12 D. D. Wick, K. A. Reynolds and W. D. Jones, J. Am. Chem. Soc.,
since they both predict a linear dependence of kobs on [CO]. If a
dissociative and associative mechanism were operating simul-
taneously then eqn. (A6) would be valid where kCO = ka ϩ k1Ј.
Therefore a plot of kobs versus [CO] would yield a linear plot
1999, 121, 3974.
13 There has been one report of alkane co-ordination to a cage
porphyrin complex (D. R. Evans, T. Drovetskaya, R. Bau, C. A.
Reed and P. D. W. Boyd, J. Am. Chem. Soc., 1997, 119, 3633).
However, it appears that the alkane is trapped inside the cage in the
crystal structure, with no evidence of co-ordination in solution
(D. R. Evans, personal communication).
with slope equal to kCO
.
kobs = [ka ϩ k1Ј][CO] = kCO[CO]
(A6)
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To separate ka and k1 requires experiments to be performed at
different alkane concentrations. In alkane solution it is impos-
sible to vary the solvent concentration in this way. However,
such experiments have previously been performed in liquid
krypton at low temperature to elucidate the reaction mechan-
ism for the decay of Ni(CO)3(N2),50 and in supercritical CO2 to
probe the reactivity of W(CO)5(CO2).51
To analyse our data for the decay of M(η5-C5R5)(CO)2-
(alkane) we have considered two extreme cases, where either
ka ӷ k1Ј or k1Ј ӷ ka. If k1Ј ӷ ka, then we expect the ∆H‡ value
to approximate to the M–alkane BDE since the rate determin-
ing step is the breaking of the M–alkane bond. We would also
expect ∆S‡ to be positive. If ka ӷ k1Ј, then we expect the ∆H‡
value to be less than the M–alkane BDE and ∆S‡ to be
negative.
In the absence of an excess of CO, M(η5-C5R5)(CO)2(alkane)
could react either with photoejected CO to reform M(η5-
C5R5)(CO)3 or with parent to form the known45 dimer M(η5-
C5R5)2(CO)5. The reaction of M(η5-C5R5)(CO)2(alkane) with
photoejected CO will display second-order kinetics irrespective
of whether the reaction is dissociative or associative since the
concentrations of CO and M(η5-C5R5)(CO)2(alkane) are
the same. The reaction of M(η5-C5R5)(CO)2(alkane) with par-
ent to form M2(η5-C5R5)2(CO)5 will occur via pseudo first-order
kinetics since the concentration of M(η5-C5R5)(CO)3 is much
greater than that of M(η5-C5R5)2(CO)2(alkane). Our low tem-
perature data are fit more accurately by first order kinetics,
therefore we have used an exponential fit to allow comparison
of the stabilities of the different alkane complexes.
31 F. P. A. Johnson, M. W. George, V. N. Bagratashvili, L. N.
Vereshchagina and M. Poliakoff, Mendeleev Commun., 1991, 1,
26.
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37 J. W. Chambers, A. J. Baskar, S. G. Bott, J. L. Atwood and
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Acknowledgements
We would like to thank Professor J. J. Turner for helpful discus-
sions, and Mr. M. Guyler and Mr. K. Stanley for their technical
support. We thank the referees for helpful suggestions and
Mr. K. Dost for his help in obtaining mass spectra. We are
grateful to the EPSRC, Nicolet Instruments Ltd., Isle of Man
Government (DCG) and the University of Nottingham (GIC)
for financial support. We are particularly grateful to the EPSRC
Lasers for Science Facility laser loan pool for the loan of the
Spectron SL805G Nd:YAG laser.
38 J. Makranczy, K. Begyery-Balog, L. Rusz and L. Patyi, Hung. J. Ind.
Chem., 1976, 4, 269.
39 IUPAC Solubility Data Series, Pergamon Press, Oxford, 1990,
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40 A. I. Cooper, PhD Thesis, University of Nottingham., 1994.
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43 X.-Z. Sun, S. M. Nikiforov, J. Yang, C. S. Colley and M. W. George,
Appl. Spectrosc., submitted.
44 T. E. Bitterwolf, K. A. Lott, A. J. Rest and J. Mascetti, J. Organomet.
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46 G. E. Ball, personal communication.
47 J. K. Klassen, M. Selke, A. A. Sorensen and G. K. Yang, J. Am.
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