The crystal and molecular structures of two transition-metal ion complexes with tetramethylallene have been determined by single-crystal X-ray diffractometry. They are acetylacetonatobis(tetramethylallene)rhodium(I)(C5H7O2)RhI(C7H12)2, and the dimer of dichloro(tetramethylallene)platinum(II)–carbon tetrachloride, [Cl2Pt(C7H12)]2,2CCl4. In both cases the tetramethylallene ligand is perpendicular to the co-ordinating plane and functions as a π-bonded mono-olefin. The co-ordinated allene ligand is not linear and in the rhodium complex the central C–C bonds differ in length. In the platinum complex both C–C bonds are longer than in the free-ligand.