Prepd by the reaction of magnesium boride with hydrochloric or phosphoric acid: Stock, Kuss, Ber. 56B, 789 (1923); from dihydropentaborane: Burg, Schlesinger, J. Am. Chem. Soc. 55, 4009 (1933); from diborane: Stock, Mathing, Ber. 69B, 1456 (1936); Klein et al., J. Am. Chem. Soc. 80, 4149 (1958). Review: Greenwood in Comprehensive Inorganic Chemistry vol. 1, J. C. Bailar, Jr. et al., Eds. (Pergamon Press, Oxford, 1973) pp 785-791.