the 31P signals associated
with cis-[Pt(PMe3)Me(MeOH)]+ and the parallel and matching
increase in the signal of the corresponding trans complex, which
appears as a singlet with platinum satellites at δ -6.4 (1JPtP
)
2789 Hz). The rate of cis to trans conversion proved to be too
fast to be followed for the complexes containing P(Pri)3, PCy3,
and P(2-MeOC6H4)3, and therefore, soon after the addition of
acid, even at the lowest temperatures, only the 31P signals due
The dialkyl complexes were used as precursors for the
synthesis “in situ” of cationic cis solvento [PtL2(Me)(MeOH)]+
complexes. For the complexes containing the bulky P(Pri)3,
P(Cy)3, and P(2-MeOC6H4)3 phosphines the conversion of [PtL2-
(Me)(MeOH)]+ from the cis to the trans form was too fast to
be followed, even at the lowest temperatures and, therefore, the
spectral characteristics of these elusive cis species could not be
1
to the trans isomers were observed. A collection of H and
31P NMR data for the dialkyl complexes and for the monoalkyl
cis and trans solvento isomers is reported in Table 1.
Spectrophotometric Kinetic Studies. 1H and 31P NMR
spectra taken during the course of the reactions showed that
the process under study was the simple uncatalyzed isomeriza-
tion of the cis monoalkyl solvento complex. The amount or
the nature of the acid used (provided the counteranion is not a
nucleophile, as for HBF4, HClO4, or 4-toluenesulfonic acid) had
no effect on the reaction. All isomerization reactions were
followed spectrophotometrically using methanol as the solvent.
The conversion was 100% complete, and the spectral changes
showed isosbestic points (see Supporting Information Figure
SI2). For the complexes containing P(Pri)3, PCy3, and P(2-
MeOC6H4)3 the two consecutive processes, protonolysis and
isomerization, were complete soon after the mixing of the
reagents. The isomerization follows a first-order rate law.
Specific rate constants, ki/s-1, measured at different tempera-
tures, were analyzed by least-squares regression of linear Eyring
plots (primary kinetic data for isomerization at different tem-
peratures are given as Supporting Information in Table SI1).
The values of ∆Hq (kJ mol-1) and ∆Sq (JK-1 mol-1) are
1
detected. The H and 31P experiments indicate quite clearly
that at 253 K the protonolysis of the dialkyls substrates is seen
as a single-step conversion to the cis-monoalkyl products, with
no evidence throughout of possible reaction intermediates such
as platinum(IV) hydrido-alkyl species of the type recently
reported in the literature.30-32
(27) (a) Immirzi, A.; Musco, A. Inorg. Chim. Acta 1977, 25, L41-L42.
(b) Porzio, W.; Musco, A.; Immirzi, A. Inorg. Chem. 1980, 19, 2537.
(c) Cameron, T. S.; Clark, H. C.; Linden, A.; Nicholas, A. M.;
Hampden-Smith, M. J. Inorg. Chim. Acta 1989, 162, 9.
(28) (a) Ferguson, G.; Roberts, P. J.; Aylea, E. C.; Khan, M. Inorg. Chem.
1978, 17, 2965. (b) Alyea, E. C.; Dias, S. A.; Ferguson, G.; Restivo,
R. J. Inorg. Chem. 1977, 16, 2329. (c) Alyea, E. C.; Ferguson, G.;
Somogyvari, A. Inorg. Chem. 1982, 21, 1639. (d) Smith, J. D.; Oliver,
J. D. Inorg. Chem. 1978, 17, 2585.
(29) (a) Clark, H. C.; Hampden-Smith, M. J. Coord. Chem. ReV. 1987, 79,
229 and references therein. (b) Immirzi, A.; Musco, A.; Mann, B. E.
Inorg. Chim. Acta 1977, 21, L37-L38.

