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

ABSTRACT 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 (L i Pr ) 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 ‐[Tc III Cl 3 (κ 4 ‐ As , CC , As ‐L i Pr )], mer ‐[Tc V NX 2 (κ 4 ‐ As , CC , As ‐L i Pr )] (X = Cl, Br) and cis,trans , mer ‐[Tc V N(CN)Cl(κ 4 ‐ As , CC , As ‐L i Pr )] alongside their rhenium homologs mer ‐[Re V Cl 3 (κ 4 ‐ As , CC , As ‐L i Pr )] and mer ‐[Re V NCl 2 (κ 4 ‐ As , CC , As ‐L i Pr )] 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 ‐[Re V Cl 3 (κ 4 ‐ As , CC , As ‐L i Pr )] 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 99m Tc proves the viability of alkyne donors as building blocks for stable chelation of technetium at the tracer level.