Electron-rich olefins of general type [[double bond, length half m-dash][graphic omitted]R]2(LR2; R = Me, Et, Ph, 4-MeC6H4, 4-MeOC6H4, or 2-MeOC6H4) undergo reaction with a variety of rhodium(I) precursors via ligand displacement or chloride-bridge cleavage to afford monocarbenerhodium(I) complexes, such as [RhCl(LR)(PPh3)2], [Rh(cod)Cl(LR)], or [Rh(CO)Cl(LR)(PPh3)][LR=[double bond, length half m-dash][graphic omitted]R, cod = cyclo-octa-1,5-diene]; complexes [RhCl(L′Me)(PPh2)X][L′Me=[double bond, length half m-dash][graphic omitted]Me, X = CO or PPh3] have similarly been obtained from the olefin L′Me2. From these, further complexes may be obtained by ligand (neutral or anionic) exchange processes: trans-[RhBr(LR)(PPh3)2], trans-[Rh(CO)(LR)(PPh3)2]X (X = Br, Cl, ClO4, or I), [Rh(CO)X(LR)(PPh3)](X = BH4 or ClO4), cis-[Rh(CO)2X(LR)](X = Cl or NO3), [Rh(cod)X(LR)](X = CH2SiMe3, ClO4, or NO3), cis-[Rh(cod)-(LR)(PPh3)][ClO4], and [Rh(CO)3(LR)][ClO4]. In many of the reactions some of these ligand displacements at RhI proceed without retention of stereochemistry and it is likely that the observed product is the thermodynamically preferred isomer. Other chemical properties of the monocarbene-rhodium(I) complexes relate to (i) rare examples of the displacement of LR from Rh by PPh3 or Ph2PCH2CH2PPh2 under rather forcing conditions, and (ii) oxidative addition (not particularly facile) of HCl, [NMe2CHCl]Cl, or C2(CN)4. The 45 new complexes have been characterised by analysis and spectroscopy (i.r. and 1H and 31P n.m.r.) and, where appropriate, relative molecular mass determination, and electrical conductivity. From J(31P–103Rh) coupling constants it is concluded that LR has a greater trans influence than PPh3[also indicated by ν(Rh–Cl)] but a lower cis influence.

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