STRUCTURES OF THE P21-C PHASES OF [(CH3)4P]2COI4 AND [(CH3)4P]2COBR4

Bis(tetramethylphosphonium) tetraiodo-cobaltate(II), [C4H12P]2[CoI4], M(r) = 748.8, monoclinic, P2(1)/c, a = 9.859 (2), b = 16.762 (2), c = 13.716 (1) angstrom, beta = 90.05 (1)-degrees, V = 2266.6 (6) angstrom 3, Z = 4, D(x) = 2.19 g cm-3, lambda(Mo K-alpha) = 0.7107 angstrom, mu = 62.7 cm-1, F(000) = 1364, T = 293 K, final R = 0.073 for 3481 unique reflections with F > 3-sigma(F). Bis-(tetramethylphosphonium) tetrabromocobaltate(II), [C4H12P]2[CoBr4], M(r) = 560.8, monoclinic, P2(1)/c, a = 9.489 (2), b = 15.985 (3), c = 13.106 (2) angstrom, beta = 89.52 (2)-degrees, V = 1987.9 (7) angstrom 3, Z = 4, D(x) = 1.87 g cm-3, lambda(Mo K-alpha) = 0.7107 angstrom, mu = 89.7 cm-1, F(000) = 1076, T = 293 K, final R = 0.088 for 2019 unique reflections with F > 3-sigma(F). The two structures are of the slightly distorted beta-K2SO4 type. [(CH3)4P]2CoI4 is virtually isostructural with the P2(1)/c phase of several [(CH3)4N]2MX4 compounds, characterized by a counter-clockwise rotation around [010] of all ions with respect to an assumed, high-temperature Pmcn structure. [(CH3)4P]2CoBr4 is uniquely characterized by a clockwise rotation of all ions, if the coordinate system with beta = 90.48-degrees is chosen for reference. The actual structural differences, however, are quite small, as is shown by considering the structure in reference to the same unit cell, but with the alternative coordinate system having beta = 89.52-degrees.