(2.2.2-Cryptand)potassium tetra­kis­(η(2)-ethyl­ene)cobaltate(−I)

The title salt, [K(C(18)H(36)N(2)O(6))][Co(C(2)H(4))(4)], is one of only two known homoleptic ethyl­enemetalates. The cation and anion are well separated, which gives an unperturbed tetra­hedral anion as is expected for a formally Co(−I) d (10) metal center. The considerable elongation of the C=C bonds of the ethyl­ene ligands [average 1.401 (6) Å], relative to that of free ethyl­ene (1.333 Å), is consistent with metal→π* back-bonding models. One arm of the 2.2.2-cryptand (4,7,13,16,21,24-hexa­oxa-1,10-diaza­bicyclo­[8.8.8]hexa­cosa­ne) complexant is disordered and was modeled over two positions with a refined occupancy ratio of 0.559 (2):0.441 (2). In the crystal, the cationic K(2.2.2-cryptand) units are linked via C—H⋯O hydrogen bonds, forming inversion dimers. There are no other significant inter­molecular inter­actions in the crystal structure.