Substituted cyclopentadienyl compounds. Part V. NMR and extended Hückel studies of the isomeric preferences of some main group metal cyclopentadienide compounds and a reinvestigation of the crystal structure of thallium(I) tricyanovinylcyclopentadienide

Solution NMR studies have been performed on a series of mono-substituted cyclopentadienide salts, [M+(C5H4X−)] (M=Li, Na, K, Cs, Tl; X=CONMe2, COOMe, COC6H5, COMe, CHO, COCO2C2H5, NO2 and C(CN)C(CN)2). Population of a fulvenoid isomer in which the dominant bonding mode involves interaction between the cation and the ring substituent heteroatom is enhanced when X is strongly electron accepting and the cation is a hard Lewis acid. An alternate isomer involving a bonding interaction between the cyclopentadienyl carbon atoms and the metal ion predominates when X is weakly accepting and the cation is a soft Lewis acid. A range of compounds between these two isomeric extremes show fluxional behaviour in dimethyl sulphoxide (DMSO) solvent. Within the series of alkali metal salts and for a given X group, the maximum barrier for interconversion of isomers is achieved when M is potassium. This phenomenon is attributed to the balance between Lewis character and solvation of the cation in DMSO. Molecular orbital calculations based on the extended Hückel method were performed to examine the effect of the chemical nature of X and M on the relative energies of the isomers. These suggest that the energy separation is more sensitive to the nature of X than of M, but that the electronegativity of M also plays a role by its effect in shifting the energies into critical π-MO energy ranges. The single crystal X-ray structure of a representative derivative, Tl[C5H4C(CN)C(CN)2] has been reinvestigated. It shows a complex 3D structure in which each thallium is bonded to four nitrogen atoms and a single cyclopentadienide ring, a situation entirely consistent with the behaviour of this compound in solution in which the fulvenoid isomer is dominant.