[Ru(O)₄]

The four oxo groups as a tetrahedral set are classified as X₈L (neutral representation, 10 electrons, σ and π-bonding) for the d-block metal rather than X₈ (four X₂ groups). The four oxygen atoms possess 16 valence orbitals (24 electrons) and may be treated as sp hybridised. Four (occupied) point away from the metal and are non-bonding (8 electrons). Three of the π-orbitals are represented as T₁, have no corresponding combination on the metal (6 electrons), and so are non-bonding. The remaining ten electrons constitute the X₈L class. Overall there are nine bonding orbitals (A₁, E, 2T₂) in point group (T_d) connecting the four oxo groups set with the metal. See:

• T.Ziegler, A.Rauk, and E.J.Baerends, Chem. Phys., 1976; 16, 209-217, https://doi.org/10.1016/0301-0104(76)80056-0.
• H.B.Gray, Coord. Chem. Rev., 1966; 1, 2-6, https://doi.org/10.1016/S0010-8545(00)80146-0.
• M.L.H. Green, J. Organomet. Chem., 1995, 500, 127-148, https://doi.org/10.1016/0022-328X(95)00508-N.