Total energy calculations based on the generalized gradient approximation to the density functional theory reveal that the Ni(benzene) and Ni(benzene)(2) anions are unstable against autodetachment of the additional electron while other anion complexes containing more than one Ni atom are stable. Although the adiabatic electron affinities increase with Ni content, they are significantly smaller than those in pure Ni clusters containing the same number of Ni atoms. The difference between adiabatic electron affinities and vertical detachment energies are around 0.2 eV in most cases, indicating that the equilibrium geometries of Ni-n(benzene)(m)(-) are not significantly altered from their corresponding neutral geometries. The vertical transitions from the anion to the neutral provide new insight into the magnetic moment of these organometallic complexes.