Crystal Structure of Dipotassium cis-Hexacyanobutenediide and the Electronic Structure of the Anion

Dipotassium cis-hexacyanobutenediide, K₂((CN)₂CC(CN)C(CN)C(CN)₂), crystallizes in the space group C₂/c with a = 20.08, b = 7.42, c = 17.36 Å and β = 117.8°. There are eight formula units in the unit cell. Data were collected on a three-circle manual diffractometer with Cu Ka radiation. The structure was solved by Patterson and heavy-atom Fourier methods and was refined by full-matrix least squares, based on 1767 unique observed reflections, to a final conventional R index of 0.072. The anions lie approximately parallel to the a, c plane and stack along b, which is the needle axis. Each potassium ion is surrounded by seven terminal nitrogen atoms from six different anions and is enclosed between the planes of anions. Observed bond lengths are in agreement with the predictions of molecular orbital theory, and the bond angles and average values of the torsion angles agree with those expected from extended molecular orbital calculations. The relatively small deviation of the anion from the C₂ symmetry that is predicted by the calculations can be attributed to interactions with neighboring potassium ions.