The asymmetric unit ( Fig. 2) consists of a dimer (A) with formula [Co(MeT)2H20]2, which has bridging MeT oxygen atoms and is structurally analogous to [NiT2H20]2, and a mononuclear species (B) with formula Co(MeT)2(H20)2 which exhibits cis configuration of water molecules. A nonbonded water molecule is also present. Hydrogen bonds exist between coordinated water molecules and ligand O atoms; both H20 molecules of B take part in this interaction but only one H20 on ,4 is involved, such that each B molecule is associated via three H-bonds (O-O distances 2.61, 2.73 and 2.75 A) with one A molecule only. It is notable that there is no interaction between separate A pairs or B pairs, other than that induced by normal van der Waals approaches and that many of the shorter intermolecular contacts involve methyl C atoms.Although the mother liquor must have contained both species, the relative concentration of each, and possibly others, in solution could have been a structure-determining factor for the solid state. However, the observed constitution of the stable lattice (the crystals were not moisture sensitive) would appear necessary to facilitate H-bonding and lattice packing which, because of the methyl substituent, is possibly not so efficiently achieved when either species is absent.We thank the National Research Council of Canada for financial support. The distribution of residual electron density in rutile crystals has been calculated from intensities carefully obtained by diffractometry. There is no indication that the charge densities of Ti 4+ (having no d electrons) are deformed in an octahedral crystal field. This result seems to support our conclusion that the aspherical charge densities of Ni 2+ (3d 8) in 7-Ni2SiO4 are due to d electrons placed in an octahedral crystal field.An indication of 3d electrons in the t2g orbitals of Ni 2 ÷ and Co 3÷ was observed in the final difference syntheses of 7-Ni2SiO4 and [Co(NHa)6][Co(CN)6] (Marumo, Isobe, Saito, Yagi & Akimoto, 1974;Iwata & Saito, 1973). In the difference maps eight small peaks were arranged at the corners of a cube around the transition metal atom, the peaks being at 0.45 A from the metal atom. To see whether or not such peaks are due to d electrons, a difference synthesis of rutile, TiO2, was calculated based on carefully measured intensity data, since Ti 4+ possesses no 3d electrons. All attempts to shape a crystal specimen into a sphere failed. The specimen used had the dimensions 0"08 × 0"08 x 0.09 mm. The intensities were collected on a Rigaku automated four-circle diffractometer. The experimental conditions were exactly the same as those for 7-Ni2SiO4, Table 1. Crystal data and atomic parameters TiO2_,, ~=0.016 (7) U=62.072/~3 at 26°C Tetragonal, P42/mnm Dm=4"264 g cm -3 a=4"5845 (1)/~ D~,=4-260 g cm -3 c = 2-9533 (1) tl, Z= 2 O at (O,u,u) with u=0"30493 (7) Isotropic extinction parameter g= 0.29 × 10 4The temperature factors are in the form:exp { --27z2(h2a .2 UII + k2b .2 U22 -[-12c'2 U33

The electron-density distribution in crystals of K2Na[Co(NO2) 6] has been determined by the single-crystal Xray diffraction method. The residual electron density around the Co atom in the octahedral complex ion, [Co(NO2)6] 3-, comprises the eight positive peaks arranged at the corners of a cube [1.69 (14) e A -3 at 0.43 A from Co] and the six negative peaks on the Co-N bond [-0.79 (13) e A -3 at 0.62 A from Co]. The central Co atom is largely neutralized, the number of electrons within a sphere of radius 1.22 A (octahedral covalent radius of Co) being 26.3 (l). The valence-electron populations of the atoms were refined and the net charges of Co, N and O atoms were estimated to be +0.06 (6), -0.07 and -0.22 (2) e, respectively. A bonding peak with height 0.32 (10) e A -3 is observed between the N and O atoms. The bonding electron in the N-O bond is largely extended perpendicularly to the plane of the nitro group, showing some ~z-bond character.

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