The reaction of HCl with [MoH3(CCBu(t))(Ph(2)PCH(2)CH(2)PPh(2))(2)]: Enforced protonation at the metal gives an alkyne

The reaction of anhydrous HCl with [MoH3(CCBu(t))(dppe)(2)] in tetrahydrofuran produced [MoH2Cl2(dppe)(2)] together with dihydrogen and Bu(t)CCH. Initial protonation of [MoH3(CCBu(t))(dppe)(2)] generates an equilibrium mixture of [MoH3(CCHBu(t))(dppe)(2)](+) and [MoH4(CCBu(t))(dppe)(2)](+), in which the predominant species is the vinylidene complex. However, it is [MoH4(CCBu(t))(dppe)(2)](+) which is the more reactive of the two protonated species, since intramolecular reductive coupling of a hydride and alkynyl ligand in this complex is more rapid than the analogous coupling of two hydride ligands in [MoH3(CCHBu(t))(dppe)(2)](+), and thus the reaction proceeds along the pathway involving [MoH3(eta(2)-CHCBu(t))(dppe)(2)](+). Further protonation of the metal in [MoH3(eta(2)-CHCBu(t))(dppe)(2)](+) labilises the site to dissociation of first dihydrogen, then Bu(t)CCH, and subsequent binding of chloride produces [MoH2Cl2(dppe)(2)]. The general factors associated with the protonation of alkynyl complexes at either (i) the metal to give an alkyne or (ii) the alkynyl ligand to give an alkylidyne complex are discussed.