A five-membered metallacycloallene complex, 1-zirconacyclopenta-2,3-diene, was prepared from low-valent zirconocene and 2,4-bis(trimethylsilyl)but-1-en-3-yne. The complex reacted with carbonyl compounds to afford alkynyl alcohols after hydrolysis via C=O insertion into Zr-C bond. Similarly, nitriles inserted into the Zr-C bond to give the corresponding ketones after hydrolysis. The insertion of C=O and CN selectively took place at zirconium-sp 3 carbon of the metallacycle. nium.Recently, we and Erker's group independently reported stable five-membered cycloallenes, 1-metallacyclopenta-2,3-diene compounds (Fig. 2) [7][8][9][10][11][12]. These complexes are stable at room temperature (rt). It should be noted that replacement of one carbon atom with a transition metal made the strained compounds much more stable, and it must be interesting to study their reactivity. The fivemembered zirconacycloallene 1 was synthesized by transformation of a conjugated 1-zirconacyclopent-3-yne complex. The 1-zircona-and 1-hafnacyclopenta-2,3-dienes 2 were prepared from the corresponding bis(alkynyl)metallocenes via 1,1-hydroboration. The preparative methods of these complexes, however, were specific and unsuitable for large-scale synthesis. Therefore, more versatile synthetic routes have been desired in order to study the reactivity of five-membered metallacycloallenes. Bertrand and co-workers recently reported several bent allene compounds [13][14][15], including small cycloallenes. These compounds are of interest in relation to the metallacycloallenes, although the allene character of these is yet controversial [16,17]. Straightforward synthesis of unsaturated metallacyclesFor the syntheses of metallacyclic compounds such as 1-metallacyclpent-3-enes, 1-metallacyclopenta-2,3,4-trienes, and 1-metallacyclopent-3-ynes, low-valent metallocenes and 1,3-dienes, 1,3-diynes, and 1,2,3-trienes are employed (i-iii, Scheme 1) [8]. This methodology realized the synthesis of stable fivemembered metallacyclotrienes (ii) [18][19][20], as well as metallacycloalkynes (iii) [21][22][23][24], which have been thought to be highly reactive and unisolable. According to these protocols one must consider that 1,3-enynes would be possible starting compound for cycloallenes (iv, Scheme 1), as predicted by N. SUZUKI et al.