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Inorganica Chimica Acta 331 (2002) 136– 142

Determination of the nucleophilic reactivity constants for a series

of N-(n-propyl)-N%-(para-R-benzoyl)thioureas towards
trans-[Pt(pyridine)2Cl2]
Klaus R. Koch *, Jörn D.S. Miller, Oliver Seidelmann
Department of Chemistry, Uni6ersity of Stellenbosch, P Bag X1, Matieland 7602, South Africa

Received 2 July 2001; accepted 8 October 2001

Dedicated to Professor A.G. Sykes

Abstract

The relative nucleophilic reaction constants npt, for a series of N-(n-propyl)-N%-(para-R-benzoyl)thioureas (R = H, CH3O,
(CH3)3C, Br, Cl, C7H15O, I, NO2) have been determined for the substrate trans-[PtCl2(pyridine)2] in methanol– water (75:25 v/v
at 30 °C and v= 0.03), and are compared with that of unsubstituted thiourea. The bimolecular substitution reaction constants k2
were found to vary from 12.1 91.3 to 6.0 9 0.4 (for thiourea), corresponding to relative nucleophilicity constants of 4.08–3.78.
A small but detectable influence of the substituent has been found on these constants, the npt values decreasing in the order
H CH3O\Cl\C7H15Ot-butI\Br\NO2 \thiourea. Essentially however, the nucleophilicity of the series of N-(n-pro-
pyl)-N%-(para-R-benzoyl)-thioureas resembles that of thiourea. © 2002 Elsevier Science B.V. All rights reserved.

Keywords: Kinetics and mechanism; Platinum complexes; Pyridine complexes; Thiourea complexes

1. Introduction as potentially selective ligands, especially for the late

transition metal ions. Some potential analytical applica-
N,N-Dialkyl-N%-acyl(aroyl)thioureas (HL) and N-al- tions of N,N-dialkyl-N%-acyl(aroyl)thiourea complexes
kyl-N%-acyl(aroyl)-thioureas (H2L), with the general of the platinum group metals Pt(II), Pd(II) and Rh(III)
formula RCONHCSNR%R%% have been known for have recently been reviewed [7]. Depending on the
more than 100 years [1], and are easily synthesised in metal ion and the substitution pattern of R, R% and R%%
good yields [2]. Nevertheless, the first investigations of in the ligand RCONHCSNR%R%%, various modes of
their coordination chemistry with transition metals coordination have been observed for these ligands.
were only carried out in the early 1960s [3]. These stable Whereas N,N-dialkyl substituted derivatives of aro-
molecules, which have at least three potential donor matic acids (R= Ph, ferrocene (Fc)) predominantly give
atoms (N, O, S), have been found to display a remark- uncharged O,S-chelates with most of the transition
ably rich coordination chemistry, showing a more metal ions [4–8], monodentate S-bound complexes are
varied coordination behaviour than the structurally re- formed with the d10 ions of Cu(I), Hg(II), Ag(I) and
lated b-diketones, which have been thoroughly Au(I) [9,10]. Monodentate S-coordination has also
investigated. been observed in cis-[Pt(H2L-S)2Cl2] (where H2L=N-
More recent systematic studies of these ligands by the (n-butyl)-N%-benzoylthiourea) [6b], as well as with N-
research groups of Hoyer/Beyer [4], Schuster [5] and (n-propyl)-N%-benzoylthiourea in cis/trans-[M(H2-
Koch [6] have confirmed the utility of these compounds LS)2X2] complexes with M= Pt(II) or Pd(II) and X=
Cl−, Br− or I− [6h]. Moreover, an unusual binuclear
* Corresponding author. Tel.: +27-21-808 3331; fax: + 27-21-808
Rh(I) complex in which N-phenyl-N%-benzoylthiourea
3849. binds as a bridging N,S,O-coordinated ligand has also
E-mail address: krk@maties.sun.ac.za (K.R. Koch). been described recently [11].

