[μ-2,3,5,6-Tetrakis(2-pyridyl)pyrazine κ⁶N ⁶,N¹,N²:N³,N⁴,N ⁵]bis[diaqua(dihydrogen m-phenylenediphosphonato κo) nickel(II)] dihydrate

The title compound [Ni₂(C₆H₆O ₆P₂)₂(C₂₄H₁₆N ₆)(H₂O)₄]·₂H₂O or [Ni₂(tpyprz)(1,3-HO₃PC₆H₄PO ₃H)₂(H₂O)₄]· ₂H₂O [tpyprz = tetra-kis-(2-pyrid-yl)pyrazine, C ₂₄H₁₆N₆] 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 {NiO₃N ₃} octa-hedral coordination, with the three nitro-gen donors of each terminus of the tpyprz ligand in a meridional orientation. An aqua ligand occupies the position trans to the pyrazine nitro-gen donor, while the second aqua ligand is trans to the oxygen donor of the dihydrogen-1,3-phenyl-diphospho- nate 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 organo-phospho-nate ligand are evident in the significantly longer P - O bonds compared to the unprotonated sites. In the crystal structure, the complex mol-ecules and the solvent water mol-ecules are linked into a three-dimensional hydrogen-bonded framework through O - H⋯O inter-actions between the aqua ligands, the protonated organo-phospho-nate oxygen atoms and the water mol-ecules of crystallization. Intra-molecular π-stacking between the phenyl group of the phospho-nate ligand and a pyridyl group of the tpyprz ligand, at a distance of 3.244 (5) Å between ring centroids, is also observed.