Abstract Reactions between [(η-C 5H 5)Ni] 2Fe 2(CO) 6(μ 4-C 2Ph 2) and PR 3 (R Me or OMe) afford [Ni(PR 3) 2(η-C 5H 5)][(η-C 5H 5)NiFe 2(CO) 6(μ 3-C 2Ph 2)], which has been characterised by a single-crystal X-ray study. The cation has distorted trigonal geometry, with NiC(cp) 2.113 Å (mean), NiP 2.151, 2.163 Å, and PNiP 98.9°. The anion contains an Fe 2Ni triangle, to which are attached 6 CO ligands (to Fe) and the C 5H 5 group (to Ni); the C 2Ph 2 moiety is attached via two σ bonds (to one Fe and the Ni) and a π bond (to the second Fe), with the CC bond parallel to the FeNi bond. Significant bond lengths include: FeFe 2.506(1), FeNi 2.453, 2.474(1), CC 1.383(7) Å. The formation of the square pyramidal Fe 2NiC 2 core in the complex anion by formal addition of Ph - to (η-C 5H 5)NiFe 2(CO) 6(C 2Ph) (containing a trigonal bipyramidal Fe 2NiC 2 core) is discussed.