Density functional approach has been employed to investigate the ground state geometries and stabilities of RuO n species (n = 1 to 5) in neutral as well as in anionic forms. It is shown that Ru can bind stably with four O atoms indicating the maximum oxidation state of Ru as high as +8. The adiabatic electron affinities of RuO n reveal the superhalogen behaviour of RuO4. The interaction of RuO4 superhalogen with an alkaline metal Ca forms a stable CaRuO4 complex. In this complex, RuO4 unit closely mimics the behaviour of O atom when compared with CaO. This provides an opportunity to synthesize new class of complex compounds by interaction of RuO4 with appropriate metal atom.