The concurrent short Ru-Ru and Ru-L bond lengths (L = axial ligand) and the stronger oxidation resistance found in [Ru{eta3-HB(tz)3}(CO)2]2 (1) (tz = 1,2,4-triazolyl) relatively to [Ru{eta3-HB(pz)3}(CO)2]2 (2) (pz = 1-pyrazolyl) lead to the recognition that the HOMO is probably a sigma orbital for 1 and 2, though both compounds have the same filled orbitals of sigma, pi, delta, delta* and pi* as [Ru2(CO)4(O2CMe)2L2].