Reaction between MHCl(CO)(PPh 3) 3 (M=Ru, Os) and the o-halophenyl mercury compounds, Hg(C 6H 4X-2) 2 (X=Cl, Br, I) gives the five co-ordinate complexes, M(C 6H 4X-2)Cl(CO)(PPh 3) 2 ( 1a M=Ru, X=Cl; 1b M=Ru, X=Br; 2a M=Os, X=Cl; 2b M=Os, X=Br; 2c M=Os, X=I). In these complexes there is a significant bonding interaction between the coordinatively unsaturated metal centre and the o-halo-substituent on the σ-bound phenyl group as revealed by crystal structure determinations of 2a, 2b, and 2c. Each of the five-coordinate complexes readily adds CO forming the corresponding six-coordinate dicarbonyl complexes, M(C 6H 4X-2)Cl(CO) 2(PPh 3) 2 ( 5a M=Ru, X=Cl; 5b M=Ru, X=Br; 6a M=Os, X=Cl; 6b M=Os, X=Br; 6c M=Os, X=I). Crystal structure determination of 6a confirms regular octahedral geometry for these six-coordinate complexes with no interaction between the metal centre and the o-halo-substituent on the σ-bound phenyl group. The complexes 1a, b, 2a– c, 5a, b, 6a– c, are potentially precursors of benzyne complexes through reduction (removal of ClX) but all attempts at reduction were unsuccessful. The related thiocarbonyl complexes, Os(C 6H 4X-2)Cl(CS)(PPh 3) 2 ( 7a X=Cl; 7b X=Br) and Os(C 6H 4X-2)Cl(CO)(CS)(PPh 3) 2 ( 8a X=Cl; 8b X=Br), have been prepared similarly beginning with OsHCl(CS)(PPh 3) 3. The crystal structure of 8a has been determined. Both 8a and 8b undergo a slow migratory-insertion reaction upon heating to yield the corresponding η 2-thioacyl complexes, Os(η 2-C[S]C 6H 4X-2)Cl(CO)(PPh 3) 2 ( 9a X=Cl; 9b X=Br), the crystal structures of both of which have been determined. Once the o-halophenyl group is no longer directly bonded to the metal, as in 9a and 9b, normal reactivity returns to the o-halo substituent and 9b undergoes lithium–bromine exchange when treated with n-butyllithium and the resulting lithiated material, when treated with Sn n Bu 3Cl, gives Os(η 2-C[S]C 6H 4Sn n Bu 3-2)Cl(CO)(PPh 3) 2 ( 10).