PROTONATION OF S-[MO(ETA(2)-MECCH)(2)(PH(2)PCH(2)CH(2)PPH(2))(2)] - MECHANISM OF FORMATION OF TRANS-[MOX(CHCHME)(PH(2)PCH(2)CH(2)PPH(2))(2)] (X=CL OR BR) OR TRANS-[MOF(CCH(2)ME)(PH(2)PCH(2)CH(2)PPH(2))(2)]

The reaction between s-[Mo(eta(2)-MeCCH)(2)(Ph(2)PCH(2)CH(2)PPh(2))(2) ] and an excess of anhydrous HX (X = Cl or Br) in tetrahydrofuran give s trans-[MoX(CHCHMe)(Ph(2)PCH(2)CH(2)PPh(2))(2)] and the evolution of 1 mol equivalent of MeCCH. Mechanistic studies indicated that initial protonation of s-[Mo(eta(2)-MeCCH(2))(Ph(2)PCH(2)CH(2)PPh(2))(2)] occu rs at a propyne ligand to form the vinyl species. HMe)(eta(2)-MeCCH)(P h(2)PCH(2)CH(2)PPh(2))(2)](+), and at low concentrations of acid rate- limiting dissociation of the other trans-propyne, followed by attack o f halide ion at the molybdenum, produces trans-[MoX(CHCHMe)(Ph(2)PCH(2 )CH(2)PPh(2))(2)]. At high concentrations of acid further rapid proton ation of the vinyl ligand occurs to give Me)(eta(2)-MeCCH)(Ph(2)PCH(2) CH(2)PPh(2))(2)](2+). This second protonation further labilises the tr ans-propyne which is lost in the rate-limiting step and subsequent att ack of halide gives ans-[Mo(CHCH(2)Me)(Ph(2)PCH(2)CH(2)PPh(2))(2)](+). Dissociation of a proton gives the product, trans-[MoX(CHCHMe)(Ph(2)P CH(2)CH(2)PPh(2))(2)]. The reaction between s-[Mo(eta(2)-MeCCH)(2)(Ph( 2)PCH(2)CH(2)PPh(2))(2)] and HBF . OEt(2) gives a mixture of trans-[Mo F(CHCHMe)(Ph(2)PCH(2)CH(2)PPh(2))(2)] and the alkylidyne complex: tran s-[MoF(CCH(2)Me)(Ph(2)PCH(2)-CH(2)PPh(2))(2)]. The alkylidyne complex has been isolated pure by fractional crystallisation. The-factors whic h discriminate between the formation of the vinyl and alkylidyne speci es are discussed.