The usual assumption of the extra stability of icosahedral boranes ( 2) over pentagonal-bipyramidal boranes ( 1) is reversed by substitution of a vertex by a group 13 metal. This preference is a result of the geometrical requirements for optimum overlap between the five-membered face of the ligand and the metal fragment. Isodesmic equations calculated at the B3LYP/LANL2DZ level indicate that the extra stability of 1-M-2,4-C2B4H7 varies from 14.44 kcal/mol (M = Al) to 15.30 kcal/mol (M = Tl). Similarly, M(2,4-C2B4H6)(2)(1-) is more stable than M( 2,4-C2B9H11)(2)(1-) by 9.26 kcal/mol (M = Al) and by 6.75 kcal/ mol (M = Tl). The preference for (MC2B4H6)(2) over (MC2B9H11)(2) at the same level is 30.54 kcal/ mol (M = Al), 33.16 kcal/mol (M = Ga) and 37.77 kcal/mol (M = In). The metal-metal bonding here is comparable to those in CpZn-ZnCp and H2M-MH2 ( M) Al, Ga, and In).