Although simple theoretical considerations suggest that the B-B distances should be shorter in five-vertex close deltahedra than in the six-vertex deltahedra, boron-containing five-vertex deltahedra show a wide range of B-B distances in the equatorial belt of the trigonal bipyramid which are longer than those in their square-bipyramid analogs. High-quality ab initio molecular orbital calculations at the Hartree-Fock and MP2 levels have been performed on (BX)(n)(Y)(2) systems (n = 3 and 4) varying the apical group Y and substituent X on the equatorial boron. Geometrical optimizations show how these distances broadly correlate with the electronegativity of the axial atoms. For the case of (BX)(n)(Y)(2) molecules with n 3 with X = H and Y = N, this B-B distance is calculated to be quite short (1.65 Angstrom) but for Y = SiH and X = NH2 much longer (1.96 Angstrom). This calculated variation in B-B distance, in accord with experimental values when available, is significantly larger than when n = 4. The result is two sets of molecules, those which appear to obey Wade’s rules and form regular deltahedra (n = 4 for all X, Y, and n = 3 for X, Y = H, N) and those where the B-B distance is either so long or the computed B-B overlap population negative so that each of the equatorial boron atoms are essentially three-coordinate. (This occurs for Y not equal N, all X, and n = 3.). There is a strong effect of the electronegativity of the axial atoms. These electronic differences are readily understood by examination of the orbitals of the molecular building blocks.