Investigation of the main configurations of these two low-lying quartet states shows that they differ by the occupation of the d x 2 -y 2 and d xy orbitals, which are either doubly or singly occupied (see supporting information Fig. ). This had been noted before. The coupling of these two configurations via the spin-orbit operator results in two states with a strong orbital angular momentum along the main molecular axis, which couples to the electron spins to give either an enhanced total magnetic momentum along the main axis or a reduced one. Because the orbital momentum is created by a hole in the d x 2 -y 2 /d xy subshell, the spin-orbit coupled configuration with enhanced projection along the main axis is energetically more stable, in agreement with Hund's rules. These observations suggest using an extended spin Hamiltonian (denoted 'model m2' in the following) of the form