The dimeric dichloropentamethylcyclopentadienyl-rhodium and -iridium complexes, [{M(C5Me5)Cl2}2], reacted in ethanol in the presence of sodium carbonate with butadiene, cycloheptatriene, and 6,6-diphenylfulvene, giving pentamethylcyclopentadienyl-rhodium(III) or -iridium(III) complexes of π-1-methylallyl, π-cyclohepta-2,4-dienyl, and (1,1-diphenylmethyl) cyclopentadienyl. Under similar conditions with [{Rh(C5Me5)Cl2}2], norbornadiene and dicyclopentadiene gave pentamethylcyclopentadienyl-rhodium(I) diene complexes; ethylene gave bis(ethylene)-rhodium(I) and -iridium(I) complexes. A hydrido-intermediate is implied in these reactions and both the hydrido- and deuterio-chloro(triphenyl phosphine) pentamethylcyclopentadienyliridium complexes were isolated and characterised. With cyclopentadiene, [{Rh(C5Me5)Cl2}2] gave the cation and (cyclopentadienyl)-endo-H-pentamethylcyclopentadienerhodium(I). (Cyclopentadienyl)exo-H-pentamethylcyclopentadienerhodium(I) was obtained by reduction of [Rh(C5Me5)(C5H5)]+ with sodium borohydride and was much more reactive than the endo-H-isomer. With halogenating agents [Rh(C5Me5)(C5H5)]+ was the product, whereas the endo-H-isomer underwent cleavage to [{Rh(C5Me5)X2}2] and [{Rh(C5H5)X2}2]. [{Ir(C5Me5)Cl2}2] reacted with cyclopentadiene to give only the [Ir(C5Me5)(C5H5)]+ cation, which on reduction, gave a mixture of two isomers, exo-H-[Ir(C5Me5H)(C5H5)] and [Ir(C5Me5)(C5H6)]. The mass spectra of some new complexes and of (C5Me5H) are recorded.