KINETICS AND MECHANISM OF ADDITION OF TERTIARY PHOSPHINES AND PHOSPHITES TO THE CARBONYL(ETA(5)-CYCLOPENTADIENYL)-(ETA-ETHENE)IRON CATION

Kinetic studies of the addition of a range of tertiary phosphine and p hosphite nucleophiles PR(3) to the cation [Fe(cp)(CO)(2)(eta-C2H4)](+) 1 (cp = eta(5)-C5H5) [equation (i)] revealed the general rate law, Ra te = k(1)[Fe][PR(3)]. The second-order rate constants k(1) decrease ma rkedly down the order P(C(6)H(4)OMe-2)(3) > PBu(3)(n) > P(C(6)H(4)OMe- 4)(3) > P(C(6)H(4)Me-4)(3) > P(C(6)M(4)Me-4)Ph(2) > PPh(3) > P(C2H4CN- 2)Ph(2) > P(C2H4CN-2)(3) > P(C6H4Cl-4)(3) > P(OBu(n))(3). This reactiv ity order parallels that-of decreasing electron availability at the ph osphorus centre, as shown quantitatively by the good correlation betwe en log k(1) and the Tolman Sigma(chi) values. An excellent fit to the Hammett and Bronsted equations is also observed for reaction (i) with the nucleophiles P(C(6)H(4)X-4)(3). The moderate Bronsted slope alpha of [Fe(cp)(CO)(2)(eta-C2H4)](+) + PR(3) --> [Fe(cp)(CO)(2)(C(2)H(4)PR( 3))](+) (i)0.46 establishes the importance of phosphine basicity in de termining nucleophilicity towards the ethene ligand in cation 1. These results, together with the large negative entropy of activation with PPh(3) (Delta S-1(double dagger) = -103 J K-1 mol(-1)), are interprete d in terms of direct addition (k(1)) of the phosphorus nucleophiles to the ethene ligand in 1 and suggest a transition state in which there is build-up of positive charge on the phosphorus centre and considerab le phosphorus-carbon bond formation.