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'