A novel 1 1 co-crystal of bis(1 10-phenanthroline)( 1 10-phenanthroline -5 6-dione)nickel(II) hexafluorophosphate and tris(1 10-phenanthroline)nickel(II) hexafluorophosphate complexes Ni(phen)2(phen-dione) Ni(phen)3 (PF6)4