Forcing the Side‐on π‐Coordination of a C≡C Triple Bond to Technetium Using the As,$CC$,As Alkyne Pincer Ligand 1,2‐Bis(2‐(diisopropylarsaneyl)‐4‐(trifluoromethyl)phenyl)Ethyne

Alkyne complexes are known for transition metals across the d-block with exception of the radioelement technetium despite considerable synthetic efforts. DFT calculations suggest that this is not inherent to the transition metal but a consequence of the overall ligand sphere. The arsenic-based tolane ligand 1,2-bis(2-(diisopropylarsaneyl)-4-(trifluoromethyl)phenyl)ethyne (LiPr) forces a coordination of the central alkyne moiety onto the metal through ligand design. The stable, crystalline Tc(III) and Tc(V) alkyne complexes mer-[TcIIICl₃(κ⁴-As,CC,As-LiPr)], mer-[TcVNX₂(κ⁴-As,CC,As-LiPr)] (X = Cl, Br) and cis,trans,mer-[TcVN(CN)Cl(κ⁴-As, CC, As-LiPr)] alongside their rhenium homologs mer-[ReVCl₃(κ⁴-As,CC,As-LiPr)] and mer-[ReVNCl₂(κ⁴-As,CC,As-LiPr)] have been prepared and fully characterized. According to spectroscopic and DFT analyses, the technetium complexes represent robust, classical 2e^− alkyne complexes, while a different situation was found for mer-[ReVCl₃(κ⁴-As,CC,As-LiPr)] with a formally oxidized metal ion and reduced 4e^− donor ligand. This has general implications for π-ligand coordination in group 7 and potentially for neighboring elements. Successful translation to the medicinally relevant nuclear isomer ⁹⁹ᵐTc proves the viability of alkyne donors as building blocks for stable chelation of technetium at the tracer level.