The ground state multiplicity of 2,4-dimethylene five-member heterocycles is dependent on the protonation of the heteroatom. The relative singlet and triplet energy at pi-CASSCF-optimized geometries Varies and is dependent on the number of protons attached to the heteroatom. The 2,4-dimethylenecyclopentadiene anion (X = CH-) and the 2,4-dimethylenepyrrole anion (X = N-) are ground state singlets while the 2,4-dimethylenecyclopentadiene (X = CH2) and 2,4-dimethylenepyrrole cation (X = NH2+) are ground state triplets. The neutral 2,4-dimethylenepyrrole (X = NH) is a ground state triplet with a weaker preference than that calculated for the 2,4-dimethyrenepyrrole cation. Similarly, the protonated 2,4-dimethylenefurane (X = OH+) is a strongly preferred ground stale triplet while the neutral 2,4-dimethylenefurane (X = OH) has a weaker preference for the triplet ground state. Analogous relationships exist for the systems with the third-row elements. These properties of the 2,4-dimethylene five-member heterocycles allow for the construction of molecular organic ferromagnets with pH-controlled magnetism.