loop_ _valence_param.atom_1 _valence_param.atom_1_valence _valence_param.atom_2 _valence_param.atom_2_valence _valence_param.Ro _valence_param.B _valence_param.ref_id _valence_param.details Cu 2 O -2 1.679 0.37 a . Cu 2 O -2 1.649 0.37 j . Cu 2 N -3 1.64 0.37 m '2-coordinate N' Cu 2 N -3 1.76 0.37 m '3-coordinate N' loop_ _valence_ref.id _valence_ref.reference a 'Brown & Altermatt (1985), Acta Cryst. B41, 244-247' j 'Liu & Thorp (1993), Inorg. Chem. 32, 4102-4205' m 'See, Krause & Strub (1998), Inorg. Chem. 37, 5369-5375'