A diruthenium μ-phosphane complex, {Cp‡Ru­(μ-H)}2(μ-η2-HP t Bu2) (2a; Cp‡ = 1,2,4-C5 t Bu3H2), which features an agostic Ru–H–P interaction at one ruthenium center, was synthesized by the reaction of Cp‡Ru­(μ-H)4RuCp‡ (1) with P t Bu2H. The agostic Ru–H–P interaction was stabilized by the bulky Cp‡ and t Bu groups. The agostic Ru–H–P interaction in 2a was unambiguously confirmed by X-ray diffraction. The rapid site exchange at the P–H and Ru–H–Ru positions indicated that the η2-P–H bond underwent oxidative addition in solution, yielding a μ-phospanido isomer, {Cp‡Ru­(μ-H)}2(H)­(μ-P t Bu2) (3a). The population of the μ-phosphanido isomer increased when the size of the substituent at the phosphorus atom was reduced, and μ-phosphanido complex 3b was exclusively obtained by the reaction of 1 with PPh3.