SYNTHESES, FIR SPECTRA, AND CRYSTAL-STRUCTURES OF THE PENTASELENIDES K2SE5 AND [NA(15-CROWN-5)]2SE5

K2Se5 has been prepared by crystallization from a potassium polyselenide solution in DMF. In a similar way [Na(15-crown-5)]2Se5 can be prepared by crystallization from a sodium polyselenide solution in DMF in the presence of 15-crown-5. Both compounds were characterized by FIR spectroscopy as well as by X-ray structure determinations. K2Se5: Space group P2(1)2(1)2(1), Z = 4, 1659 observed independent reflections, R = 0.061. Lattice dimensions at -90-degrees-C: a = 671.5(5), b = 694.9(5), c = 1776.4(9) pm. The compound crystallizes with the K2S5 type structure. [Na(15-crown-5)]2Se5: Space group P1BAR, Z = 2, 3026 observed indenpendent reflections, R = 0.140. Lattice dimensions at -100-degrees-C: a = 853.7(5), b = 1042.8(10), c = 2011.0(10) pm; alpha = 88.95(5)-degrees, beta = 79.42(5)-degrees, gamma = 66.15(5)-degrees. The compound forms an ion triple, in which the sodium ions are coordinated by the five oxygen atoms of the crown ether molecules as well as by the terminal selenium atoms of the pentaselenide chain. The rather high R value is a result of the disordered crown ether molecules.