4826 Inorganic Chemistry, Vol. 36, No. 21, 1997
Romeo and Alibrandi
Table 2. Ligand Properties of Phosphorus(III) Compounds, Rate Constants, and Activation Parameters for the Spontaneous Cis to Trans
Isomerization of the Solvento [PtL2(Me)MeOH]+ Species in Methanol
c
no.
phosphine (L)
øa
θb
103k1
∆Hq(VTK)d
125 ( 1h
∆Hq(CTK)e
∆Sq(VTK)f
84 ( 1
∆Sq(CTK)g
1
2
3
4
5
PMe3
8.55
10.6
6.3
5.4
12.1
118
122
132
132
136
0.018
0.017
2.95
127 ( 2
128 ( 3
106 ( 3
123 ( 2
122 ( 2
92 ( 6
92 ( 10
63 ( 12
128 ( 4
84 ( 4
PPhMe2
PEt3
108 ( 1h
119 ( 1h
125 ( 1h
127 ( 1i
121 ( 1h
122 ( 1i
119 ( 1h
120 ( 1i
129 ( 1h
127 ( 1i
120 ( 1h
120 ( 1h
120 ( 1i
119 ( 1h
124 ( 1i
67 ( 1
116 ( 1
94 ( 3
P(Prn)3
PPh2Me
9.26
0.071
103 ( 1
120 ( 1
121 ( 1
119 ( 1
123 ( 1
124 ( 1
117 ( 1
134 ( 1
116 ( 1
118 ( 1
80 ( 1
6
7
8
PPh3
13.2
11.5
16.8
10.5
145
145
145
5.31
17.9
119 ( 1
115 ( 5
125 ( 2
111 ( 2
106 ( 15
113 ( 5
P(4-MeC6H4)3
P(4-ClC6H4)3
0.543
9
11
P(4-MeOC6H4)3
P(3-MeC6H4)3
145
148
62.0
7.97
117 ( 1
120 ( 2
124 ( 5
117 ( 5
12
P(3-ClC6H4)3
18.4
145
0.141
116 ( 2
69 ( 7
96 ( 1
a Values in cm-1 taken from ref 34; for P(Pri)3 and P(Cy)3, ø ) 3.45 and 1.4, respectively. b Cone angle in deg data taken from ref 42; for P(Pri)3
and P(Cy)3, θ ) 160 and 170°, respectively. c First-order rate constants (s-1) for isomerization at 298.16 K. d Enthalpies of activation (kJ mol-1
)
)
from variable-temperature kinetics. e Enthalpies of activation (kJ mol-1) from constant-temperature kinetics. f Entropies of activation (J K-1 mol-1
from variable-temperature kinetics. g Entropies of activation (J K-1 mol-1) from constant-temperature kinetics. h Temperature gradient R ) 0.0166
°C/s-1 i Temperature gradient R ) 0.0083 °C/s-1
. .
Quantitative Separation of Steric and Electronic Effects.
To correlate electronic or steric effects with chemical and
physical properties it is necessary to dissect the overall effect
into its steric and electronic components. In the last few years,
numerous papers by Giering, Prock, and co-workers11a,b and by
Poe¨ and co-workers11c-g have reported methods to perform a
quantitative analysis (QALE) of the stereoelectronic properties
of ligands of the type AR3, ARnAr3-n, and A(4-XC6H4)3. A
large body of kinetic and physicochemical properties can be
analyzed successfully with the general equation33
analysis of ligand effect data, based essentially on the use of
data obtained from graphical analysis to control the results of
the regression analysis. To perform the stereoelectronic analysis
of the NMR and kinetic data in Tables 1 and 2 we followed the
protocol suggested by Giering.33 Attempts to apply Drago’s
electrostatic/covalent model37 were unsuccessful, since the
properties being analyzed depend heavily upon steric effects
and to a minor extent on aryl effects. The limits of Drago’s
E/C model in correlation analyses involving phosphines have
been recently discussed.38
Stereoelectronic Analysis of 1JPtP Coupling Constants. The
property ) a(ø) + b(θ) + b′(θ - θst)λ + c(Ear) + d (3)
1
values of JPtP coupling constants of the cis-[PtL2(Me)2]
complexes in Table 1 are in the range typical of a phosphorus
atom trans to carbon in platinum(II) compounds, and setting
apart the rogue value for complex 14, they encompass a range
of 160 Hz. Values of ø, θ, and Ear were used in the analysis as
stereoelectronic parameters of the ligands, but in principle, other
related parameters could be used, such as pKa39 or pKa′ 11c values
of PR3H+, Bodner’s δ(13CO) values,40 Brown’s ER values,41 or
Coville’s solid angle values,42 but their use does not bring about
any significant improvement in the analysis.
