Coordination polyhedra CoXn (X = F, Cl, Br, I) in crystal structures

The Voronoi-Dirichlet polyhedra (VDP) and the method of intersecting sphere s were used to perform crystal-chemical analysis of compounds containing co ordination polyhedra CoXn (X = F, Cl, Br, I). It was found that cobalt atom s display coordination numbers (C.N.s) 6 or 4 with respect to the halogen a toms (thus giving octahedral and tetrahedral complexes, respectively). The C.N. of cobalt (IV, III, and II) atoms in fluorides is always 6. Only Co(II ) atoms can be found in other halides; they exhibit a C.N. of 6 or 4 in chl orides, and only 4 in bromides and iodides. An increase in the metal oxidat ion number in CoF6 octahedra, as well as the increase in the C.N. upon the (CoCl4)-Cl-II --> (CoCl6)-Cl-II transition, is accompanied by a decrease in the cobalt VDP volume (by 1.6-2.0 and 3.0 Angstrom(3), respectively), The Co-X distance for the same halogen atom changes by similar to 0.1-0.2 Angst rom, whereas the radius of the sphere whose volume is equal to the VDP volu me of Co changes only by 0.01-0.04 Angstrom. Nonvalence Co ... Co interacti ons in the crystal structures are discussed.