0020-1693/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 0 - 1 6 9 3 ( 0 1 ) 0 0 7 7 7 - 0
 K.R. Koch et al. / Inorganica Chimica Acta 331 (2002) 136–142 137

The overwhelming majority of O,S-chelates derived trans-[Pt(py)2Cl2]+ H2Ln

from N,N-dialkyl-N%-benzoylthioureas have been found
to have the S,S-cis stereochemical configuration with d8 “ trans-[Pt(py)2(H2LnS)Cl]+ + Cl −
metal ions, or the S-facial stereochemical configuration We have specifically chosen this series of ligands, in
with octahedral metal ions such as Rh(III) [12] and view of their tendency to coordinate to Pt(II) and
Ru(III) [13]. To the best of our knowledge, only one Pd(II) through the S-atom only. Coordination by the
example of a trans configuration in bis(N,N,-dibutyl- amidic oxygen atom of these ligands is inhibited by an
N%-naphthoylthioureato)platinum(II) has been charac- intramolecular hydrogen-bond which prevents coordi-
terised by means of X-ray diffraction [6a], while there is nation by the amidic oxygen atom as shown below
NMR spectroscopic evidence for the existence of other [6,6h] under appropriate conditions. For comparison
S,S-trans complexes of Pt(II) derived from N,N-di- we have also included unsubstituted thiourea,
alkyl-N%-(trimethoxy-benzoyl)thioureas [14]. The reason (NH2)2C(S), in our study.
for the paucity of trans-chelates is, however, not yet
clearly understood. Simple steric considerations based
on the possible repulsion of bulky substituents R or R%,
R%% do not lead to a satisfactory explanation of this
phenomenon, since no evidence of trans complexes was
found for ligands containing the bulky ferrocene moiety
(R =Fc) [8], whereas N,N-dialkyl-N%-(trimethoxy-ben-
zoyl)thioureas do form some trans-[Pt(L-S)2Cl2] com-
In this sense, these potentially bidentate ligands are
plexes, at least as minor products [14]. These
expected to resemble the coordination of simple unsub-
observations suggest that electronic properties of par-
stituted thiourea, under conditions in which the in-
ticularly the aroyl (R) substituents may subtly influence
tramolecular hydrogen bond in the former remains
the mode of coordination of these ligands.
intact.
Attempts to explain the formation of the preferred
S,S-cis complexes with d8 metal ions, which are pre-
sumably thermodynamically favoured, usually invoke
the energetically more favourable formation of two 2. Experimental
orthogonal dp –dp bonds between the metal and sulfur
atoms [15a]. Furthermore, the rather short SS dis- The series of N-(n-propyl)-N%-(para-R-benzoyl)-
tances between adjacent ligands observed in the crystal thioureas were synthesised under an atmosphere of
structures of several cis complexes, suggest that weak dinitrogen according to the method reported by Dou-
interligand SS interactions may also play a role in glass and Dains [2]. These molecules were purified from
these complexes [15b]. On the other hand, given the by-products using column chromatography, with Silica
relatively high trans-effect of unsubstituted thioureas Gel 60 as stationary phase, and chloroform or methyl-
[16], it is somewhat surprising so few trans-S,O chelates ene chloride mixed with between 0–10% acetone (v/v)
as eluent.
of Pt(II) with N,N-dialkyl-N%-acyl(aroyl)thioureas have
Melting points were determined on a Reichert–Jung
been isolated. If the S,S-cis isomers are thermodynami-
thermovar attached to a DP-4 digital thermometer. 1H
cally favoured, the corresponding trans complex may
and 13C spectra of the ligands were recorded in CDCl3
presumably be a kinetically favoured product of the
at 25 °C using a Varian VXR-200 Fourier transform
substitution and subsequent ring closure reactions [6a].
spectrometer operating at 200 and 50.32 MHz, respec-
In order to gain deeper insight into the mechanistic
tively. The 1H NMR spectra of the complexes trans-
aspects of the coordination chemistry of N-alkyl-N%-
[Pt(py)2Cl2] and trans-[Pt(py)2(N-(n-propyl)-N%-
aroylthioureas, we have determined the substitution
benzoylthiourea)2] were measured in CD3OD/D2O
kinetics of a series of N-(n-propyl)-N%-(para-R-ben-
(75:25) using a UNITY-400 at 399.95 MHz. Elemental
zoyl)thioureas (designated here for simplicity as H2Ln,
analyses for % C, H, N and S were carried out using a
with n= 1 –8 corresponding to R =H, CH3O, (CH3)3C,
Fission Elemental Analyser EA 1108. The substrate
Br, Cl, C7H15O, I, NO2) toward the classical substrate
trans-[Pt(py)2Cl2] was synthesised according to the pub-
trans-[PtCl2(pyridine)2] pioneered by Belluco, Basolo
lished method [18].
and Pearson [17], with a view to determining their
relative nucleophilicity, npt in methanol – water (75:25
v/v).1 2.1. Ligand synthesis

