The novel cyaphide complex trans-[Ru-(dppe)(2)Me(C P)] is obtained in excellent yields and exhibits the first instance of controlled reactivity of any terminal cyaphide complex. Its treatment with ZnX2/PPh3 effects selective metathesis of the methyl moiety to afford the unprecedented halocyaphide complexes trans-[Ru-(dppe)2(X)(C P)] (X = Cl, Br, I), which are structurally characterized (X = CI, Br). Exemplified with the trans -bromide, these compounds are susceptible to substitution of the halides by nucleophilic reagents-illustrated with Me2Mg-and also readily undergo halide abstraction by TIOTf to afford the first hypocoordinate cyaphide complex, viz., [Ru(dppe)(2)(C P)]center dot OTf, which is isolable in bulk and exhibits good stability. NMR spectroscopic and crystallographic data reveal the latter to adopt a square-pyramidal geometry with an accessible coordinate vacancy, which is susceptible to the addition of nucleophiles. This is illustrated analytically by reactions with Me2Mg and LiC CPh and with its facile bulk carbonylation to afford trans-[Ru(dppe)(2)(CO)(C P)](+).