The reaction of 1,2‐dipiperidinoacetylene (1) with 0.5 equivalents of SnCl2 or GeCl2⋅dioxane afforded the 1,2,3,4‐tetrapiperidino‐1,3‐cyclobutadiene tin and germanium dichloride complexes 2 a and 2 b, respectively. A competing redox reaction was observed with excess amounts of SnCl2, which produced a tetrapiperidinocyclobutadiene dication with two trichlorostannate(II) counterions. Heating neat 1 to 110 °C for 16 h cleanly produced the dimer 1,3,4,4‐tetrapiperidino‐3‐buten‐1‐yne (3); its reaction with stoichiometric amounts of SnCl2 or GeCl2⋅dioxane furnished the 1,3,4,4‐tetrapiperidino‐1,2‐cyclobutadiene tin and germanium dichloride complexes 4 a and 4 b, respectively. Transition‐metal complexes containing this novel four‐membered cyclic bent allene (CBA) ligand were prepared by reaction of 3 with [(tht)AuCl], [RhCl(CO)2]2, and [(Me3N)W(CO)5] to form [(CBA)AuCl] (5), [(CBA)RhCl(CO)2] (6), and [(CBA)W(CO)5] (7). The molecular structures of all compounds 2–7 were determined by X‐ray diffraction analyses, and density functional theory (DFT) calculations were carried out to rationalise the formation of 3 and 4 a.

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A series of symmetric and asymmetric tetraaminocyclopentadienone iron tricarbonyl complexes were prepared from Fe­(CO)5 and the diaminoacetylenes (DAA) R2NCCNR2 (NR2 = piperidinyl, 4-methylpiperidinyl, homopiperidinyl) via intermediate ferracyclobutenone complexes. In the presence of trimethylamine-N-oxide, the reactions with acetonitrile afforded the corresponding iron dicarbonyl acetonitrile complexes, which served as highly active precatalysts for the transfer hydrogenation of aldehydes, ketones, and imines with isopropanol as the hydrogen source and for the hydrogenation of aldehydes and ketones with dihydrogen under comparatively mild reaction conditions (3 bar H2 pressure, room temperature). Density functional theory (DFT) calculations were performed to reveal the mechanism of the isopropanol-mediated transfer hydrogenation of benzaldehyde and acetophenone, which is likely to involve a catalytically active hydroxycyclopentadienyl iron dicarbonyl hydride species. Dihydrogen transfer from the latter onto benzaldehyde and acetophenone occurs in a concerted manner with exceptionally low activation barriers following the established outer-sphere mechanism.