The heterobimetallic complexes [(η⁵-C₅R₅)Ru(CO)(μ-dppm)M(CO)₂(η⁵-C₅H₅)] (M = Mo, W; R = H, CH₃) (1–4) are prepared by metathetical reactions between (η⁵-C₅R₅)Ru(dppm)Cl and Na⁺[(η⁵-C₅H₅)M(CO)₃]⁻. IR spectroscopic and X-ray structural studies show that each of these complexes contains a semi-bridging carbonyl ligand. The low activity of these complexes in catalytic CO₂ hydrogenation to formic acid might be attributed to their non-facile reactions with H₂ to yield the active dihydride species. The metal–metal bonds can be protonated to form the cationic complexes, which contain strong Ru–H–M bridges. The bridging hydrogen atom is weakly acidic since it can be removed by a strong base, and it protonates BPh₄⁻ to give BPh₃ and benzene. It also reacts with the hydridic hydrogen of Et₃SiH to yield H₂. The bridging hydrogen, however, cannot be removed by hydride scavengers such as Ph₃C⁺OTf⁻ and Me₃Si⁺OTf⁻. The sluggishness of the catalytic formic acid decomposition by 1–4 is attributable to the stability of the protonated bimetallic intermediate [(η⁵-C₅R₅)Ru(CO)(μ-dppm)(μ-H)M(CO)₂(η⁵-C₅H₅)]⁺HCOO⁻ formed during the catalysis.