MECHANISMS OF ORGANOMETALLIC SUBSTITUTION-REACTIONS .3. KINETICS OF THE REACTION OF ORGANONITRILES WITH TRICARBONYL(CYCLOHEPTATRIENE) COMPLEXES OF METALS IN GROUP-VI

A kinetic study of the displacement of cyclohepta-triene from [(η6-C7H8)M(CO)3] (M = Cr, 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]2. With tungsten kinetic behaviour intermediate between these two extremes is observed. These results are rationalised by a general mechanism involving stepwise attack by two benzo-nitrile nucleophiles via the steady-state intermediate [(η4-C7H8)M(CO)3(RCN)]. Attack by a third benzo-nitrile rapidly leads to the products. Analogous kinetic studies of the reaction of [(η6-C7H8)M(CO)3] (M = Cr, Mo) with o-toluo-nitrile 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-3 to second-order at higher [RCN]. Electronic factors are seen to be more important than steric considerations in these reactions. © 1979.