Seemingly simple group 8 cyclopentadienyl dicarbonyl metal halides: From little things, interesting things grow

Single crystal X-ray studies are presented for the ‘piano-stool’ structures [CpRu,Os(CO)₂Cl], [Cp′Ru(CO)₂X] (X = Cl, Br, I (redetermination)), [Cp*Ru(CO)₂Cl] (two phases), enabling the assembly of comparative geometric data for the [CpM(CO)₂Cl] (M = Fe, Ru, Os), [Cp,Cp′Ru(CO)₂X] (X = Cl, Br, I) and [Cp^xRu(CO)₂Cl] (Cp^x = Cp (η–C₅H₅), Cp′ (η–C₅H₄CH₃), Cp* (η–C₅CH₃)₅,Cp^Ph (η–C₅(C₆H₅)₅)). In the [Cp′Ru(CO)₂X] arrays, the methyl groups are found to consistently lie ‘eclipsed’ vis-a-vis the halogen substituents, with a concomitant tilt of the Cp ring; the reasons for this are explored theoretically.