We propose a mechanism for the cage-opening reaction of a four atoms shared closo/closo B20H16 (1) with MeCN to a face shared closo/nido macropolyhedron B20H16(MeCN)(2) (4) through a diamond-square-diamond rearrangement. Even though only one isomer of the product has been reported experimentally, our computational studies at the B3LYP/6-31G* level predict the possibility of the formation of the other isomers. Depending upon the position of the attack of the MeCN ligand on the polyhedral skeleton, different products are formed. The energetics of the reactions of B20H16 with Me2S and H2O are comparable.