Irradiation with u.v. light of mixtures of [Cr(CO)3(η-arene)](arene = C6H6, C6H3Me3-1,3,5, or C6Me6) and [Rh(CO)2(η-C5Me5)] in tetrahydrofuran (thf) affords the dimetal compounds [CrRh(µ-CO)2(CO)2(η-C5Me5)(η-arene)]; the complex [CrRh(µ-CO)2(CO)2(η-C9H7)(η-C6H3Me3-1,3,5)](C9H7= indenyl) has been similarly prepared from [Rh(CO)2(η-C9H7)] and [Cr(CO)2(thf)(η-C6H3Me3-1,3,5)]. The i.r. and n.m.r. data for these species are reported and discussed in relation to the molecular structure of [CrRh(µ-CO)2(CO)2(η-C5Me5)(η-C6H6)] which has been established by an X-ray diffraction study : crystals are orthorhombic, space group Pnma(no. 62), in a unit cell with a= 10.635(4), b= 11.690(3), c= 15.275(5)Å, and Z= 4. The structure has been refined to R 0.051 from 1 584 independent intensities [I 2.5σ(I)]. The molecule is constrained crystallographically to Cs, symmetry, the mirror plane being defined by the terminal carbonyl ligand on each metal atom, the Rh–Cr bond [2.757(2)Å], and the centroids of the two cyclic ligands. The pentamethylcyclopentadienyl ligand on the Rh atom is in a trans relationship to the η6-benzene ligand on the Cr atom, and both lie astride (perpendicular to) the mirror plane. The two other carbonyl ligands are terminal to the Cr atom, but are strongly semi-bridging to the Rh atom [Cr–C 1.902(7), Rh–C 2.200(7)Å], and define planes which are nearly perpendicular to the mirror plane. Some distortion of the Rh–C5 geometry towards a ‘diolefin’ type attachment is noted and discussed. Reaction of the compounds [M(CO)5(thf)](M = Cr or W) and [Mo(NCMe)(CO)5] with [Rh2(µ-CO)2(η-C5Me5)2] affords the heteronuclear trimetal cluster complexes [MRh2(µ-CO)2(CO)5(η-C5Me5)2] in high yield. These species may be regarded as molecules in which an M(CO)5 fragment, isolobal with CH2, is ‘complexed’ by an ethylene-like [Rh2(µ-CO)2(η-C5Me5)2] fragment.