To answer the question, "Can a quartet be the ground state of heteroatom analogues of trimethylenemethane (TMM)?", B3LYP, CASSCF, and CASPT2 calculations have been performed on the lowest doublet and quartet states of the positive and negative radical ions of TMM and of several heteroatom-substituted analogues. Of the molecules on which calculations were performed, all those containing three pi electrons, including AlO3., were found to have doublet ground states. However, a quartet ground state was computed for O(SiH2)(3)(.+), a radical ion containing five pi electrons. Calculations on H-3(.) and HeH3. models revealed a similar difference between the three-and five-electron cases. A doublet was computed to be the ground state of H-3(.) at all D-3h, geometries, but at some He-H distances the quartet was computed to be the ground state of D-3h HeH3.. The model calculations lead to an explanation of why radicals containing three pi electrons are all predicted to have doublet ground states; whereas, the quartet is computed to be the ground state of at least one radical containing five pi electrons.