Silver(I) diaquanickel(II) catena-borodiphosphate(V) hydrate, (Ag0.57Ni0.22)Ni(H2O)2[BP2O8]·0.67H2O

The structure framework of the title compound, (Ag0.57Ni0.22)Ni(H2O)2[BP2O8]·0.67H2O, is the same as that of its recently published counterpart AgMg(H2O)2[BP2O8]·H2O. In the title structure, the Ag, Ni, B and one O atom are located on special positions (sites symmetry 2). The structure consists of infinite borophosphate helical [BP2O8]3− ribbons, built up from alternate BO4 and PO4 tetrahedra arranged around the 65 screw axes. The vertex-sharing BO4 and PO4 tetrahedra form a spiral ribbon of four-membered rings in which BO4 and PO4 groups alternate. The ribbons are connected through slightly distorted NiO4(H2O)2 octahedra, four O atoms of which belong to the phosphate groups. The resulting three-dimensional framework is characterized by hexagonal channels running along [001]. However, the main difference between the structures of these two compounds lies in the filling ratio of Wyckoff positions 6a and 6b in the tunnels. Indeed, in this work, the refinement of the occupancy rate of sites 6a and 6b shows that the first is occupied by water at 67% and the second is partially occupied by 56.6% of Ag and 21.6% of Ni. In the AgMg(H2O)2[BP2O8]·H2O structure, these two sites are completely occupied by H2O and Ag+, respectively. The title structure is stabilized by O—H⋯O hydrogen bonds between water molecules and O atoms that are part of the helices.

The anionic partial structure of the title compound contains one-dimensional infinite helices, [BP 2 O 8 ] 3-, which are wound around 6 5 axis. It is built up from alternate borate (BO 4 ) and phosphate (PO 4 ) tetrahedra, forming a spiral ribbon. The Ni 2+ cation is six fold coordinated by four oxygen atoms originating from four phosphate groups and two water molecules as shown in Fig.1. The ribbons are interconnected through slightly distorted NiO 4 (H 2 O) 2 octahedra. The resulting 3-D framework shows hexagonal tunnels running along c direction where water molecules are located (Fig.2).
The +I, +II, +III and +V oxidation states of the Ag, Ni, B and P atoms were confirmed by bond valence sum calculations (Brown & Altermatt, 1985). The calculated values for the Ag + , Ni II+ , B III+ and P V+ ions are as expected, viz. 1.06, 1.89, 3.06 and 4.97, respectively.
The main difference between the structure of this compound and that of his counterpart AgMg(H 2 O) 2 [BP 2 O 8 ],H 2 O lies in the filling ratio of the Wyckoff positions 6a and 6b in tunnels. Indeed, in this work, the refinement of the occupancy rate of the following sites (symmetry 2) 6a (x, 0, 0) and 6b (x, 2x, 3/4) (space group P6 5 22) shows that the first is occupied by water at 67% and the second is partially occupied by 56.6% of Ag + and 21.6% of Ni 2+ for a total of 78%. While in the case of AgMg(H 2 O) 2 [BP 2 O 8 ],H 2 O structure these two sites are completely occupied by H 2 O and Ag + respectively. The structure is stabilized by O-H···O hydrogen bonds between water molecules and O atoms that are part of the helices ( Fig.1 and Table 1).

Experimental
The compound was hydrothermally synthesized at 453 K for 7 days in a 25 ml Teflon-lined steel autoclave from the mixture of NiCO 3 , H 3 BO 3 , H 3 PO 4 (85%), AgNO 3 and 5 mL of distilled water in the molar ratio of 1:4:6:1:165. The brilliant colourless octahedral crystals were recovered and washed with hot water, then dried in air.
Except for boron and hydrogen the presence of the elements were additionally confirmed by EDAX measurements.

Refinement
The refinement with the sites of Ag and O6 fully occupied led to R = 0.06, Rw = 0.20 and two peaks in the Fourier difference map +4.7 and -3.4 at 0.18 Å from Ni and at 0.35 Å from Ag respectively. While the refinement of the occupancy rate of Ag(0.350 (2)) and O6(0.34 (1)) leads to R = 0.027, Rw = 0.07 and two peaks +1.21 and -0.66 at 0.65Å and 0.52Å from Ag.
In the final refinement cycles, the occupancy of O6 was fixed to 0.667. However, the large atomic displacement parameter for this atom indicate disorder or atomic movement along the partially empty tunnel.
The electrical neutrality of the molecule led us to put some nickel in the site of Ag. The refinement of this model leads to the ratio Ag/Ni about 0.47. In the site occupied by a mixture of Ag + and Ni 2+ , the cations are constrained to have the same positional and displacement parameters and the sum of the occupancy rate is restrained to fit the charge balance. The highest peak and the minimum peak in the difference map are at 0.66 Å and 0.52 Å respectively from Ag1 atom. The O-bound H atoms were initially located in a difference map and refined with O-H distance restraints of 0.86 (1). In the last cycles they were refined in the riding model approximation with U iso (H) set to 1.5U eq (O). The 779 Friedel opposite reflections are not merged. Fig. 1. Partial plot of (Ag 0.57 Ni 0.22 )Ni(H 2 O) 2 [BP 2 O 8 ],0.67(H 2 O) crystal structure showing plyhedra linkage. Displacement ellipsoids are drawn at the 50% probability level. Symmetry codes: (i) -y + 1, -x + 1, -z + 13/6; (ii) y -1, -x + y, z + 1/6; (iii) y -1, x, -z + 5/3; (iv) x, x-y + 1, -z + 11/6; (v) -x + y -1, y, -z + 3/2; (vi) -x, -x + y, -z + 4/3; (vii) y, x + 1, -z + 5/3; (viii) x-y + 1, -y + 2, -z + 2.

Special details
Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The