The compound [Fe(C₄H₄N)(C₅H₅)] reacts with [Os₃(CO)₁₀(MeCN)₂] to give the air-stable orthometallated compound [Os₃H{(C₄H₃N)Fe(C₅H₅)}(CO)₁₀]1, the crystal structure of which has been determined. The Os₃ cluster is linked to the Fe atom by the eight-electron donating C₄H₃N ligand. Cyclic voltammetric studies showed that the cation 1⁺ is formed reversibly with E_½ of 0.32 V (vs. ferrocene–ferrocenium). The IR spectrum of 1⁺ formed at –25°C in a spectroelectrochemical cell is closely related to that of 1 indicating that there is no geometric rearrangement on oxidation. The extent of interaction between the cluster and the azaferrocenyl group is estimated by the shift of v(CO) on oxidation. The shift of 8–14 cm^–1 to higher energy indicates that, although the electron is removed from the azaferrocenyl group, there is nevertheless a significant change in orbital energies at the cluster on oxidation. Cation 1⁺ decomposes within 1 min at –25°C, yielding a number of unidentified carbonyl species. The related orthometallated compound [Os₃H{(C₄H₃N)Mn(CO)₃}(CO)₁₀]2 was correspondingly synthesised and characterised by X-ray diffraction.