 Small hydride sources, however, add hydride to an axial position for reasons which are still disputed.The Cieplak effect explains this phenomenon by postulating that hyperconjugation of the forming σ*C–H orbital with geometrically aligned σ orbitals is the stabilizing interaction that controls stereoselectivity.In an equatorial approach, the bonds that are geometrically aligned antiperiplanar to the forming C–H bond are the C–C bonds of the ring, so they donate electron density to σ*C–H.