Gravar-mail: μ-2,3,5,6-Tetra-2-pyridylpyrazine-κ(3) N (1),N (2),N (6):κ(3) N (3),N (4),N (5)-bis­[(methanol-κO)(nitrato-κ(2) O,O′)(nitrato-κO)cadmium(II)]