Vinylidene Transition–Metal Complexes, XX[1] Synthesis, Reactions, and Molecular Structure of Bis(arsane) and Bis(stibane) Rhodium(I) Complexes trans‐[RhCl(L)(EiPr3)2] (EAs, Sb) Including the Rhodium–Mediated Rearrangement of Alkynes to the Isomeric Allenes Reaction of the dimeric bis(olefin)rhodium(I) compounds [RhCl(olefin)2]2 [olefinC8H14 (2), C2H4 (5)] with AsiPr3 and SbiPr3 gives mono‐ and dinuclear olefin(arsane) and olefin(stibane) complexes [RhCl(olefin)(EiPr3)]2 [olefinC8H14 (3, 4), C2H4 (6, 7)] and trans‐[RhCl(C2H4)(EiPr3)2] (8, 9). Treatment of 8 and 9 with CO, H2, and CPh2N2 leads to the displacement of the ethylene ligand with formation of the corresponding carbonyl (10, 11), dihydrido (12, 13), and diazoalkane (14) rhodium derivatives. In solution, compound 14 loses CPh2 to give trans‐[RhCl(N2)(AsiPr3)2] (15). The alkyne complexes trans‐[RhCl(HCC̊)(AsiPr3)2] (16–18), which are prepared either from 8 or [RhH2Cl(AsiPr3)2] (12) and HCC̊ (RH, Ph, CO2Me), rearrange in solution to produce the vinylidenerhodium isomers trans‐[RhCl(CCHR)(AsiPr3)2] (19–21). Reaction of 8 with HCCMe and HCCtBu leads directly to the vinylidene complexes 22, 23. Compound 8 reacts with excess propyne, but–2‐yne, or 2,2–dimethylpent–2‐yne to give the rhodium allenes trans‐[RhCl(η2‐CH2CCHR)(AsiPr3)2] (25–27) in nearly quantitative yields. With DCCMe as a substrate, it is shown that two propyne molecules are involved in the rearrangement to generate the coordinated allene. The crystal and molecular structure of trans‐[RhCl(η2‐CH2CCH2)(AsiPr3)2] (27) has been determined.