Almasi, Lucreţia (1971). "The Sulfur–Phosphorus Bond". In Senning, Alexander (ed.). Sulfur in Organic and Inorganic Chemistry. Vol. 1. New York: Marcel Dekker. pp. 51–55. ISBN0-8247-1615-9. LCCN70-154612, although note that Almasi identifies the chemoselective axis as hard-softness instead.