A Rhenium–Cyclohexane Complex with Preferential Binding of Axial CH Bonds: A Probe into the Relative Ability of CH, CD, and CC Bonds as Hyperconjugative Electron Donors?

Which bonds bind better? According to NMR experiments with the alkane complex [CpRe(CO)2(cyclohexane)] at low temperatures, complexation with an axial proton is preferred over that through an equatorial one by about 1.5 kJ mol−1. Calculations support this order of binding energies. The preference is proposed to be due to stronger electron donation through hyperconjugation from a CH bond than from a CC bond.