A, and 0.003 rad (0.2 °) in the final cycle, the r.m.s, average of the translations of the rigid bodies totaled over all the rings is 0.082 A. The same average over the rotations is 0.029 rad (1.7°). A second measurement of the similarily between the observed and calculated structures is the r.m.s, displacement of the atoms. Since the phenyl rings were refined as rigid bodies, the comparison must be made between the final atomic coordinates of the calculated structure and the idealized coordinates based on Daly's work. The r.m.s, displacements between the atoms of the two structures are then 0.174, 0.117 and 0.195 A,, respectively, for the three phenyl rings. The phosphorus atom is translated by 0.147 A,, and the total r.m.s, displacement over all the atoms is 0.165 A,. These shifts are small; the experimental structure appears to be well reproduced.

The crystal structure of koechlinite has been redetermined by single-crystal X-ray analysis. The space group is Pea21, with cell dimensions: a = 5.487 (2); b = 16.226 (6); e = 5.506 (2) A,. There are four molecular units per unit cell. The structure was refined with block-matrix least-squares methods. The final R on F value was 8.7 % for 788 independent reflexions, including the unobserved reflexions. Molybdenum is octahedrally surrounded by six oxygen ions. These octahedra share four corners with neighbouring octahedra in one plane. The other two apex ions points towards a BiO layer of the type that occurs, for example, in BiOC1.

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