In analogy with Sanderson's electronegativity equalization principle, it is possible to postulate a principle of spin potential equalization in the E[N(alpha), N(beta)] representation of the spin polarized density functional theory, where N(alpha) and N(beta) refer to the number of electrons with spins alpha and beta, respectively. The principle provides simple expressions to evaluate the energy changes DeltaE between two interacting molecules, A and B, together with the electron transfer, DeltaN(alpha) and DeltaN(beta). The model is illustrated for a series of addition reactions of electrophilic, nucleophilic, and ambiphilic carbenes to alkenes in their singlet and triplet multiplicities. The results are in a consistent qualitative agreement with the experimental reactivity established for these systems.