The Reaction of the Ruthenium(II) Indenyl Complex [Ru(n5-C9H7)[k3(P,C,C)-PPh2(CH2CH=CH2)](PPh3)][PF6] with Terminal Alkynes. Mechanisms of 1-Alkyne to n1-Vinylidene Transformation and Kinetic Detection of Hemilability of the Allylphosphine Ligand

The reaction of the complex [Ru(eta(5)-C9H7){(3)(K)(P,C,C)-PPh2(CH2CH=CH2)}(PPh3)][PF6] (1) with p-XC(6)H(4)CdropCH (X = H, Cl) yields the transient and observable vinylidene species (Ru(eta(5)-C9H7){(1)(K)(P)-PPh2(CH2CH=CH2)}(PPh3)(=C=CH(p-XC6H4))](+), which react further by an intramolecular [2 + 2] cycloaddition process, forming bicyclic alkylidene compounds. The formation of the vinylidene intermediates, associated with a change of the binding mode of the allylphosphine ligand from K-3(P,C,C) to monodentate K-1(P) coordination, has been investigated by kinetic measurements carried out in chloroform-d at 38 degreesC. Plots of first-order k(obs) values vs concentration of arylalkyne are linear with a positive intercept on the y axis. The reaction proceeds via two parallel pathways, one which is first order in complex 1 and first order in arylalkyne, second order overall, and one which is zero order in arylalkyne, overall first order. The second-order pathway implies rate-determining nucleophilic attack of the arylalkyne on complex 1, k(2) = [5.5(+/-0.4)] x 10(-4) (X = H) and [2.8(+/-0.3)] x 10(-4) M-1 S-1 (X = Cl) being the corresponding rate constants, associated with displacement of the allylic double bond or a haptotropic shift of the indenyl ring, The first-order pathway involves rate-determining formation of a transient 16-electron intermediate arising from complex 1 by reversible decoordination of the allylic double bond and rapid reaction with the incoming arylalkyne. The rate of this route is independent of the concentration and nature of the alkyne (k(1) = [9.5(+/-2.5)] x 10(-5) s(-1) for X = H; [6.9(+/-1.5)] x 10(-5) s(-1) for X = Cl) and corresponds to the rate of formation of the intermediate species [Ru(eta(5)-C9H7){(1)(K)(P)PPh2(CH2CH=CH2)}(PPh3)](+). The vinylidene complexes [Ru(eta(5)-C9H7)(C=C=RR'){(1)(K)(P)PPh2(CH2CH CH2)}(PPh3)][BF4] (R' = H, R = Ph (3a), p-MeC6H4 (3b), P-ClC6H4 (3c); R' = Me, R = Ph (4a), p-MeC6H4 (4b)) have been independently synthesized by electrophilic attack of HBF4 or MeSO3CF3 on the alkynyl derivatives [Ru(eta(5)-C9H7)(CdropCR){K-1(P)-PPh2(CH2CH=CH2)}(PPh3)] (R = Ph (2a), p-MeC6H4 (2b), p-ClC6H4 (2c)), obtained in turn from [Ru(eta(5)-C9H7)Cl{(1)(K)(P)-PPh2(CH2CH=CH2)}(PPh3)].