where ø is an infrared parameter34 that measures the σ donicity
of the ligand (the electron-donor ability decreases as ø increases),
θ is Tolman’s cone angle,35 which measures the steric require-
ments of the ligand, θst is the steric threshold, below which no
steric effects are evident, λ is a switching function that equals
0 when θ < θst and equals 1 when θ > θst, and Ear is the aryl-
effect parameter,36 which depends on the number of pendent
aryl groups of ARnAr3-n and A(4-XC6H4)3 but is independent
of any substituents on the aryl rings. a, b, b′, and c are
regression coefficients that measure the relative importance of
electronic, steric, and aryl factors in the process. The response
of the property to ø is assumed to be linear over the entire range
of ligands, while the response to the steric parameter (θ - θst)λ
is not linear. The presence of so many variables makes the
results from regression analysis questionable, especially in the
detection of a meaningful value for the steric threshold. Having
in mind this difficulty, very recently Giering, Prock, et al.33
have proposed a combined method of graphical and regression
The results of a regression analysis of all data points (n )
13), performed using eq 3 and the MicroMath SCIENTIST
program, were as follows: a ) -6.00 ( 1 Hz cm; b ) + 4.0
( 0.4 Hz deg-1; b′ ) -8.9 ( 1 Hz deg-1; c ) 23.9 ( 5 Hz;
d ) 1338 ( 58 Hz; θst ) 149°. The correlation coefficient of
the linear plot of 1JPtP(calc) Vs 1JPtP(obs) (where 1JPtP(calc) is obtained
1
by applying eq 3 and JPtP(obs) is determined experimentally)
(37) (a) Drago, R. S. Organometallics 1995, 14, 3408. (b) Drago, R. S.;
Joerg, S. J. Am. Chem. Soc. 1996, 118, 2654. (c) Drago, R. S.
Applications of Electrostatic/CoValent Models in Chemistry; Surfside
Scientific Publishers: P.O. Box 13413, Gainesville, FL, 1994. (d)
Drago, R. S.; Dadmun, A. P.; Vogel, G. C. Inorg. Chem. 1993, 32,
2473.
(30) (a) Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 1996,
118, 5961. (b) Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem.
Soc. 1995, 117, 9371.
(31) Hill, G. S.; Rendina, L. M. Puddephatt, R. J. Organometallics 1995,
14, 4966.
(32) De Felice, V.; De Renzi, A.; Panunzi, A. Tesauro, D. J. Organomet.
Chem. 1995, 488, C13-C14.
(33) Bartholomew, J.; Fernandez, A. L.; Lorsbach, B. A.; Wilson, M. R.;
Prock, A.; Giering, W. P. Organometallics 1996, 15, 295.
(34) Bartik, T.; Himmler, T.; Schulte, H. G.; Seevogel, K. J. Organomet.
Chem. 1984, 272, 29.
(38) Fernandez, A.; Reyes, C.; Wilson, M. R.; Woska, D. C.; Prock, A.;
Giering, W. P. Organometallics 1997, 16, 342.
(39) (a) Streuli, C. A. Anal. Chem. 1960, 32, 985. (b) Henderson, W. A.;
Streuli, C. A. J. Am. Chem. Soc. 1960, 82, 5791. (c) Allman, T.; Goel,
R. G. Can. J. Chem. 1982, 60, 716. (d) Bush, R. C.; Angelici, R. J.
Inorg. Chem. 1988, 27, 681.
(40) Bodner, G. M.; May, M. P.; McKinney, L. E. Inorg. Chem. 1980, 19,
1951.
(35) Tolman, C. A. Chem. ReV. 1977, 77, 313.
(36) Wilson, M. R.; Woska, D. C.; Prock, A.; Giering, W. P. Organome-
tallics 1993, 12, 1742.
(41) (a) Lee, K. J.; Brown, T. L. Inorg. Chem. 1992, 31, 289. (b) Brown,
T. L.; Lee, K. J. Coord. Chem. ReV. 1993, 128, 89.
(42) White, D.; Coville, N. J. AdV. Organomet. Chem. 1994, 36, 95-158.

