The reactions of the eight-coordinate rhenium(V) polyhydride complex ReH4(mq)(PPh(3))(2) (mq = the anion of 2-mercaptoquinoline) with terminal alkynes RC=CH (R = H, Et, n-Pr, i-Pr, n-Bu, Ph, p-tol, CH(2)Ph) in the presence of an electrophile (H+ as HPF6(aq), or Ph(3)CPF(6)) and dichloromethane as the reaction solvent provide a facile and high yield route to a new class of alkylidyne complexes of composition [Re(=CCH(2)R)H-2(mq)(PPh(3))(2)]PF6 (1) These complexes are easily deprotonated to form their neutral monohydrides Re(=CCH(2)R)H(mq)(PPh(3))(2) (2), which can in turn be reprotonated (by HPF6(aq)) to reform 1 quantitatively. When these same reactions are carried out with the use of an internal alkyne which is an isomer of one of the aforementioned terminal alkynes, then these same alkylidyne complexes are formed. For example, 1-, 2- and 3-hexyne afford the same complex, [Re(=C(CH2)(4)CH3)H-2(mq)(PPh(3))(2)]PF6, with no perceptible difference in reaction time or product yield. The allene 3-methyl-1,2-butadiene H2C=C=CMe(2), which is an isomer of 3-methyl-1-butyne, affords the same product as the butyne, viz. [Re(=CCH(2)i-Pr)H-2(mq)(PPh(3))(2)]PF6. This observation; along with results of deuterium labeling studies, suggests that the isomerization of internal to terminal alkynes occurs via eta(2)-allene intermediates, The subsequent and sequential formation of eta(2)-RC=CH and vinylidene intermediates is likely. The structural identity of these alkylidyne complexes was established by X-ray crystal structure determinations on three of the six-coordinate monohydrides of type 2 (R = n-Pr, i-Pr, and Ph) in which the PPh(3) ligands are trans to one another and the hydride and allcylidyne ligands are in a cis arrangement. When ReH4(mq)(PPh(3))(2) is treated with an electrophile (H+ or Ph(3)C(+)) in the absence of an alkyne, the dirhenium(V) complex [Re2H6(mu-mq)(2)(PPh(3))(4)](PF6)(2) is formed. Attempts to grow crystals of this complex led to its conversion to [Re2H6(mu-mq)(2)(PPh(3))(4)](H2PO4)(2) . 2(CH3)(2)CO, the structure of which was determined by X-ray crystallography. The dirhenium(V) dication contains two eight-coordinate metal centers linked through a fairly symmetrical Re-2(mu-S)(2) unit. The Re ... Re distance of 3.9034(8) Angstrom accords with the absence of a Re-Re interaction between the two 18-electron rhenium centers.