Nickelation of [3-Et-7,8-Ph₂-7,8-nido-C₂B₉ H₈]^2-: Synthesis and characterization of 1,2 → 1,2 and 1,2 → 1,7 isomerized products

The reaction between [3-Et-7,8-Ph₂-7,8-nido-C₂B₉ H₈]^2- and Ni(dppe)Cl₂ in tetrahydrofuran affords major and minor products, identified spectroscopically and crystallographically. The major species is 1,2-Ph₂-4-dppe-6-Et-4,1,2-closo-NiC₂B₉ H₈, which arises from the presumed initial product 1,2-Ph2-3-dppe-6-Et-3,1,2-closo-NiC₂B₉H₈ by a sterically induced 1,2 ? 1,2 C atom isomerization, typical of overcrowded nickelacarboranes. The minor species is 1,8-Ph₂-2-dppe-4-Et-2,1,8-closo-NiC₂B₉ H₈, arising from an unexpected 1,2 ? 1,7 carbon-atom isomerization of the initial product. The fact that one boron atom is labelled with an Et group, coupled with the fact that these isomerizations occur at low temperatures, allows comment on the isomerization mechanisms. The minor isomer cannot be explained by reference to Wales' sequential diamond - square - diamond mechanism for 1,2-closo- C₂B₁₀H₁₂ ? 1,7-closo-C₂B₁₀H₁₂. A simple rationalization of the formation of both major and minor isomers is available through triangle face rotation. Copyright © 2003 John Wiley & Sons, Ltd.