Isomerization of Platinum(II) Solvento Complexes
Inorganic Chemistry, Vol. 36, No. 21, 1997 4827
constants to the inductive effects brought about by substituents
on phosphorus and can be constructed by subtracting the
contributions of all the terms of the regression equation, except
1
that of the variable of interest, i.e. a(ø), from the JPtP
experimental data, according to the following equation:
(1JPtP)(ø) ) (1JPtP obs
)
- [b(θ) + b′(θ - θst)λ + c(Ear) + d]
(4)
Likewise, the steric profile (Figure 1B) can be constructed using
the equation
(1JPtP)(θ) ) (1JPtP obs
)
- [a(ø) + c(Ear) + d]
(5)
and finally the aryl dependency (Figure 1C) can be obtained
from the equation
(1JPtP)(Ear) ) (1JPtP obs
)
- [a(ø) + b(θ) + b′(θ - θst)λ + d]
(6)
In practice, for the sake of visual comparison of the plots,
the term d has not been subtracted, and the figures are presented
with the same absolute scale. The plots include all data points.
The most relevant feature is that, before the steric threshold,
both electron release and steric encumbrance of substituents on
phosphorus concur to increase the value of 1JPtP, with the latter
factor playing a somewhat major role. After the steric threshold,
the overload of steric congestion leads to a sharp decrease of
1JPtP that must be associated with severe distortions of the
P-Pt-P bond angle and lengthening of Pt-P bond dis-
tances.29,43 The slope of the electronic profile in Figure 2D
(-6.00 ( 1 Hz cm) is comparable to that found for isosteric
ligands in cis-[PtPh2(CO)(PRnAr3-n)] compounds (a ) -8.2
( 0.9 Hz cm)44 indicating that inductive effects are transmitted
primarily through the C-Pt-P bond axis and the nature of cis
groups does not play a significant role.
The 1JPtP values of the trans-[PtL2(Me)(MeOH)]+ complexes
encompass a range of 425 Hz, much wider than that for cis-
[PtL2(Me)2]. The results of the QALE analysis, performed by
using eq 3, were as follows: a ) 14.6 ( 2 Hz cm; b ) 3.6 (
0.6 Hz deg-1; b′ ) -3.7 ( 1 Hz deg-1; c ) 64 ( 7 Hz; d )
2240 ( 62 Hz; θst ) 151° (r2 ) 0.998, rmsd ) 7, n ) 13), in
agreement with those obtained from the graphical analysis (a
) 14.4 ( 1 Hz cm, from the electronic plot for isosteric P(4-
XC6H4)3 ligands). The electronic and steric profiles are
illustrated in Figure 2A,B, respectively. The most significant
feature that emerges is that the values of 1JPtP decrease greatly
with increasing the σ-donor power of the coordinated phos-
phines, in sharp contrast to the pattern of behavior observed
for the cis-dialkyl complexes (the values of the slopes of the
plots of the two electronic profiles are a ) -6.0 ( 1 Hz cm,
Figure 1A, and a ) +14.6 ( 2 Hz cm, Figure 2A, respectively).
Figure 1. (A) Electronic profile, showing the dependence of the
coupling constants 1JPtP upon the electronic parameter ø for cis-[Pt(PRn-
Ar3-n)2(Me)2] compounds. (B) Steric profile, showing the dependence
of JPtP upon Tolman’s cone angle. (c) Ear plot. See text for the
description of each plot. Numbers refer to the ligands as listed in Table
1. Filled squares refer to isosteric P(4-XC6H4)3 ligands.
1
was r2 ) 0.995. A supplementary statistical criterion is given
by the value of the root-mean-square deviation of 1JPtP(calc) from
1JPtP(obs) (rmsd ) 6), which gives an estimate of the scatter of
data. An appropriate index of goodness-of-fit is given also by
the correspondence between the values of a (-6.00 ( 1 Hz
cm) obtained from full regression analysis and the value of a
(-6.89 ( 0.2 Hz cm) obtained from a plot of 1JPtP(obs) vs ø for
the isosteric ligands P(4-XC6H4)3, where θ and Ear remain
constant. These statistical criteria, r2, rmsd, and correspondence
of the a values, were used throughout to quantitatively assess
the utility of the equations used in modeling the examined
property.
