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Mechanisms of organometallic substitution reactions. Part 3.a Kinetics of the reaction of organonitriles with tricarbonyl(cycloheptatriene) complexes of metals in group VI

โœ Scribed by Marian Gower; Leon A.P. Kane-Maguire

Publisher
Elsevier Science
Year
1979
Tongue
English
Weight
463 KB
Volume
37
Category
Article
ISSN
0020-1693

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โœฆ Synopsis

A kinetic study of the displacement of cycloheptatriene from I($-C,H8)M(CO)J (M = 0, MO. W) by benzonitrile shows unusual changes in rate law down the triad. While the chromium complex obeys the second-order rate law Rate = k[complex] [RCN] , the molybdenum species follows the third-order law Rate = k[complex] [RCN]?. With tungsten kinetic behaviour intermediate between these two extremes is observed. These results are rationalised by a general mechanism involving stepwise attack by two benzonitrile nucleophiles via the steady-state intermediate [(q4-C,Hs)M(CO)s(RCN)].

Attack by a third benzonitrile rapidly leads to the products. Analogous kinetic studies of the reaction of [(q'-C,H,)M(CO),] (M = 0, MO) with o-toluonitrile and 1,3,5-trimethylbenzonitrile give more complex behaviour. While second-order kinetics are uniformly observed with chromium, the rate law with molybdenum varies from third-order at [RCN] < 0.2 mol dm" to second-order at higher [RCfl .

Electronic factors are seen to be more important than steric considerations in these reactions.

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