The BPh4-salt of the carbyne complex [Cp(CO)2RmCToll+, 1, undergoes net [2 + 21 cycloaddition of cis-azobenzene, cis-azotoluene, and benzo[cl cinnoline to give new complexes 5a, 5b, and 7, respectively, which possess four-membered metallacycles. The BCld-salt of complex 1 initially reacta with cis-azobenzene and cis-azotoluene to give metallacycles Sa and 5b, but upon stirring these complexes abstract chloride from the BC14-anion and insert a CO ligand into the Re-carbon bond of the metallacycle to give new complexes possessing fivemembered metallacycles. With benzo[clcinnoline, the BC4-salt of complex 1 reacta to give initially the metallacycle 7 formed from the B P k -salt, but upon stirring, the benzo[clcinnoline group is displaced by chloride (from BCl4-) to yield the chlorocarbene complex Cp(C0)2Re=C-(C1)Tol. The metallacycles 5a and 5b react with [(Ph3P)2NlC1 to give new complexes (lOa,b) that result from loss of a CO ligand and an "NPh" group from the metallacycle, coordination of C1-to the metal, ortho-metalation of the N-aryl group, and hydrogen migration to the remaining nitrogen atom. The BCL-salt of complex 1 reacts with ethylene oxide, propylene oxide, and isobutylene oxide to form new carbene complexes of the form Cp(CO)2Re=C(Tol)OCR&R2Cl that result from chloride-induced ring opening and addition of the epoxide to the carbyne carbon. A similar ring-opening and addition of 3,3-dime'thyloxetane occurs with the BC4-salt of complex 1 to form the new carbene complex C~( C O )~R~= C ( T O~) O C H~C M~~C H~C~ (17).With 2-methylaziridine, carbyne complex 1 reacts to give an aziridinocarbene complex, and complex 1 abstracts sulfur from 2 equiv of propylene sulfide to give the +dithiocarboxylate complex [Cp(C0)2Re{~2-S2CTol)lBPh4 (19). Carbyne complex 1 and ita methylcarbyne analogue both react with benzophenone hydrazone to give the hydrazonyl carbene complexes Cp-(CO)zRe=C(R)NHN=CPh2,20 (R = Tol) and 21 (R = Me). Complex 19 reacta with PMe3 to displace the Cp ligand as the phosphonium salt [CpPMe31BPh4 and form the new complex trans-(C0)2(PMe3)zRe{~2-S2CTolj. A similar reaction of PMe3 occurs with complex 7 to give initially a cis-bis(phosphine) complex 24 which isomerizes upon heating to a trans derivative 25. Complexes 5a and 5b react with PMe3 to give similar Cp displacement reactions and form trans-bis(phosphine) complexes 26a,b in which the metallacycles have undergone a rearrangement to form s2-benzamidinato ligands. Complexes loa, 17, 20, 21, 24, 25, and 26a have been crystallographically characterized.