Ca3MnRu2O9 and Ba3MnRu2O9 were synthesized from transition metal dioxides and alkaline earth metal carbonates at 1100-1300 degreesC. Ca3MnRu2O9 adopts the prototypical GdFeO3-type perovskite structure with Mn and Ru statistically disordered over the single metal atom site. The susceptibility shows Curie-Weiss behavior above 240 K with mu (eff) = 3.14 mu (B)/metal atom, which is in excellent agreement with the expected spin-only moment of 3.20 mu (B). Below 150 K, the compound shows spin-glass-like short-range ferrimagnetic correlations. The high temperature region of the electrical resistivity reveals a small activation energy of 17(1) meV whereas the low temperature region is nonlinear and does not fit a variable range hopping model. Ba3MnRu2O9 crystallizes in the 9-layer BaRO3-type structure containing M3O12 face-shared trioctahedral clusters in which Mn and Ru are statistically disordered. Ba3MnRu2O9 shows nonlinear reciprocal susceptibility at all temperatures and is described by a variable-spin cluster model with an S = 1/2 ground state with thermally populated excited states. The low spin value of this system (S = 1/2) is attributed to direct metal-metal bonding. Below 30 K, the compound shows short-range magnetic correlations and spin-glass-like behavior. The high-temperature region of the electrical resistivity indicates a small activation energy of 8.8(1) meV whereas the low-temperature region is nonlinear. The importance of metal-metal bonding and the relationships to other related compounds are discussed.