1
Coupling constants JPtP of closely related planar platinum(II)
complexes have been regarded as a good diagnostic probe of
the extent of the covalence of the Pt-P bond, representing
primarily the change in 6s character of the hybrid orbital of
platinum used in bonding to the phosphorus atom.25 Correla-
tions with data available from other spectroscopic techniques
and from X-ray diffraction studies have shown that smaller
1
values of JPtP correspond to longer Pt-P bond distances and
to higher trans influences of groups in trans position to the
bond.45-47 Thus, ligands of high trans influence increase the
(43) Clark, H. C.; Nicholas, A.; Martin de P. Magn. Reson. Chem. 1990,
The results of QALE analysis can be now displayed as
electronic, steric, and aryl profiles. The electronic profile
28, 99.
(44) Romeo, R.; Arena, G.; Monsu` Scolaro, L. Inorg. Chem. 1992, 31,
1
(Figure 1A) represents the sensitivity of the JPtP coupling
4879.

4828 Inorganic Chemistry, Vol. 36, No. 21, 1997
Romeo and Alibrandi
which involves dissociation of the solvent (S) from the cis-
solvento species (via kD), followed by the conversion of a
T-shaped 14-electron intermediate (kT pathway).
A rate law of the form, ki ) kD/{1 + (k-D/kT)[S]}, can be
derived. The term (k-D/kT)[S] in the rate law measures the
retardation due to the capture of the first intermediate by the
bulk solvent. This effect decreases, as the donor properties of
the solvent decrease, and it can be considered approximately
constant when one compares the kinetic behavior of compounds
of strictly similar structural properties. Accordingly, the
isomerization of cis-[Pt(PEt3)2(R)(MeOH]+ complexes6 was
found to proceed through the dissociative mechanism in the
above reaction scheme, and is characterized by the following:
(i) mass-law retardation by [MeOH] in diethyl ether-methanol
mixtures; (ii) high values of ∆Hq and large positive values of
∆Sq; (iii) small steric acceleration on going from R ) methyl
or phenyl to mesityl.48
The QALE analysis for the isomerization process has been
performed according to the protocol described before. The
regression analysis of all the kinetic data in Table 2 led to the
following equation:
log ki ) (-0.27 ( 0.06 cm)(ø) +
(0.13 ( 0.03 deg-1)(θ) + (0.066 ( 0.3)(Ear) - 17.8 ( 4
(7)
r2 ) 0.995
rmsd ) 0.342 11 data points
There is a good correspondence between the value of a )
-0.27 ( 0.06 cm and the slope of the line for isosteric ligands
P(4-XC6H4)3 with θ ) 145° and a ) -0.32 ( 0.04 cm. It is
worth remembering that this analysis does not include data
points for highly reactive complexes, formed by the most
sterically demanding ligands. Thus, the absence of a steric
threshold could be due only to an insufficient variation of the
steric parameter of the ligands. Assuming there is no steric
threshold and eq 7 applies to the entire set of ligands, it is
possible to predict the rates of isomerization of the complexes
containing P(Pri)3 (ki,298 ) 117 s-1) and PCy3 (ki,298 ) 8300
s-1). The electronic profile (Figure 3A) and the steric profile
(Figure 3B) for the overall set of ligands were constructed by
using the parameters in eq 7 and the same absolute vertical scale
for the two plots. As expected, the rates of isomerization
increase with increasing electron-donating ability of the sub-
stituents on the phosphorus atoms. Electron donation facilitates
the departure of MeOH with its previously bonding electron
pair and stabilizes the electron-deficient transition state. The
process appears to be particularly sensitive in revealing elec-
tronic interactions between the metal and phosphine ligands.
These findings are in keeping with the results of a study on the
isomerization of cis-[Pt(PEt3)2(YC6H4)X] complexes,4a where
the rate constants ki were correlated to the Hammett parameters
of the substituents Y, in the meta or para position on the
aromatic ring.
Figure 2. (A) Electronic profile, showing the dependence of the
1
coupling constants JPtP upon the electronic parameter ø for trans-
[Pt(PRnAr3-n)2(Me)(MeOH)]+ compounds. (B) Steric profile. Numbers
refer to the ligands as listed in Table 1.
Pt 6s character of their bond at the expense of the trans Pt-P
bond which uses the same hybrid orbital. Our findings can be
rationalized in terms of the competition of the two trans
phosphines for the same orbital along the P-Pt-P bond axis
and of the lengthening of the Pt-P bond. At the steric threshold
(θst ) 151°), there is still an inversion in the direction of the
steric effects. However, as expected for the less congested trans
geometry, the negative steric effect (b′ ) -3.7 ( 1 Hz deg-1
)
is smaller than that for the cis compounds (b′ ) -8.9 ( 1 Hz
deg-1).
