This paper describes the reactivity of permethylzirconocene and permethyltitanocene toward different 1,3-butadiynes. A pointed dependence on the metals and the diyne substituents was found. Unusual, but still stable, five-membered zirconacyclocumulenes (eta(4)-diyne complexes, zirconacyclopenta-2,3,4-trienes) Cp*Zr-2(eta(4)-1,2,3,4-RC4R), R = Ph and SiMe3, were prepared using two new and effective synthetic routes. One starts with the permethylzirconocene bisacetylides Cp*Zr-2(C equivalent to CR)(2), R = Ph (1a), SiMe3 (1b), which rearrange in sunlight to form the stable five-membered zirconacyclocumulenes Cp*Zr-2(eta(4)-1,2,3,4-RC4R), R = Ph (2a), SiMe3 (2b). The alternative route to 2a and 2b is the reduction of Cp*2ZrCl2 with Mg in the presence of the adequate disubstituted butadiynes RC equivalent to C-C equivalent to CR. Both methods failed to produce the analogous titanacyclocumulenes, which seemed extremely unstable. Nevertheless, we were able to obtain distinct products employing the reduction pathway with permethyltitanocene. For R = SiMe3, the novel titanacyclopropene (eta(2)-complex) Cp*Ti-2(eta(2)-1,2-Me3SiC2C equivalent to CSiMe3) (3) was isolated. For R = Ph, an activation of both pentamethylcyclopentadienyl ligands was observed resulting in the complex [eta(5)-C5Me4-(CH2)-]Ti[-C-(=CHPh)C(=CHPh)CH2-eta(5)-C5Me4] (4). The reaction of 4 with carbon dioxide led to the Cp*-substituted titanafuranone Cp*Ti[-OC(=O)C(Ph)C(-)C(=CHPh)CH2-eta(5)-C5Me4] (5). The zirconacyclocumulene 2b surprisingly inserted two molecules of CO2 to give the unprecedented cumulenic dicarboxylate Cp*Zr-2 [-OC(=O)C(SiMe3)=C=C=C(SiMe3)C(=O)O-] (6). The eta(2)-complex 3 (titanacyclopropene) took up one molecule of carbon dioxide to afford the titanafuranone Cp*Ti-2 [OC(=O)C(SiMe3)=C(C equivalent to SiMe3)-] (7).