The rhodium allenylidenes trans-[RhCl{=C=C=C(Ph)R}(P/Pr-3)(2)] [R = Ph (1), p-Tol (2)] react with NaC5H5 to give the half-sandwich type complexes [(eta(5)-C5H5)Rh{=C=C=C(Ph)R}(P/Pr-3)] (3, 4). The reaction of 1 with the Grignard reagent CH2=CHMgBr affords the eta(3)-pentatrienyl compound [Rh(eta(3)-CH2CHC=C=CPh2)(P/Pr-3)(2)] (6), which in the presence of CO rearranges to the eta(1)-pentatrienyl derivative trans-[Rh{eta(1)-C(CH=CH2)=C=CPh2}(CO)(P/Pr-3)(2)] (7). Treatment of 7 with acetic acid generates the vinylallene CH2=CH-CH=C=CPh2 (8). Compounds 1 and 2 react with HCl to give the five-coordinate allenylrhodium(III) complexes [RhCl2{CH=C=C(Ph)R}(P/Pr-3)(2)] (10, 11). An unusual [C-3 + C-2 + PI coupling process takes place upon treatment of 1 with terminal alkynes HC=CR', leading to the formation of the eta(3)-allylic compounds [RhCl{eta(3)-anti-CH(P/Pr-3)C(R') (C=C=CPh2}(P/Pr-3)] [R' = Ph (12), p-Tol (13), SiMe3 (14)]. From 12 and RMgBr the corresponding phenyl and vinyl rhodium(l) derivatives 15 and 16 have been obtained. The previously unknown unsaturated ylide /Pr3PCHC(Ph)=C=C=CPh2 (17) was generated from 12 and CO. A [C-3 + P] coupling process occurs on treatment of the rhodium allenylidenes 1, 2, and trans-[RhCl {=C=C=C(p-Anis)(2)}(PPr3)(2)](20) with either Cl-2 or PhlCl(2), affording the ylide-rhodium(III) complexes (RhCl3{C(P/Pr-3)C=C(R)R'}(P/Pr-3)] (21-23). The butatrienerhodium(I) compounds trans-[RhCl{eta(2)-H2C= C=C=C(R)R'}(P/Pr-3)(2)] (28-31) were prepared from 1, 20, and trans-[RhCl{=C=C=C(Ph)R}(P/Pr-3)(2)] [R = CF3 (26), tBu (27)] and diazomethane; with the exception of 30 (R = CF3, R' = Ph), they thermally rearrange to the isomers trans-[RhCl{eta(2)-H2C=C=C=C(R)R'}(P/Pr-3)(2)) (32, 33, and syn/anti-34). The new 1,1-disubstituted butatriene H2C=C=C=C(tBU)Ph (35) was generated either from 31 or 34 and CO. The iodo derivatives trans-[Rhl(eta(2)-H2C=C=C=CR2)(P/Pr-3)(2)] [R = Ph (38), p-Anis (39)] were obtained by an unusual route from 1 or 20 and CH(3)l in the presence of Kl. While the hydrogenation of 1 and 26 leads to the allenerhodium(I) complexes trans-[RhCl{eta(2)-H2C=C=C(Ph)R}(PPr3)(2)] (40, 41), the thermolysis of 1 and 20 produces the rhodium(I) hexapentaenes trans-[RhCl(eta(2) -R2C=C=C=C=C=CR2)(P/Pr-3)(2)] (44, 45) via C-C coupling. The molecular structures of 3, 7, 12, 21, and 28 have been determined by X-ray crystallography.