Valence structures and magnetic interactions of 1-D mixed-metal complexes, [Ni 1−x Pd x X(chxn) 2 ]X 2 (0.0 ≤ x ≤ 1.0, chxn = (1 R ,2 R )-diaminocyclohexane; X = Cl, Br) studied by 15 N solid-state NMR (Valence structures and magnetic interactions of 1-D mixed-metalcomplexes, [Ni1?xPdxX(chxn)2]X2(0.0 ? x ? 1.0,chxn = (1R,2R)-diaminocyclohexane; X = Cl, Br) studied by15Nsolid-state NMR)