1
Although traditionally pure methanol is used as solvent for 2.1.1. N-(n-Propyl) -N%-benzoylthiourea, H2L 1,
determining npt [17], ligand solubility compelled us to use a 75:25 C11H14N2OS
methanol– water mixture in our study at 30 °C and v = 0.03. Product isolated in 60.9% yield, m.p. 131–132 °C.
138 K.R. Koch et al. / Inorganica Chimica Acta 331 (2002) 136–142

Anal. Found: C, 59.31; H, 6.49; N, 11.57; S, 13.95. (br. unres. m, 8H), 1.65– 1.86 (br. unres. m, 4H), 3.66
Calc. for C11H14N2OS: C, 59.42; H, 6.36; N, 12.60; S, (m, 2H), 4.01 (t, 2H), 6.95 (d, 2H), 7.78 (d, 2H), 8.90 (s,
14.42%. l(1H)/ppm CDCl3: 1.02 (t, 3H), 1.74 (m, 2H), 1H), 10.78 (br s, 1H). l(13C)/ppm CDCl3: 11.46, 14.04,
3.69 (m, 2H), 7.51 (m, 2H), 7.62 (m, 1H), 7.83 (d, 2H), 21.59, 22.56, 25.89, 29.02, 31.71, 47.55, 68.46, 114.81,
9.02 (br s, 1H), 10.74 (br s, 1H). l(13C)/ppm, CDCl3: 123.41, 129.51, 163.50, 166.27, 179.94.
11.40, 21.50, 47.54, 127.34, 129.05, 131.79, 133.43,
166.80, 179.71. 2.1.7. N-(n-Propyl) -N%-(4 -iodobenzoyl) -thiourea H2L 7,
C11H13IN2OS
2.1.2. N-(n-Propyl) -N%-(4 -methoxybenzoyl) -thiourea Product isolated in 44.5% yield, m.p. 122– 124 °C.
H2L 2, C12H16N2O2S Anal. Found: C, 38.32; H, 3.76; N, 8.22; S, 9.03. Calc.
Product isolated in 90.0% yield, m.p. 74– 76 °C. Anal. for C11H13IN2OS: C, 37.94; H, 3.77; N, 8.05; S, 9.21%.
Found: C, 56.93; H, 6.80; N, 11.23; S, 12.38. Calc. for l(1H)/ppm CDCl3: 1.01 (t, 3H), 1.73 (m, 2H), 3.65 (m,
C12H16N2O2S: C, 57.11; H, 6.40; N, 11.10; S, 12.71%. 2H), 7.54 (d, 2H), 7.85 (d, 2H), 9.00 (br s, 1H), 10.64
l(1H)/ppm CDCl3: 1.00 (t, 3H), 1.73 (m, 2H), 3.65 (m, (br s, 1H). l(13C)/ppm CDCl3: 11.42, 21.50, 47.60,
2H), 3.86 (s, 3H), 6.96 (d, 2H), 7.80 (d, 2H), 9.00 (br s, 101.22, 128.75, 131.21, 138.38, 166.14, 179.51.