Stereoelectronic Analysis of Kinetic Data. (a) Uncatalyzed
Isomerization. The isomerization of cis-[Pt(L)2(Me)(MeOH)]+
complexes is described by the simple reaction scheme
The linear plot of Figure 3B describes the steric dependence
of the rates of these reactions, and the slope of the plot measures
the sensitivity of the isomerization to steric effects. An increase
of 10° in the cone angle of the phosphines accounts for more
than 1 order of magnitude increase in reactivity. The steric
dependence is the result of the different response of the energy
(45) (a) Mather, G. G.; Pidcock, A.; Rapsey, J. N. J. Chem. Soc., Dalton
Trans 1973, 2095. (b) Meek, D. W.; Mazanec, T. J. Acc. Chem. Res.
1981, 14, 266.
(46) Appleton, T. G.; Bennett, M. A. Inorg. Chem. 1978, 17, 738.
(47) Appleton, T. G.; Clark, H. C.; Manzer, L. E. Coord. Chem. ReV. 1973,
10, 335.
(48) In an associative process, the destabilization of the five-coordinate
transition state, produced by the o-methyl groups in the mesityl ring,
provokes a decrease in rate of at least 5 orders of magnitude. See:
Faraone, G.; Ricevuto, V.; Romeo, R. Trozzi, M. J. Chem. Soc., Dalton
Trans 1974, 1377.

Isomerization of Platinum(II) Solvento Complexes
Inorganic Chemistry, Vol. 36, No. 21, 1997 4829
Figure 4. (A) Electronic profile for phosphines acting as entering
groups in the nucleophilic substitution reaction of 5-Aq from the
complex cis-[PtPh2(CO)(5-Aq)] (5-Aq ) 5-aminoquinoline; primary
rate data taken from ref 44). (B) Steric profile for the same reaction.
Numbers refer to the ligands as listed in Table 1. (15) refers to P(But)3,
and (16), to P(2-MeC6H4)3.
Figure 3. Stereoelectronic analysis of the rate constants ki/s-1 for the
uncatalyzed isomerization of cis-[Pt(PRnAr3-n)2(Me)(MeOH)]+ com-
pounds, at 298.16 K in methanol: (A) electronic profile; (B) steric
profile. Numbers refer to the ligands as listed in Table 1.
of the ground and the transition states to changes in the size of
the “spectator” ligands. For a purely dissociative process, the
idealized steric profile is made up by lower and upper plateau
regions, connected by a straight line with a positive slope.49
The first horizontal lower part, for very small ligands, limits a
region where steric effects are not operative both in the ground
and in the transition state. A steric threshold marks the onset
of steric destabilization of the ground state, and the rates increase
with ligand size and steric congestion of the substrate. The
second horizontal upper part, for the most sterically demanding
ligands, reflects the case in which the energies of the ground
state and of the more flexible transition state are sterically
influenced to a comparable extent. Therefore, the plot of Figure
3B, found for ligands of intermediate size, where the rate
increases with increasing steric destabilization of the ground
state, is only a part of a more complicated steric profile. There
are literature examples of a complete fitting of the experimental
data to the expected profile, such as for the rates of dissociative
substitution of CO from mer-[Ru(CO)3(SiCl3)2L] (L ) phos-
phine),50 analyzed by Giering et al. with the QALE method.49
For a number of other dissociative processes of carbonyl
compounds the steric profile is a straight line as shown in Figure
3B.51 A reviewer has observed that the value of b ) 0.13 (
0.03 deg-1 for isomerization is very large compared to the values
reported for CO dissociation from a variety of carbonyl
complexes, where it was claimed51,52 that the steric effect
decreased with the coordination number of the complexes. It
could also increase as the transition state comes later, and since
the electronic effect is much larger than in the CO dissociation
reactions, this supports a very late transition state for the
isomerization with MeOH being totally free. This is in contrast
to CO dissociation reactions where the electronic and steric
effects are both small and the extent of bond breaking is
presumably much less.
