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2. G. WILL, 1969. Acta Cryst., B 25, 1219; Idem, 1967. Ibid., 23, 1071.
3. Structure Reports, 9, 154; 17, 315; 23, 270; 26, 67.
4. J.L. HOARD and R.E. HUGHES, 1967. The Chemistry of Boron and its Compounds. pp. 62, 88. New York, Wiley.
5. A.J. PERROTTA, W.D. TOWNES and J.A. POTENZA, 1969. Acta Cryst., B 25, 1223.