1H), 10.79 (br s, 1H). l(13C)/ppm CDCl3: 11.44, 21.57,
47.53, 55.57, 114.35, 123.74, 129.54, 163.82, 166.24,
179.90. 2.1.8. N-(n-Propyl) -N%-(4 -nitrobenzoyl) -thiourea H2L 8,
C11H13N3O3S
2.1.3. N-(n-Propyl) -N%-(4 -t-butylbenzoyl) -thiourea Product isolated in 27.4% yield, m.p. 109–110 °C.
H2L 3, C15H22N2OS Anal. Found: C, 49.34; H, 5.12; N, 10.75; S, 11.93.
Product isolated in 72.8% yield, m.p. 86– 88 °C. Anal. Calc. for C11H13N3O3S: C, 49.42; H, 4.94; N, 15.72; S,
Found: C, 64.33; H, 8.27; N, 10.16; S, 11.36. Calc. for 12.00%. l(1H)/ppm CDCl3: 1.02 (t, 3H), 1.74 (m, 2H),
C15H22N2OS: C, 64.69; H, 7.98; N, 10.06; S, 11.52%. 3.65 (m, 2H), 8.03 (d, 2H), 8.35 (d, 2H), 9.33 (br s, 1H),
l(1H)/ppm CDCl3: 1.02 (t, 3H), 1.33 (s, 9H), 1.74 (m, 10.57 (br s, 1H). l(13C)/ppm CDCl3: 11.41, 21.47,
2H), 3.66 (m, 2H), 7.50 (d, 2H), 7.76 (d, 2H), 8.97 (br 47.68, 124.18, 128.86, 137.37, 150.58, 165.08, 179.25.
s, 1H), 10.78 (br s, 1H). l(13C)/ppm CDCl3: 11.45,
21.56, 31.01, 35.18, 47.57, 126.08, 127.31, 128.84, 2.1.9. trans-[Pt(py)2Cl2]
157.49, 166.73, 179.85. Anal. Found: C, 28.10; H, 2.63; N, 6.45. Calc. for
C10H10Cl2N2Pt: C, 28.31; H, 2.38; N, 6.60%. l(1H)/
2.1.4. N-(n-Propyl) -N%-(4 -bromobenzoyl) -thiourea ppm, CD3OD + D2O (75+ 25): 7.46–7.50 (m, m-py,
H2L 4, C11H13BrN2OS 4H), 7.96 (tt, p-py, J1 = 6.2 Hz, J2 = 1.4 Hz, 2H),
Product isolated in 54.4% yield, m.p. 101– 103 °C. 8.77–8.80 (m, o-py, 4H); 3J(195Pt – 1H) couplings very
Anal. Found: C, 43.95; H, 4.21; N, 8.90; S, 10.26. Calc. broad due to CSA broadening at high field.
for C11H13BrN2OS: C, 43.86; H, 4.36; N, 9.30; S,
10.65%. l(1H)/ppm CDCl3: 1.01 (t, 3H), 1.73 (m, 2H),
3.65 (m, 2H), 7.63 (d, 2H), 7.70 (d, 2H), 9.01 (br s, 1H), 2.1.10. trans-[Pt(py)2(H2L 1S)2]Cl2
