Dissociative phosphine exchange for cyclopentadienylmolybdenum(III) systems. Bridging the gap between Werner-like coordination chemistry and low-valent organometallic chemistry

Several phosphine exchange processes on 17-electron CpMoCl(2)(PR(3))(2) systems have been investigated. The exchange of two PPh(3) ligands with either two PMe(3) ligands or with Ph(2)PCH(2)CH(2)PPh(2) (dppe) is complete within a few minutes at - 80 degrees C. Equally fast is the exchange of two PEt(3), ligands with two PMe(3) ligands. On the other hand, the exchange of two PEt(3) ligands with dppe is much slower (t(1/2) approximate to 15 min to a few hours at r.t.), with excess dppe accelerating the exchange and free PEt, retarding it. The self-exchange reaction of PMe(3) is extremely slow (less than 25% exchange at r.t. in 6 h at r.t.) and an analysis of the initial rate of this reaction shows a two-term rate law with one [PMe(3)]-dependent and one independent term. Finally, PMe(3) self-exchange on Cp*MoCl2(PMe(3))(2) proceeds over one order of magnitude faster than for the corresponding Cp system, with a substantially [PMe(3)]-independent rate law. All these data are indicative of a dominant dissociative exchange mechanism involving rupture of the Mo-PR(3) bond in the slow step and formation of a 15-electron intermediate. The rate of phosphine dissociation qualitatively correlates with the Mo-P distance in the 17-electron starting complex. Only for the Cp-MoCl2(PMe(3))(2) system is phosphine dissociation sufficiently slowed down so that the alternative associative exchange pathway becomes competitive. Possible reasons for a low activation barrier in these dissociative exchanges are discussed.