Further investigations into the chemistry of the rhenacyclobutadiene complexes (CO) 4Re(η 2-C(R)C(CO 2Me)C(X)) ( 1: R=Me, X=OEt ( 1a), O(CH 2) 3CCH ( 1b), NEt 2 ( 1c); R=CHEt 2, X=OEt ( 1d); R=Ph, X=OEt ( 1e)) are reported. Reactions of 1 with alkynes at reflux temperature of toluene and at ambient temperature either under photochemical conditions or in the presence of PdO yield ring-substituted η 5-cyclopentadienylrhenium tricarbonyl complexes, 2. The symmetrical alkynes R ′CCR ′ (R ′=Ph, Me, CO 2Me) afford the pentasubstituted complexes (η 5-C 5(Me)(CO 2Me)(OEt)(Ph)(Ph))Re(CO) 3 ( 2d), (η 5-C 5(Me)(CO 2Me)(OEt)(Me)(Me))Re(CO) 3 ( 2e), (η 5-C 5(Me)(CO 2Me)(OEt)(CO 2Me)(CO 2Me))Re(CO) 3 ( 2f), and (η 5-C 5(Me)(CO 2Me)(NEt 2)(CO 2Me)(CO 2Me))Re(CO) 3 ( 2i) on reaction with the appropriate 1, whereas the unsymmetrical alkynes R ′CCR″ (R ′=Ph; R″=H, Me) give either only one, (η 5-C 5(Me)(CO 2Me)(OEt)(Ph)H)Re(CO) 3 ( 2a)), or both, (η 5-C 5(Me)(CO 2Me) (OEt)(Ph)(Me))Re(CO) 3 ( 2b) and (η 5-C 5(Me)(CO 2Me)(OEt)(Me)(Ph))Re(CO) 3 ( 2c), (η 5-C 5(Ph)(CO 2Me)(OEt)(Ph)H)Re(CO) 3 ( 2g) and (η 5-C 5(Ph)(CO 2Me)(OEt)(H)(Ph))Re(CO) 3 ( 2h), of the possible products of [3 + 2] cycloaddition of alkyne to η 2-C(R)C(CO 2Me)C(X). Thermolysis of (CO) 4Re(η 2-C(Me)C(CO 2Me)C(O(CH 2) 3CCH)) ( 1b) containing a pendant alkynyl group proceeds to (η 5-C 5(Me)(CO 2Me)(O(CH 2) 3)H)Re(CO) 3 ( 2j), a η 5-cyclopentadienyl-dihydropyran fused-ring product. Competition experiments showed that each of PhCCH and MeO 2CCCCO 2Me reacts faster than PhCCPh with 1a. The results with unsymmetrical alkynes are rationalized by steric properties of substituents at the CC and ReC bonds and by a preference of ReC(Me) over ReC(OEt) to undergo alkyne insertion. A mechanism is proposed that involves substitution of a trans CO by alkyne in 1, insertion of alkyne into ReC bond to give a rhenabenzene intermediate, and collapse of the latter to 2. Complexes 1a and 1d undergo rearrangement in MeCN at reflux temperature to give rhenafuran-like products, (CO) 4Re(κ 2-OC(OMe)C(CHCR 2)C(OEt)) (R=H ( 3a) or Et ( 3b)). The reaction of 1d also proceeds in EtCN, PhCN, and t-BuCN at comparable temperature, but is slower (especially in t-BuCN) than in MeCN. In pyridine at reflux temperature, 1a undergoes a similar rearrangement, with CO substitution, to give (CO) 3(py)Re(κ 2-OC(OMe)C(CHCEt 2)C(OEt)) ( 4). A mechanism is proposed for these reactions. The sulfonium ylides Me 2SCHC(O)Ph and Me 2SC(CN) 2 (Me 2SCRR ′) react with 1a in acetonitrile at reflux temperature by nucleophilic addition of the ylide to the ReC(Me) carbon, loss of Me 2S, and rearrangement to a rhenafuran-type structure to yield (CO) 4Re(κ 2-OC(OMe)C(C(Me)CRR ′)C(OEt)) (R=H, R ′=C(O)Ph ( 5a); R=R ′CN ( 5b)). All new compounds were characterized by a combination of elemental analysis, mass spectrometry, and IR and NMR spectroscopy.