10.63 (br s, 1H). l(13C)/ppm CDCl3: 11.43, 21.52, Not isolated; Prepared in situ by reaction of 1 equiv.
47.61, 128.64, 128.94, 130.67, 132.42, 165.94, 179.53. of with 2 equiv. of H2L1 in CD3OD + D2O: l(1H)/ppm
CDCl3: 7.53–7.58 (m, m-ph, 4H), 7.66 (tt, p-ph, J1 =
2.1.5. N-(n-Propyl) -N%-(4 -chlorobenzoyl) -thiourea 7.2 Hz, J2 = 1.6 Hz, 2H), (7.65–7.69 (m, m-py, 4H),
H2L 5, C11H13ClN2OS 7.89– 7.92 (m, o-ph, 4H), 7.99 (tt, p-py, J1 = 7.0 Hz,
Product isolated in 66.9% yield, m.p. 86– 88 °C. Anal. J2 = 1.4 Hz, 2H), 8.56–8.59 (m, o-py, 4H), 9.9 (br, NH,
Found: C, 51.76; H, 5.07; N, 10.84; S, 12.26. Calc. for 2H), 11.0 (br, NH, 2H).
C11H13ClN2OS: C, 51.45; H, 5.11; N, 10.91; S, 12.49%.
l(1H)/ppm CDCl3: 1.01 (t, 3H), 1.73 (m, 2H), 3.65 (m, 2.2. Kinetic studies
2H), 7.48 (d, 2H), 7.76 (d, 2H), 9.02 (br s, 1H), 10.66
(br s, 1H). l(13C)/ppm CDCl3: 11.38, 21.47, 47.56, All kinetic measurements were carried out with solu-
128.81, 129.38, 130.15, 140.02, 165.76, 179.52. tions (water:methanol 25:75) containing 0.03 mol dm − 3
tetrabutyl–ammonium perchlorate (v= 0.03) at a tem-
2.1.6. N-(n-Propyl) -N%-(4 -heptoxybenzoyl) -thiourea perature of 30(90.1) °C. Deionised water and AnalaR
H2L 6, C18H28N2O2S grade methanol, freshly distilled prior to use, was used.
Product isolated in 95.2% yield, m.p. 84– 86 °C. Anal. The concentration of the substrate, trans-[Pt(py)2Cl2],
Found: C, 64.55; H, 8.50; N, 8.53; S, 9.49. Calc. for was set at 2.0× 10 − 6 mol dm − 3. The concentration of
C18H28N2O2S: C, 64.23; H, 8.40; N, 8.32; S, 9.53%. the ligand solutions were varied between 4.0×10 − 5
l(1H)/ppm CDCl3: 0.88 (t, 3H), 1.01 (t, 3H), 1.25–1.50 mol dm − 3 and 2.0× 10 − 4 mol dm − 3.
 K.R. Koch et al. / Inorganica Chimica Acta 331 (2002) 136–142 139

