The carbon-13 NMR spectra of twenty four acyclic and cyclic η4-1,3-, 1,4- and 1,5-diene-rhodium(I) complexes of type η5-C5H4XRh (η4-diene), (X =H, Cl, Ph, CO2CH3 or CHO) are described. Each diene type can be easily distinguished from the values of its 103Rh13C coupling constants and olefincarbon coordination shifts. The effects of methyl substituents on the relative chemical shifts of the acyclic η4-1,3-diene carbon atom are broadly similar to that in analogous η4-dienetricarbonyliron systems, but the metalcarbon coupling constants imply a greater degree of retrodative bonding in the case of the rhodium complexes. The largest coordination shifts are found for the β olefin carbons of the 1,4-diene complexes. The unusual metalcarbon coupling in these complexes are a reflection of the strained conformation adopted on bonding to rhodium, and do not suggest conjugative interaction between the two alkene functions. The spectra of the 1,5-diene complexes show the expected trend resulting from a reduction in strain energy relative to that for the previous case. The spectra of the η5-cyclopentadienyl groups in these complexes as discussed in terms of a small contribution from either an η3-allyl-η2-ene or a η4-diolefin-η1-alkyl rotamer to the metalring bonding scheme; the rotamer type is governed by the cyclopentadienyl ring substituent and counter diene ligand. Several functionally-substituted cyclopentadienide salts have been examined by variable temperature hydrogen-1 and carbon-13 NMR spectroscopy. The fluxional behaviour of Tl1(C5H4NO2 and K(C5H4CHO) demonstrate how the Lewis acid character of the metal ion and the electron-accepting nature of the ring substituent control the structural preference for a form in which the substituent is either co-planar or orthogonal with respect to the cyclopentadienide group. Two dimensional 1H13C NMR spectra and simulation techniques peritted unambiguous assignment of ring nuclei. The chemical ionisation and fast atom bombardment mass spectra of several of the thallium(I) complexes reveal considerable association in the vapour state.