The effects of relativity on the bonding between heavy elements of groups 13 and 17 have been investigated. Using extensive energy minimized basis sets, calculations have been carried out within several models which include relativistic effects at various levels-4-component Dirac-Hartree-Fock, spin-free relativistic, Levy-Leblond, mass-velocity-Darwin perturbation theory, as well as nonrelativistic Hartree-Fock calculations. The relativistic effects on spectroscopic constants are dramatic, and dominated by the spin-orbit splitting and the relativistic contraction of the valence p-orbitals. Particularly noteworthy is the spin-orbit induced change of sign of the dipole moment for [113][117]. This highlights the importance of including the spin-orbit effect in the variational optimization of the wave function for heavy elements.