The kinetics of the reactions was followed by period- monitored at various reagent concentrations; this is
ically recording the UV– Vis-spectra of the reaction most conveniently done by means of UV-spectroscopy.
mixture in 1 cm quartz cells in the wavelength range In the presence of an excess of the nucleophile Y, kobs
250–400 nm, using a Hewlett Packard 8452A diode-ar- can be determined experimentally assuming a pseudo
ray spectrophotometer, capable of scanning this spec- first-order rate law. In general, for each nucleophile in
tral range within 0.1 s. Spectra were recorded this study, plots of ln(Z) against time (where Z=([Xo]/
immediately after mixing appropriate reagent solutions ([Xo]− [P]), [Xo]= substrate concentration at t=0) re-
(at 30 °C) directly in the quartz cells. Changes in producibly gave straight lines confirming pseudo-first-
absorbance were measured at 320 nm for reactions with order conditions, as shown typically for a fixed excess
the ligands H2L1 – 8. This wavelength was chosen since it concentration of H2L2 in Fig. 1.
afforded the highest degree of sensitivity and reproduci- Optimum conditions for determining values of kobs
bility for these experiments. In order to control the with respect to easily detectable changes in the UV-
concentration and ensure the purity of the substrate, spectra were found using a substrate (trans-[Pt(py)2Cl2])
the UV-spectrum of the substrate solutions were concentration of 2.0×10 − 6 M, with nucleophile
recorded before every kinetic experiment, and com- (H2Ln) concentrations ranging from 4.0× 10 − 5 to
pared to the spectrum of a freshly prepared solution of 2.0× 10 − 4 M. The rate of reaction was monitored by
the substrate. The maximum absorbance at the chosen the formation of the product (P), resulting in an in-
wavelength for the kinetic experiment was determined crease in absorbance at 320 nm for all nucleophiles
for each ligand after a time=‘infinity’ experiment, studied. 1H NMR spectroscopy showed that even in the
representing the completed substitution reactions. This presence of a large excess of nucleophile (H2Ln), only
absorbance maximum was found to be reproducible, the replacement of chloride ions took place, the pyri-
and independent of the ligand concentration. dine molecules remaining bound to Pt(II) throughout
All kobs values were determined from an average of at these experiments.
least two repeat experiments. Care was taken to esti- Values for kobs can be determined from the slopes of
mate the relative errors of k1 and k2 at 95% confidence these plots and then used to determine k1 and k2 by
from the linear regression analysis of [nucleophile] ver- plotting kobs against varying [Y]. Fig. 2 shows the
sus kobs plots using recommended statistical procedures resulting graphs of all the nucleophiles investigated at
[20]. five different concentrations each (4× 10 − 5, 6×10 − 5,
8 × 10 − 5, 1× 10 − 4, 2× 10 − 4 M), while Table 1 lists
the experimental values obtained for k1 and k2 from
3. Results and discussion linear regression analysis, together with an estimate of
the standard error of k2.
The mechanism of nucleophilic substitution reactions As expected, the values of k1 are essentially indepen-
at square planar platinum(II) complexes of the type dent of the nature and concentration of the nucleophile
trans-[Pt(py)2Cl2] has been extensively studied and is (H2Ln), and are found to vary only slightly within an
well understood [16,17]. In general for such nucleophilic estimated error range of 91.8×10 − 4 s − 1. Our values
substitution reactions, two simultaneous reaction paths for k1, representing the solvent pathway of the substitu-
have to be taken into consideration, one being first tion reaction are somewhat higher than the literature
order (k1) reflecting the nucleophilic attack of the sol- value of 1× 10 − 5 s − 1 [17], ranging in our case from 9
vent on the platinum(II) complex, the other being sec- to 11×10 − 4 s − 1, which if significant, may reasonably
ond order (k2) ascribed to a rapid substitution by the be ascribed to the use of mixed 25:75 water–methanol
entering group Y. This pseudo-first-order nucleophilic
reaction pathway of the solvent is therefore indepen-
dent of the reagent concentration, while the second
order reaction pathway consists of a direct bimolecular
reaction between Y and the substrate trans-[Pt(py)2Cl2].
In general, under conditions of an excess of the nucle-
ophile Y, the kinetics of this reaction may be described
by a pseudo-first-order rate equation for nucleophilic
attack of the reagent Y on the substrate [17]
kobs =k1 +k2[Y] (1)
This rate law requires that a sufficient excess of
Fig. 1. Typical plot of ln(Z) against time for N-(n-propyl)-N%-(4-
reagent Y, at fixed substrate concentration be used, and methoxybenzoyl)-thiourea with [H2L2]= 4.0 ×10 − 5 M and [sub-
that either the decreasing concentration of substrate, or strate] = 2.0 ×10 − 6 M, at 30 °C confirming pseudo-first-order
the increasing concentration of product (P) has to be kinetics.
140 K.R. Koch et al. / Inorganica Chimica Acta 331 (2002) 136–142

