Using a gradient-corrected density functional method, we studied computationally how single impurity atoms affect the structure and the properties of a Ni₄ cluster. H and O atoms coordinate at a Ni–Ni bond, inducing small changes to the structure of bare Ni₄ which is essentially a tetrahedron. For a C impurity, we found three stable structures at a Ni₄ cluster. In the most stable geometry, the carbon atom cleaves a Ni–Ni bond of Ni₄, binding to all Ni atoms. Inclusion of the impurity atom leads to a partial oxidation of the metal atoms and, in the most stable structures, reduces the spin polarization of the cluster compared to bare Ni₄. An H impurity interacts mainly with the Ni 4s orbitals, whereas the Ni 3d orbitals participate strongly in the bonding with O and C impurity atoms. For these impurity atoms, Ni 3d contributions dominate the character of the HOMO of the ligated cluster, in contrast to the HOMO of bare Ni₄ where Ni 4s orbitals prevail. We also discuss a simple model which relates the effect of a H impurity on the magnetic state of metal clusters to the spin character (minority or majority) of the LUMO or HOMO of the bare metal cluster.