[μ-2,3,5,6-Tetra­kis(2-pyrid­yl)pyrazine-κ(6) N (6),N (1),N (2):N (3),N (4),N (5)]bis­[diaqua(dihydrogen m-phenylene­diphospho­nato-κO)nickel(II)] dihydrate

The title compound [Ni(2)(C(6)H(6)O(6)P(2))(2)(C(24)H(16)N(6))(H(2)O)(4)]·2H(2)O or [Ni(2)(tpyprz)(1,3-HO(3)PC(6)H(4)PO(3)H)(2)(H(2)O)(4)]·2H(2)O [tpyprz = tetra­kis­(2-pyrid­yl)pyrazine, C(24)H(16)N(6)] is a binuclear complex with a crystallographic inversion center located at the center of the pyrazine ring. The equivalent nickel(II) sites exhibit a distorted {NiO(3)N(3)} octa­hedral coordination, with the three nitro­gen donors of each terminus of the tpyprz ligand in a meridional orientation. An aqua ligand occupies the position trans to the pyrazine nitro­gen donor, while the second aqua ligand is trans to the oxygen donor of the dihydrogen-1,3-phenyl­diphospho­nate ligand. The Ni—O and Ni—N bond lengths fall in the range 2.011 (3) to 2.089 (3) Å. The protonation sites on the organo­phospho­nate ligand are evident in the significantly longer P—O bonds compared to the unprotonated sites. In the crystal structure, the complex mol­ecules and the solvent water mol­ecules are linked into a three-dimensional hydrogen-bonded framework through O—H⋯O inter­actions between the aqua ligands, the protonated organo­phospho­nate oxygen atoms and the water mol­ecules of crystallization. Intra­molecular π-stacking between the phenyl group of the phospho­nate ligand and a pyridyl group of the tpyprz ligand, at a distance of 3.244 (5) Å between ring centroids, is also observed.