Inorganic Cobalt Sandwich Complex [(η ⁵ -P ₅ )Co(η ³ -P ₃ )] ⁻

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.