Fig. 2. Rates of reaction of different nucleophiles with trans-[Pt(py)2Cl2] in water/methanol (25:75 v/v, v =0.03) at 30 °C as a function of
nucleophile concentrations (tu = thiourea, and numerals indicate different ligands corresponding to H2Ln in Table 1).

solvent system with v =0.03. The bimolecular constant guishable from that obtained by Belluco et al. [17],
k2, on the other hand, shows small but distinct differ- which lends confidence to the experimental values of k2
ences for the various nucleophiles studied, ranging from in Table 1. Shown also in Table 1, are the nucleophilic
6.7 9 0.5 to 12.19 1.3 s − 1 M − 1. Moreover, our value reactivity constants, npt, calculated from npt =log(k2/
for k2 =6.09 0.4 s − 1 M − 1 for thiourea is indistin- k1)o for the series of nucleophiles studied here. Our
value for thiourea is found to be 3.7890.08, which is
significantly smaller than the reported value of 5.78
Table 1
Experimental rate constants k1, and k2 as well as the nucleophilic
[17]. This discrepancy in npt is not too surprising, since
constant, npt for thiourea and H2Ln in methanol/water (75:25 v/v npt strongly depends on the value of k1, and our values
v =0.03 (t-butyl-ammonium perchlorate)) at 30 °C of k1 were found to be consistently somewhat larger
than the literature value, presumably due to the differ-
Ligand R k1 (s−1) a k2 (s−1 mol−1) b npt c ing solvent composition and ionic strength in our ex-
H2L1 H 0.0010 12.1 91.3 4.08 9 0.09
periments. Moreover if we use the literature value of
H2L2 CH3O 0.0010 11.791.5 4.079 0.09 k1 = 1×10 − 5 s − 1, we obtain npt = 5.789 0.5, assuming
H2L5 Cl 0.0011 10.4 90.5 4.02 9 0.08 a relative error of 50% in k1, in good agreement with
H2L6 C7H15O 0.0011 9.1 90.8 3.96 9 0.09 the literature. In any case, it is clear from our data that
H2L3 t-but 0.0011 8.99 1.2 3.919 0.10 the relative nucleophilic reaction constants npt of the
H2L7 I 0.0011 8.4 9 0.7 3.929 0.09
H2L4 Br 0.0009 7.39 0.8 3.86 9 0.09
para-substituted N-(n-propyl)-N%-benzoylthioureas are
H2L8 NO2 0.0009 6.79 0.5 3.83 9 0.08 very similar to that of a simple thiourea, corroborating
Thiourea 0.0009 6.090.4 3.78 9 0.08 the premise that these ligands closely resemble thiourea
from the nucleophilic substitution at Pt(II) point of
a
Relative error estimated 18% at 95% confidence from linear view.
regression analysis uncertainty of intercept.
b
95% confidence interval.
It is also evident from data in Table 1 that the
c
Maximum uncertainty estimate based on error propagation the- para-benzoyl substituent R in these molecules does
ory. have a small, but detectable influence on the npt con-
 K.R. Koch et al. / Inorganica Chimica Acta 331 (2002) 136–142 141

stants of these molecules. This is remarkable in view of Acknowledgements

the eight intervening bonds between the sulphur donor
atom and the para-benzoyl substituent R. Inspection of Financial support from the National Research Foun-
the npt values for the nucleophiles H2Ln, shows that dation, Angloplatinum Ltd. and the THRIP pro-
within the margin of experimental error, roughly three gramme are gratefully acknowledged. O.S. thanks the
groupings emerge. Thiourea gives the lowest npt values, Federal Republic of Germany for a ‘DAAD Dok-
followed closely by the nitro- and bromo-substituted torandenstipendium’. J.M. thanks Ranger Oil
ligands; the second grouping consists of the iodo-, (Namibia) Ltd. and TUCSIN Namibia for financial
tert-butyl- and C7H15O-substituted ligands, followed by support.
the chloro- methoxy- and unsubstituted N-(n-propyl)-
N%-benzoylthiourea. Although the relative differences of
the npt values are small, the three groupings of nucle- References
ophiles appear to be significant after careful consider-
ation of the estimated errors associated with these [1] K. Neucki, Ber. 6 (1873) 598.
[2] L.B. Douglass, F.B. Dains, J. Am. Chem. Soc. 56 (1934) 719.
values. These groupings can be seen graphically from a [3] (a) S.N. Banerjee, A.C. Sukthankar, J. Ind. Chem. Soc. 39
plot of kobs versus [Y], shown in Fig. 2. (1962) 197;
Nevertheless, any correlation between known macro- (b) A. Livingstone, Q. Rev. 19 (1965) 4.
scopic or electronic properties of the substituents R and [4] (a) L. Beyer, E. Hoyer, H. Hennig, R. Kirmse, H. Hartmann,
the corresponding npt-values is poor. Neither the Ham- J. Liebscher, J. Prakt. Chem. 317 (1975) 829;
(b) L. Beyer, E. Hoyer, H. Hartmann, Z. Chem. 21 (1981) 81.
mett constants of the substituents [16], nor the order of
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