Reversible dissociation of a sterically distorted dimer of a phosphagermylenylidene

Accessing heavier analogues of isonitriles remains challenging because inefficient π overlap leads to weak and highly polarized π bonds that strongly favor dimerization or oligomerization. Herein, we report the experimentally observed dimer–monomer equilibrium of a phosphagermylenylidene in solution. Steric protection by a hydrindacenyl ligand renders the dimer highly sterically distorted and only weakly thermodynamically stabilized, enabling reversible dissociation to the monomer. The equilibrium has been studied by variable-temperature nuclear magnetic resonance spectroscopy in benzene-d6 solution, affording a modest driving force for dimerization (ΔHΘ = 16.26( ± 0.83) kcal mol−1 and ΔSΘ = 3.98( ± 0.26) × 10−2 kcal mol−1 K−1) through van’t Hoff analysis. The transient monomer can be trapped by the weak Lewis base tetrahydrofuran without substantial perturbation of the intrinsic P=Ge bonding. Both the dimer and the weakly coordinated monomer function as phosphagermylenylidene synthons, enabling C ≡ C and Si–H bond activation. These findings demonstrate that weak coordination combined with steric control provides a general strategy for accessing reactive heavier isonitrile analogues while preserving their intrinsic reactivity. Accessing heavier analogues of isonitriles remains challenging because inefficient π overlap leads to weak and highly polarized π bonds that strongly favor dimerization or oligomerization. Herein, the authors report the reversible dimer–monomer equilibrium of a phosphagermylenylidene in solution.