THE SYNTHESIS AND CRYSTAL-STRUCTURE OF [K(18-CROWN-6)]+-[NCS-CENTER-DOT-SO2]- (18-CROWN-6 = 1,4,7,10,13,16-HEXAOXACYCLO-OCTADECANE)

Reaction of 18-crown-6 (1,4,7,10,13,16-hexaoxacyclooctadecane) with potassium thiocyanate in liquid sulfur dioxide gives, on recrystallisation from diethyl ether-sulfur dioxide, yellow crystals of the complex [K(18-crown-6)]+[NCS.SO2]- in 98% yield. Vibrational spectroscopy points to the formation of a weak donor-acceptor complex anion [NCS.SO2]-, and this was confirmed by X-ray crystallography: triclinic, P1BAR (no. 2), a = 7.635(2), b = 9.595(2), c = 8.171(1) angstrom, alpha = 102.95(2), beta = 95.68(2), gamma = 111.91(2)-degrees, Z = 1. The centrosymmetric [K(18-crown-6)]+ cations are centred on special positions on the corners of the unit cell, and the complex anion is disordered about the inversion centre at (1/2, 1/2, 1/2). The nature of the disorder makes it difficult to locate the positions of some light atoms in the [NCS.SO2]- anion, although the S-S bond length, 2.736(9) angstrom, is consistent with the lability of the complex. Dissociation pressure measurements at different temperatures were used to determine a binding energy for gaseous [NCS.SO2]- of 66 kJ mol-1, a value which is compared with those of related sulfite complexes in relation to Pearson's concept of 'absolute hardness'.