The metal-metal multiply bonded complexes Cp₂M₂(CO)₄ (M = Mo, W; M =M), Os₃(CO)₁₀H₂ (0s =0s) and Cp*₂Rh₂(C0)₂ (Rh=Rh) were reacted with phosphines containing either P-H or P-X bonds (X = Cl, Br, I). The reactions were studied in situ at variable temperatures using ¹H, ¹⁹F or ³¹P n.m.r. spectroscopy. Cp₂M₂(C0)₄ reacted with PR¹R²H (R¹ = R² = H, Me; R¹ = H, R² = Me) to form the adducts Cp₂M₂(CO)₄(PR¹R²H)₂ at -90 °C, which rearranged on warming with the loss of PR¹R²H to afford the species Cp₂M₂(C0)₄(μ₂H)(μ₂PR¹R²). When Cp₂M₂(C0)₄ was reacted with PF₂H at -90 °C, the adducts Cp₂M₂(CO)₄(PF₂H)₂ were formed and remained stable up to 110°C, Reaction of Cp₂M₂(C0)₄ with PF₂H at 25°C yielded the species CP₂M₂(CO)₄(μ₂H)(μ₂PF₂). PF₂Cl and PF₂Br reacted with Cp₂M₂(CO)₄ to afford the species Cp₂M₂(C0)₄(μ₂PF₂)X (X = Cl, Br). PF₂I reacted with Cp₂W₂(C0)₄ at -90°C to form Cp₂W₂(C0)₄(μ₂PF₂)I which rearranged on warming to -13 °C to afford Cp₂W₂(C0)₄(μ₂PF₂)(μ₂I). Os₃(C0)₁₀H₂ reacted with PR¹R²H (R¹ = R¹ = H, Me; R¹ = H, R² = Me) at 25 °C to afford the species HOs₃(C0),₁₀(μ₂H)(PR¹R²H). These species rearranged on warming with the loss of H₂ to afford Os₃(CO)₁₀(μ₂H)(μ₂PR¹R²). If R¹ = H, then these complexes reacted further with the loss of CO to yield the species Os₃(CO)9(μ₂H)₂(μ₃PR²). PF₂Cl and PMe₂Cl reacted with Os₃(C0)₁₀H₂ to yield 0s₃(C0)₁₀(µ₂H)(µ₂PF₂) and Os₃(C0)₁₀(µ₂H)(µ₂PMe₂), respectively. PF₂H reacted with Os₃(CO)₁₀H₂to form Os₃(CO)₁₀(µ₂H)(µ₂PFH). Cp*₂Rh₂(CO)₂ reacted with PR¹R²H (R¹ = H, R² = Me; R¹ = R² = H, Me) or PE₂NH₂ to afford Cp*₂Rh₂(CO)₂(PR'R²H) or Cp*₂Rh₂(C0)₂(PF₂NH₂) at -90°C. These species decomposed upon warming. When PR₂Cl (R = Me, Ph, OEt) or PF₂Cl were reacted with Cp*₂Rh₂(C0)₂, the species [Cp*₂Rh₂(C0)₂(µ₂PR₂)] +Cl- or [Cp*₂Rh₂(C0)₂(µ₂PF₂)]⁺Cl⁻ were formed. The former species were stable at 25°C whilst the latter decomposed at -70°C. PF₂H reacted with Cp*₂Rh₂(C0)₂ to afford [Cp*₂Rh₂(C0)₂(µ₂PFH)]⁺F⁻ at -80°C. This species decomposed at -60°C. HPF₂0 reacted with Cp*₂Rh₂(CO)₂ to yield presumably Cp*₂Rh₂(CO)₂(PF₂0)(H) while HPF₂S reacted with Cp*₂Rh₂(C0)₂ to yield presumably [Cp*₂Rh₂(C0)₂(HPFS)]⁺F⁻. The single crystal x -ray structures of HOs₃(C0)₁₀(µ₂H)(PMe₂H), Os₃(C0)₁₀(µ₂H)(µ₂PMe₂), Os₃(CO)₉(µ₂H)₂(µ₃PMe) and [Cp*₂Rh₂(CO)₂(µ₂PMe₂)]⁺ were solved.