Dipotassium hexaaquanickel(II) bis[hexafluoridozirconate(IV)]

Single crystals of the title compound, K2[Ni(H2O)6][ZrF6]2, were grown by slow evaporation of a 40% aqueous HF solution in which a stoichiometric mixture of NiCl2·6H2O, ZrF4 and KCl was dissolved. The monoclinic structure is isotypic with its K2Cu, K2Zn, Cs2Zn and Cs2Cu analogues. The structure is built up from isolated, slightly elongated octahedral [Ni(H2O)6]2+ complex cations (symmetry ) and dimeric [Zr2F12]4− complex anions (symmetry ) that are also isolated from each other. The [Zr2F12]4− anion results from the association of two distorted pentagonal–bipyramidal [ZrF7] coordination polyhedra by sharing an equatorial edge passing through an inversion center of the unit cell. Both isolated [Ni(H2O)6]2+ and [Zr2F12]4− complex ions are situated in planes parallel to (010). They are connected by the eight-coordinated K+ ions into a three-dimensional structure. An intricate O—H⋯F hydrogen-bonding network consolidates the structure.

A careful examination of the geometry of the [Zr 2 F 12 ] 4complex anion in isotypic structures, refined from single-crystal data, shows this anion being quasi unvarying for all the members ( Table 2). The distortion index (bond length) (Momma & Izumi, 2008) is the same for all the K compounds (0.0265) and is only very slightly higher (0.02985) for the Cs analogue.
It is also worth noting that the higher the index of distortion of the [M II (H 2 O) 6 ] cationic polyhedron, the lower the index of distortion of the counter cation K + (0.04429). This observation is obvious because water molecules are only shared between K + and M 2+ ions.

Experimental
Single crystals of the title compound were obtained by reacting a mixture of NiCl 2 .6H 2 O, ZrF 4 and KCl in the molar ratio 1:2:2 with a 40% aqueous HF boiling solution in a platinum crucible. Then the solution was poured out into a PTFE beaker and slowly evaporated to dryness using a sand bath. Green single-crystals of the title compound were extracted from the dry residue.

Refinement
The highest residual peak in the final difference Fourier map was located 0.60 Å from the Zr atom and the deepest hole was located 0.86 Å from the same atom. H atom parameter were fefined freely.

Special details
Geometry. All e.s. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.