After a short treatment of the ionic model of ROBERTSON and McCONNELL, two methods for the calculation of the coefficients of the spin Hamiltonian are given and discussed in connection with HUNDS rule and the magnitude of the effective total spin S. In addition, the dependence of the effective magnetic moment on those coefficients in shown. A quantitative calculation of the energy levels, wave functions and magnetic resonance properties of V (C5H5) 2, the latter being in good agreement with the experiments of McCONNELL, concludes this section. In Part II the magnetic-resonance behaviour of Ni (C5H5)2 and Co (C5H5)2 is investigated theoretically under various assumptions and as far as possible compared with experiment.