S. Otto et al. / Inorganic Chemistry Communications 9 (2006) 764–766
765
hand [9], their UV/vis spectra in chloroform differ suffi-
ciently to enable a spectrophotometric equilibrium study.
However, in UV/vis experiments, the determination of
the total ethene concentration under exactly the same solu-
tions used to collect the absorbance spectra is complicated.
This problem was circumvented by use of the simple device
shown in Fig. 1, which was constructed from a 5 mm NMR
tube and a 1.00 cm quartz cuvette joined by a tight-fitting
Teflon cuvette stopper with a drilled hole. Ethene was bub-
bled through a CDCl3 solution containing a known con-
centration of 1, as well as a small amount of C6H6 used
as internal standard for the determination of free ethene
by integration of the NMR signals. Enough ethene was
added to ensure virtually complete conversion to 2, the
cuvette and the NMR tube were then connected and sha-
ken to allow the equilibrium to be established (s to min).
Fig. 2. Spectral changes for the bridge cleavage reaction of trans-
[PtCl2(C2H4)]2 (1) ([Pt] = 1.60 mmol dmꢀ3) by C2H4 in CDCl3 at 298 K.
The inset shows the least-squares fit at 450 nm yielding K = 6.8
0.6 molꢀ1 dm3.
1
Next, the H NMR spectrum (non-spinning sample!) was
recorded, enabling the ethene concentration to be calcu-
lated, followed by collection of the UV/vis spectrum. The
NMR tube/cuvette was opened for a short while to allow
some ethene to escape, then reassembled and shaken. The
procedure was repeated until enough NMR/absorbance
data were collected for a satisfactory determination of the
equilibrium constant by use of the appropriate equations
[10]. The spectral changes and least-squares fit of the data
are presented in Fig. 2, yielding K = 6.8 0.6 molꢀ1 dm3.
The loss of solvent through evaporation during the exper-
iment was negligible.
crystal structure of 3 has been documented previously in
a preliminary report in Chinese, a re-determination was
considered justified [3]. It is concluded that 3 is significantly
less soluble/thermodynamically more stable than 2 under
these conditions, resulting in complete and slow displace-
ment of the cis/trans equilibrium towards 3. The cis orien-
tation is obvious, and the Pt–C and Pt–Cl bonds compare
favourably with those found in Zeise’s anion [12], cis-[PtCl2
(CH2CHC6H5)2] [2] and [PtCl2(COD)] [13]. The 1H and 13
C
In an attempt to isolate 2, a chloroform solution of 1
was slowly evaporated under an atmosphere of ethene. Sur-
prisingly, white crystals of cis-[PtCl2(C2H4)2], 3, suitable
for X-ray analysis (Fig. 3) were obtained [11]. Since the
NMR spectra [14] of 3 in CDCl3 display well-defined trip-
lets (195Pt coupling) observed in the presence of free ethene,
indicating significantly lower reactivity with respect to eth-
ene exchange as compared to 1 and 2. Chloroform can be
replaced by dichloromethane, which yielded the same prod-
ucts (2 and 3) but facilitated the evaporation process. Solu-
tions of 3 in coordinating solvents such as methanol,
acetone and acetonitrile yielded a black precipitate of
platinum after one day, indicating limited stability under
Fig. 3. Molecular diagram of cis-[PtCl2(C2H4)2] showing the numbering
scheme and thermal ellipsoids at 30% probability level. Selected geomet-
rical parameters include distances Pt–Cl(1) 2.318(3), Pt–Cl(2) 2.308(2), Pt–
C(11) 2.177(12), Pt–C(12) 2.149(10), C(11)-C(12) 1.384(14), Pt–C(21)
˚
2.166(10), Pt–C(22) 2.159(10), C(21)-C(22) 1.390(16) A; angles Cl(1)–Pt–
Fig. 1. Combination UV/vis quartz cuvette/NMR tube for determination
of the bridge-splitting stability constant of trans-[PtCl2(C2H4)]2 (1) by
ethene. Positions indicated for: (a) NMR and (b) UV/vis measurements.
Cl(2) 89.64(9), C(11)–Pt–C(21) 87.9(4), C(12)–Pt–C(22) 87.6(4), C(11)–Pt–
Cl(1) 160.2(3), C(21)–Pt–Cl(1) 162.3(3)ꢁ and torsion angles Cl(1)–Pt–
C(21)–C(22) 88.9(7) and Cl(2)–Pt–C(11)–C(12) 92.6(7)ꢁ.
Otto, Stefanus
