The unimolecular rearrangement of XSnequivalent toP (X = H, Li, BeH, BH2, CH3, NH2, OH, and F) to Sn=PX is considered using B3LYP and QCISD calculations. The theoretical findings suggest that the doubly bonded Sn=PX molecule is intrinsically more stable than the triply bonded XSnequivalent toP species, regardless of the electronegativity of the substituent X attached. On the other hand, the model calculations predict that only bulky aryl substituents can greatly stabilize XSnequivalent toP over Sn=PX due to the steric effect. (C) 2003 Elsevier B.V. All rights reserved.