(b) Associative Substitutions. The different roles of elec-
tronic effects and steric hindrance in dissociative and associative
processes can be seen clearly by comparing the pattern of
behavior illustrated above with that found for the bimolecular
displacement of 5-aminoquinoline (5-Aq) from [PtPh2(CO)(5-
Aq)]. Our previously reported QALE analysis44 of these rate
data, proposed as a series of nucleophilic reactivity constants
nPPt for phosphines reacting with platinum(II), can be improved
by omitting data points for ligands of unusual electronic effects,
such as P(C2H4CN)3, or considering that the value of cone angle
(165°) originally used for P(3-MeC6H4)3 and P(3-ClC6H4)3
greatly overstates the size of these groups. With these correc-
tions no steric threshold was observed. The reaction rates can
be correlated through a three-parameter equation:
log k2 ) a(ø) + b(θ) + c
(8)
where a ) -0.021 ( 0.02 cm, b ) -0.087 ( 0.05 deg -1, and
c ) 12.8 ( 1 (r2 ) 0.979, rmsd ) 0.354, n ) 14). The negative
(49) Ericks, K.; Giering, W. P.; Liu, H. Y.; Prock, A. Inorg. Chem. 1989,
28, 1759.
(50) Chalk, K. L.; Pomeroy, R. K. Inorg. Chem. 1984, 23, 444.
(51) Chen, L.; Poe¨, A. J. Inorg. Chem. 1989, 28, 3641.
(52) Brodie, N. M. J.; Poe¨, A. J. Can. J. Chem. 1995, 73, 1187.

4830 Inorganic Chemistry, Vol. 36, No. 21, 1997
Romeo and Alibrandi
sign of the a coefficient indicates that the transition state is
electron-demanding relative to the ground state. The negative
sign of the b coefficient indicates that increasing steric require-
ment of the ligand impedes the reaction. By using these
parameters the electronic profile (in Figure 4A) and the steric
profile (in Figure 4B) were constructed, which give a visual
comparison of the relative importance of the two parameters.
On the basis of eq 8,53 log k2 has a 94% steric contribution and
only 6% electronic contribution. The main conclusions are
when phosphines are used as entering groups in a bimolecular
substitution on [PtPh2(CO)(5-Aq)], (i) the contribution to the
rates of changes in the σ-inductive ability of the attacking
P-donor ligands is negligible and (ii) the reaction is dominated
by steric factors. The generality of this pattern of behavior for
phosphines needs to be confirmed. Shi and Elding54 have
recently reported that the rates of substitution of some square
planar platinum(II) complexes with thioethers show a significant
dependence on changes in the electronic properties of the
entering ligands. As discussed previously,55 the linear steric
profile in Figure 4B is only a part of a more complicated
hypothetical steric profile which consists of upper and lower
plateau regions connected by a straight line with a negative
slope. Along this downward sloping part of the plot, steric
effects are operative through a continuous increasing destabi-
lization of the five-coordinate transition state.
Conclusions
The overall structure-reactivity correlation for some organo-
phosphine platinum(II) compounds has been rationalized. The
QALE analysis has proved to be a useful method for evaluating
the effects of ligand basicity and steric hindrance on the NMR
parameters and the reactivity. For the complexes cis-[Pt(PRn-
1
Ar3-n)2Me2] the values of the coupling constants JPtP were
found to increase linearly with the electron releasing ability of
the substituents on the phosphorus atoms. However, the
opposite trend was observed when the two phosphines occupy
trans positions as in the trans [Pt(PRnAr3-n)2(Me)(MeOH)]+
solvento complexes. These experimental findings are in agree-
1
ment with theoretical predictions. The steric profile for JPtP
has an inverted V shape, increasing with ligands of intermediate
size and then decreasing, after the steric threshold. The decrease
is sharp, for the cis complexes, and less dramatic for the trans
compounds, as a consequence of the overload of steric conges-
tion at the central metal.
