The magnetic behaviour of the compounds containing the [Ru(2)(DPhF)(3)(O(2)CMe)](+) ion (DPhF(-)=N,N'-diphenylformamidinate) shows a strong dependence on the nature of the ligand bonded to the axial position. The new complexes [Ru(2)(DPhF)(3)(O(2)CMe)(OPMe(3))][BF(4)]0.5 CH(2)Cl(2) (1 0.5 CH(2)Cl(2)) and [Ru(2)(DPhF)(3)(O(2)CMe)(4-pic)][BF(4)] (2) (4-pic=4-methylpyridine) clearly display this influence. Complex 1.0.5 CH(2)Cl(2) shows a magnetic moment corresponding to a S=3/2 system affected by the common zero-field splitting (ZFS) and a weak antiferromagnetic interaction, whereas complex 2 displays an intermediate behaviour between S=3/2 and S=1/2 systems. The experimental data of complex 1 are fitted with a model that considers the ZFS effect using the Hamiltonian H(D)=S.D.S. The weak antiferromagnetic coupling is introduced as a perturbation, using the molecular field approximation. DFT calculations demonstrate that, in the [Ru(2)(O(2)CMe)(DPhF)(3)(L)](+) complexes, the energy level of the metal-metal molecular orbitals is strongly dependent on the nature of the axial ligand (L). This study reveals that the increase in the pi-acceptor character of L leads to a greater split between the pi* and delta* HOMO orbitals. The influence of the axial ligand in the relative energy between the doublet and quartet states in this type of complexes was also analysed. This study was performed on the new complexes 1.0.5 CH(2)Cl(2) and 2. The previously isolated [Ru(2)(DPhF)(3)(O(2)CMe)(OH(2))][BF(4)].0.5 CH(2)Cl(2) (3.0.5 CH(2)Cl(2)) and [Ru(2)(DPhF)(3)(O(2)CMe)(CO)][BF(4)].CH(2)Cl(2) (4.CH(2)Cl(2)) complexes were also included in this study as representative examples of spin-admixed and low-spin configurations, respectively. The [Ru(2)(DPhF)(3)(O(2)CMe)](+) (5) unit was used as a reference compound. These theoretical studies are in accordance with the different magnetic behaviour experimentally observed.