A study of the coordination chemistry of bis(diphenylphosphino)acetylene, Ph2P-C≡C-PPh2, with selected group 4 metallocenes is presented. By substitution of the alkyne in complexes of the type Cp'2M(L)(η(2)-Me3SiC2SiMe3) (M = Ti, no L; M = Zr, L = pyridine; Cp' = substituted or unsubstituted bridged or unbridged η(5)-cyclopentadienyl), the expected mononuclear complexes Cp*2Ti(η(2)-Ph2PC2PPh2) (4Ti), (rac-ebthi)Ti(η(2)-Ph2PC2PPh2) (5Ti), and (rac-ebthi)Zr(η(2)-Ph2PC2PPh2) (5Zr) [ebthi = ethylenebis(tetrahydroindenyl)] were obtained. When [Cp2Zr] was used in the reaction of Cp2Zr(py)(η(2)-Me3SiC2SiMe3) with Ph2P-C≡C-PPh2, the dinuclear complex [Cp2Zr(η(2)-Ph2PC2PPh2)]2 (6) was formed and isolated in the solid state. In solution, this complex is in equilibrium with the very spectacular structure of complex 7b as the first example of such a highly strained four-membered heterometallacycle of a group 4 metal, involving the rare R2PCCR' fragment in the cyclic unit. Both the stability and reactivity of heterodisubstituted alkynes X-C≡C-X (X = NR2, PR2, SR, SiR3, etc.) themselves and also of their complexes are of general interest. Complex 6 did not react with a second [Cp2Zr] fragment to form a homobimetallic complex. In contrast, for (rac-ebthi)Zr(η(2)-Ph2PC2PPh2) (5Zr) this reaction occurs. In the reaction of complex 4Ti with the Ni(0) complex (Cy3P)2Ni(η(2)-C2H4) (Cy = cyclohexyl), C-P bond cleavage of the alkyne ligand resulted in the formation of the isolated complex [(Cy3P)Ni(μ-PPh2)]2 (11). The structure and bonding of the complexes were investigated by DFT analysis to compare the different possible coordination modes of the R2P-C≡C-PR2 ligand. For compound 7b, a flip-flop coordination of the phosphorus atoms was proposed. Complexes 4Ti, 5Ti, 5Zr, 6, and 11 were characterized by X-ray crystallography.

Reactions of group 4 metallocene sources with 2-substituted pyridines were investigated to evaluate their coordination type between innocent and reductive dearomatisation as well as to probe the possibility for couplings. A dependence on the cyclopentadienyl ligands (Cp, Cp*), the metals (Ti, Zr), and the substrates (2-phenyl-, 2-acetyl-, and 2-iminopyridine) was observed. While 2-phenylpyridine is barely reactive, 2-acetylpyridine reacts vigorously with the Cp-substituted complexes and selectively with their Cp* analogues. With 2-iminopyridine, in all cases selective reactions were observed. In the isolated [Cp Ti], [Cp Zr], and [Cp* Zr] compounds the substrate coordinates by its pyridyl ring and the unsaturated side-chain. Subsequently, the pyridine was dearomatised, which is most pronounced in the [Cp* Zr] compounds. Using [Cp* Ti] leads to the unexpected paramagnetic complexes [Cp* Ti (N,O-acpy)] and [Cp* Ti (N,N'-impy)]. This highlights the non-innocent character of the pyridyl substrates.

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The crystal structure of the title compound, [Hf(C10H15)2(C4H9)2], reveals two independent mol­ecules in the asymmetric unit. The diffraction experiment was performed with a racemically twinned crystal showing a 0.529 (5):0.471 (5) component ratio. Each HfIV atom is coordinated by two penta­methyl­cyclo­penta­dienyl and two n-butyl ligands in a distorted tetra­hedral geometry, with the cyclo­penta­dienyl rings inclined to one another by 45.11 (15) and 45.37 (16)°. In contrast to the isostructural di(n-butyl)bis(η 5-penta­methyl­cyclo­penta­dien­yl)zirconium(IV) complex with a noticeable difference in the Zr–butyl bonding, the Hf—Cbut­yl bond lengths differ from each other by no more than 0.039 (3) Å.

The reactions of [Cp2Ti(η2‐Me3SiC2SiMe3)] (1; Cp = η5‐cyclopentadienyl) with 1,4‐bis(diphenylphosphanyl)but‐2‐yne (2) have been investigated and found to yield a mixture of products. From these, through the coupling of 2, the tetrasubstituted titanacyclopentadiene [Cp2Ti(CCH2PPh2)4] (3) was isolated. In addition, small amounts of very unusual complexes were obtained and characterized. In one case, the substrate 2 isomerized to the allene Ph2PC(H)=C=C(H)CH2PPh2, which formed the complex [Cp2Ti{η3‐Ph2PC(H)=C=C(H)CH2PPh2}] (4) through the coordination of a double bond and one of the phosphorus atoms. Another complex, [Cp2Ti{‐C(CH2PPh2)=C(CH2PPh2)P(Ph2)H‐}] (5), was identified to be the result of a formal hydrophosphorylation of the substrate 2 by HPPh2, and features a Ti–H–P bridge. It is not clear how HPPh2 was formed. One possible explanation is the dehydrophosphorylation of the substrate with the formation of HPPh2 and the butatriene H2C=C=C=C(H)PPh2 [tautomer of the but‐2‐en‐3‐yne HC≡C‐CH=C(H)PPh2]. The molecular structures of complexes 4 and 5 were determined by X‐ray analysis.