It is demonstrated that location of hydrogen atoms within bonding distance of a simple triatomic molecule can alter the order of energy levels relative to that of the parent system without significantly changing the behavior of the orbital energies under geometrical variation. As a result the ground state electronic configurations of H n AB2 systems often differ from those of triatomics with the same number of electrons and this fact is shown to be directly responsible for the existence of such distinctive geometrical structures as those possessed by cyclopropane, cyclopropyl and allyl cations and cyclopropene. Thus the familiar prescription of Walsh's rules which states that isoelectronic molecules possess identical geometries is seen to be valid only if the systems in question possess the same electronic configuration; in reality it may be an excited state of one system which has the same geometry as the ground state of another andvice versa. It is concluded that the methodology of Walsh's rules can be extended to a much larger class of systems than heretofore assumed simply by taking explicit account of the predictable effects of hydrogen addition upon the electronic configuration and hence on the skeletal arrangement of the heavy nuclei.