Relativistic effects on bonding

A recently proposed perturbational approach to relativistic calculations on molecules has been applied to a number of compounds containing heavy elements. Comparison of calculated spectroscopic constants with experiment and results from other theoretical models shows that the method is a viable alternative to more involved treatments. Relativistic corrections are essential for compounds containing heavy elements and result in a contraction of bond distances and a substantial change in bond energies. The perturbational approach lends itself readily to a straightforward interpretation of both effects. It appears that the bond contraction is not due to the relativistic contraction of valence atomic orbitals. It is a direct relativistic effect, the repulsion due to the rise in kinetic energy when bonds are shortened being diminished by the mass–velocity correction.