(2.2.2-cryptand)potassium tetrakis(η²-ethylene)cobaltate(-I)

The title salt, [K(C₁₈H₃₆N₂O ₆)][Co(C₂H₄)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 d¹⁰ 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.