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)]^- (<b>4</b>), 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 (<b>1</b>), and successive cleavage of the Si(nacnac') units delivers the "naked" <i>cyclo</i>-P₅/<i>cyclo</i>-P₃ sandwich framework via monosilylated intermediates <b>2</b> and <b>3</b>. The cryptate salts [M(crypt-222)]<b>4</b> (M = Na, K) were characterized by ³¹P{¹H} NMR spectroscopy, ESI-MS, and single-crystal X-ray diffraction (scXRD) for [Na(crypt-222)]<b>4</b>. In solution, <b>4</b> undergoes slow disproportionation to give the paramagnetic dianion [(η⁴-P₅)Co(η³-P₃)]^2- (<b>5</b>), isolated as [K(crypt-222)]₂<b>5</b> and authenticated by scXRD and EPR spectroscopy. Complexes <b>4</b> and <b>5</b> establish a deliberate strategy for accessing inorganic sandwich architectures and extend carbon-free metallocene chemistry to cobalt in two different oxidation states.