Cyclooctyne is the smallest unsubstituted cycloalkyne that can be isolated in the free state and it is more reactive than acyclic alkynes towards transition metal complexes. Smaller cycloalkynes such as cycloheptyne, cyclohexyne, benzyne and cyclopentyne, which are transient molecules in the free state, can be stabilized by coordination either to mononuclear, electron-rich, transition metal-containing fragments, e.g. [ZrCp,(PMe,)] and M(PR,), (M = Ni, Pt), or by formation of dinuclear or polynuclear metal complexes, e.g.[Os,H,(CO), (C,H,)] and [Pt,(p-PPh,)(p-C,H,)(PPh3)3]. The alkynes can donate between two and four n-electrons to the metal centers, the higher number being favored for the early transition metals. The metal-cycloalkyne and metal-aryne bonds in the mononuclear complexes readily insert molecules containing C = 0 , C = C, C = C and C = N bonds, a feature that may be useful in organic synthesis. The highly unsaturated species 1,4-benzdiyne acts as a bridging ligand between two metal centers in [{Ni(Cy,PCH,CH,PCy,)J,(I.I-C,H,)I.

Titanocene and zirconocene derivatives with an interannular tetramethylethylene bridge can be made by reductive coupling of 6,6-dimethylfulvene with sodium amalgam, sodium anthracenide, or magnesium metaljCCl 4 as reducing agents and subsequent reaction of the anionic tetramethyldicyclopentadienylethane product with TiCl 3 or ZrCl 4 in THF. • For Part IV see ref. 1.

The tetramethylethylene-bridged chromocene carbonyl complex (CH 3)4-C2(CsH4hCr(CO) is obtained by reaction of the Grigt\ard compound (CH 3 k C2(CsH4MgClh•4THF with CrC1 2 • THF under CO. The diamagnetic carbonyl complex is the first thermally stable chromocene ligand adduct; its structure is that of a normal wedge-shaped metallocene derivative whose ring ligands span a centroid-chromium-centroid angle of 143°; the molecule closely approximates to C 2 symmetry with respect to the Cr-C=O axis.

Cyclooctin ist das kleinste unsubstituierte Cycloalkin, das in freier Form isoliert werden kann, und es ist gegenüber Übergangsmetall‐Komplexen reaktiver als acyclische Alkine. Kleinere Cycloalkine wie Cycloheptin, Cyclohexin, Dehydrobenzol und Cyclopentin, die in freier Form instabil sind, können durch Koordination an einkernige, elektronenreiche Fragmente mit Übergangsmetallen, z. B. [ZrCp2(PMe3)] und M(PR3)2 (M = Ni, Pt), oder durch Bildung von zwei‐ oder mehrkernigen Metall‐Komplexen, beispielweise [Os3H2(CO)9(C6H4)] und [Pt2(μ‐PPh2)(μ‐C5H6)(PPh3)3], stabilisiert werden. Die Alkine können zwei bis vier π‐Elektronen an das Metallzentrum abgeben, wobei für die frühen Übergangsmetalle die höhere Anzahl bevorzugt wird. In die Cycloalkin‐Metall‐ und Arin‐Metall‐Bindungen der einkernigen Komplexe lassen sich leicht Moleküle einschieben, die CO‐, CC‐, CC‐ und CN‐Bindungen enthalten; dies ist eine Eigenschaft, die in der organischen Synthese nützlich sein kann. Das hochgradig ungesättigte 1,2,4,5‐Tetradehydrobenzol fungiert in [{Ni(Cy2PCH2CH2PCy2)}2‐(μ‐C6H2)] als verbrückender Ligand zwischen zwei Metallzentren.

In the reduction of tetramethylethylene-bridged (CH3)4C2(CsH4hTiCI2 under argon, studied by cyclic voltammetry, a first reduction to the Ti III stage occurs at a potential of-0.92 V vs. SCE, close to that of (CsHshTiCI2, and a second reduction to the Ti II stage at a potential of-2.53 V, about 0.3 V more negative than in the corresponding reduction of the unbridged analogue. Decay of the tetramethylethylene-bridged Ti II product is considerably faster than that of its unbridged analogue. Changes induced by the presence of an N2 or H2 atmosphere indicate the occurrence of secondary reactions. In the presence of CO, reduction to (CH3)4C2(CsH4hTi(COh occurs at-2.0 V. This tetramethylethylene-bridged titanocene dicarbonyl complex, which is also formed by reduction of (CH3)4C2(CsH4hTiCI2 with Mg metal under CO, undergoes irreversible reduction and oxidation at-2.78 and-0.08 V, respectively. When the latter process is conducted in the presence of Cl-, the dichloride (CH3)4C2(CsH4hTiCI2 is regenerated.