We report on the calculation of the spin-spin coupling term J in a diarylnitroxide diradical system which is a slightly simplified version of a recently synthesized stable species with a triplet ground state. We have applied our complementary space perturbative approach using virtual orbitals appropriately modified with the aim of investigating, through molecular fragmentation, the effects of the non-bridge aryls. We show how by using multireference CI techniques it is now possible to obtain accurate and reliable values of J even for large organic radicals, the basic units of longer chain polyradical systems.