The pentadienyl complexes {M(n5 C5H6R)(CO)3} {M=Mn, R=H, Me:M=Re, R=H} and {Mn(n5-syn-(l,5-Ph2C5H5)(CO)3} have been synthesised from the corresponding metal carbonyl halides M(CO)5Br {M=Mn,Re} by reaction with penta-dienyltrialkyltin reagents {R=H, Me, R=Me,n-Bu} and {MeSnCl,5-Ph2C3H3)} respectively. The organotinintermediates were prepared by reaction of organotin halides with substituted pentadienyl anions. Treatment of the dimer {RuCOCl2(PMe2Ph)2}2 with(Bu3SnC5H7) yields {RuCl(CO)(n3-C5H7)(PMe2Ph)2}. Attempts to prepare the pentadienyl compounds from pentadienyl anions and different complexes such as{RuC12(C6H6)}2, {RhCl(l,5-C8H12)}2, Mo(CO)g, {MoCl(CO)2(n3-C3H5)-(MeCN)2}, {RuCl2(l,5-CgH12)}n were unsuccessful. The complexes P(OMe)3, P(OEt)3, P(OPh)3, PMe3, PMe2Ph} and{Mn(n3-C5H7)(CO)3L}{L=PBu3, PMeg, PMe2Ph} have been prepared from {Mn(n5 -C5H7)(CO)3} using thermal and photolytic methods. The thermal substitution reactions with L show the formation of {Mn(n3 C5H7) (CO)3L} as an intermediate step followed by a decarbonylation, giving the corresponding {Mn(n5 C5H7)(CO)2L} . Only the strongest Lewis bases proved to be useful for obtaining both the n3 - and the n5 -pentadienyl complexes, {Mn(n3-C5H7)(CO)3L} and {Mn(n5 C5H7)(CO)2L} . The compounds have be characterised by infrared spectroscopy, elemental analysis and 1H-NMR, and some also by mass spectrometry and 13C-NMR. He(I) and He(II) u.v.-photoelectron spectra for {M(n5 C5H6R)(CO3)} {M=Mn, R=H, ME: M=Re, R=H} are reported. The protonation of some derivatives {Mn(n5 C5H7)(CO)2L} {L=P(C6H11)3,PPh3, CO} with strong acids was studied but the site for the proton attack was not definitely established. Photolysis of in hexane under carbon monoxide (1 atm) gives spectroscopic evidence of {Mn(n3 C5H7)(CO)4}. The preparation of n3 pentadienyl complexes (MoX(CO)2(n- 3C5H6Me) (MeCN)2} {X=Cl,Br} by oxidative addition of chloro- and bromo-2,4-hexadiene to intermediate compounds such as {Mo(CO)3(CH3CN)2} are also reported. 1H-NMR shows solvent dependent ionisation of these complexes in solution with liberation and free acetenitrile. The bromo-complex ionises to a less extent than the chloro-complex. The bidentate ligand 2,2'-bipyridine replaces the two acetonitrile molecules to yield the substitution product {MoBr(CO)2(n3 C5H6Me}.1/2 MeCN.<p
