It is shown by density functional theory using doubly polarized basis sets that the lower energy structures of silicon-bismuth clusters Bi4Si2 and Bi2Si4 have the same overall geometry and symmetry and can be formally obtained from each other by mutual substitution and optimization or by direct Bi-substitution to the corresponding neutral Si-6 cluster. Such 'structural reciprocity', which is attributed to the fluxionality of Si-6 cluster and the similarity of Bi and Si electronegativities, can be generally true for other pairs of Bi-Si clusters to a lesser degree, and could be remotely related with the metal-semiconductor transition in Bi-Si alloys. (C) 2010 Published by Elsevier B.V.