The rates of the isomerization of the solvento complexes cis-
[Pt(PRnAr3-n)2(CH3)(MeOH)]+ are accelerated both by electron
donation and steric hindrance brought about by substituents on
the “spectator” ligands. The rate-determining step is the
dissociation of the weakly bonded molecule of MeOH from
the metal. Inductive effects stabilize a flexible 3-coordinate
T-shaped 14-electron transition state, and steric effects desta-
bilize the rigid 4-coordinate square-planar ground state. High
values of the enthalpy of activation and large positive entropies
of activation are associated with isomerization. The release of
steric strain on going from the ground state to the transition
state significantly affects the rates. A plot of log ki(θ) Vs
Tolman’s cone angle of the phosphines is a straight line with a
positive slope. This plot is only a part of the expected shape
for the steric profile of a purely dissociative process and reflects
the continuous increase of steric congestion at the metal. In
contrast, when the phosphines are used as entering groups in
associative substitution processes, the steric profile is a straight
line with a negative slope.
This work gives a rational explanation of some of the reasons
which make it difficult to synthesize bis(phosphine) alkyl
solvento complexes of platinum(II) in the cis configuration. Two
factors, (i) ligand steric congestion and (ii) strong σ-electron
donation by the ligands, combine to accelerate the rate of
conversion into the more stable trans-isomers. The nature of
the bonded organic moiety (linear or branched alkyl group;
phenyl or substituted aryl group) is also of overriding impor-
tance, especially as far as the presence of â-hydrogens on a
alkyl chain is concerned. This will be the subject of a
forthcoming paper.56
Activation Parameters. The activation parameters for the
isomerization of the cis-[PtL2(Me)(MeOH)]+ solvento com-
plexes were calculated from the temperature dependence of the
reaction, using both the traditional isothermal (CTK) method
and a non-isothermal (VTK) method. The values of ∆Hq and
∆Sq obtained with the two methods, listed in Table 2, appear
to be in excellent agreement. The advantages offered by the
VTK method are straightforward: (i) It permits fast and easy
collection and processing of enormous amounts of data. (ii)
The whole set of data is from a single experiment carried-out
in homogeneous conditions. (iii) With a single kinetic run it is
possible to obtain a k(T) profile instead of a single rate constant,
saving time and chemicals. (iv) The statistical error associated
with the calculated values of ∆Hq and ∆Sq is very low.
The large values of enthalpy of activation and the positive
entropies of activation associated with isomerization give a
further indication that the reaction proceeds through the dis-
sociative mechanism shown in the earlier scheme. Setting apart
the complex with triethylphosphine, which appears as a rogue
point, the values of ∆Hq encompass a difference of 10 kJ mol-1
between the highest and the lowest value, with a mean value
of ∆Hq ) 122 ( 3 kJ mol-1. For the entropy of activation the
mean value is ∆Sq ) 111 ( 15 J K-1 mol-1, the lowest value
being 84 ( 1 J K-1 mol-1 and the highest 134 ( 2 J K-1mol-1
.
Therefore, the contribution of T∆Sq to ∆Gq, at 298 K, varies
by ∼15 kJ mol-1. These findings are in agreement with a
mechanistic picture in which the relief of steric strain on going
from the congested square planar substrate to the more flexible
T-shaped 3-coordinate transition state makes a significant
contribution to the dissociation of the bonded group from the
substrate.
Acknowledgment. We wish to thank Prof. W. P. Giering
for helpful discussions and the CNR and MURST for financial
support.
Supporting Information Available: Text providing a detailed
description of the non-isothermal method, including Figure SI1, Figure
SI2, showing spectral changes for isomerization of cis-[Pt(CH3)(PPh3)2-
(MeOH)]+ at 295 K, and Table SI1, reporting the temperature
dependence of the rates of isomerization, ki (s-1) (7 pages). Ordering
information is given on any current masthead page.
(53) (a) Szczepura, L. F.; Kubow, S. A.; Leising, R. A.; Perez, W. J.; Huynh,
M. H. V.; Lake, C. H.; Churcill, D. G.; Churchill, M. R.;Takeuchi, K.
J. J. Chem. Soc., Dalton Trans. 1996, 1463. (b) Leising, R. A.; Ohman,
J. S.; Takeuchi, K. J. Inorg. Chem. 1988, 27, 3804.
(54) Shi, T.; Elding. L. I. Inorg. Chem. 1996, 35, 5941.
(55) Romeo, R.; Arena, G.; Monsu` Scolaro, L.; Plutino, M. R.; Bruno, G.;
Nicolo`, F. Inorg. Chem. 1994, 33, 4029.
IC970210L
(56) Romeo, R.; Plutino, M. R.; Elding, L. I. Manuscript in preparation.