Inorganic Cobalt Sandwich Complex [(η5-P5)Co(η3-P3)]−

Sandwich compounds are foundational to organometallic chemistry, yet carbon-free analogs remain exceptionally rare. We report the all-phosphorus heteroleptic cobalt sandwich anion [(η5-P5)Co(η3-P3)]− (4), obtained via a stepwise P4 activation/unmasking sequence in which (nacnac′)SiP4 serves as a controllable, silicon-protected P4 synthon. Reaction with the cobalt(−I) complex [K(THF)0.2][Co(η2:η2-cod)2] furnishes a bis(silatetraphosphacyclopentadienyl) cobaltate anion (1), and successive cleavage of the Si(nacnac′) units delivers the “naked” cyclo-P5/cyclo-P3 sandwich framework via monosilylated intermediates 2 and 3. The cryptate salts [M(crypt-222)]4 (M = Na, K) were characterized by 31P{1H} NMR spectroscopy, ESI-MS, and single-crystal X-ray diffraction (scXRD) for [Na(crypt-222)]4. In solution, 4 undergoes slow disproportionation to give the paramagnetic dianion [(η4-P5)Co(η3-P3)]2– (5), isolated as [K(crypt-222)]25 and authenticated by scXRD and EPR spectroscopy. Complexes 4 and 5 establish a deliberate strategy for accessing inorganic sandwich architectures and extend carbon-free metallocene chemistry to cobalt in two different oxidation states.