Organic anions. Part 10. Hard sphere electrostatic calculations on group 1 organometallic compounds
A point charge electrostatic treatment of tetrameric organolithium compounds predicts a quotient of lithium–lithium to carbon–carbon bond lengths of 0.78, which compares with an average experimental value of 0.75. A hard sphere electrostatic (HSE) treatment of hexameric organolithium compounds predicts quotients characterising the shapes and sizes of the carbon and lithium trigonal antiprisms of 1.23, 1.23, and 1.00. The corresponding (average) experimental quotients are 1.13, 1.27, and 0.88. The HSE method is further developed for predicting the structures of group 1 Organometallic ion pairs in which a single delocalised anion is associated with several mutually interacting mobile cations. In doing so it is stressed that the critical energy surface (termed the ion-pair surface) is that described by the nucleus of the cation as it rolls over the surface of the anion. The quantitative information obtained from such calculations is crude but it is sufficient to make predictions of a general type. For example a clear division is seen between those anions that will and those that will not form solvent-separated ion pairs. In tetrahydrofuran (THF) all ion pairs R2–, Li+ will be contact-ion pairs, and most ion pairs of R2–, 2Li+ will also be contact-ion pairs but in some one of the lithiums may be solvent separated. In triple ions R–, 2Li+, on the other hand, both lithiums will be of the solvent-separated type. Calculations on various metal-ion-exchange reactions accord well with experimental findings. In the case of dianions it is predicted that mixing R12–, 2M1+ with R22–, 2M2+ will sometimes result in an exchange of both metal cations but in other cases only one will be exchanged. The dominant HSE bonding term for 1 : 1 ion pairs R–, M+(where R– is a delocalised anion) is that for the atoms immediately in contact with the metal ion and significant delocalisation (in the HSE sense) is that which removes charge from those atoms. Hence in the HSE sense the degree of charge delocalisation in methyl, allyl, and U-pentadienyl anions is the same. The W-conformer of the pentadienyl anion is, however, more charge delocalised and the metal ion is less tightly bound. Throughout this work, HSE calculations are shown to provide a useful first-order guide to the properties of group 1 organometallics.