 For example, "c","t","c","c"-[4.5.5.5]fenestrane has a trans configuration at one of the cyclopentane/cyclopentane fusions, but cis configuration at the other cyclopentane/cyclopentane fusion and at both butanepentane/cyclopentane fusions.In an extreme case the central carbon atom, which would ordinarily have tetrahedral molecular geometry for its four bonds gets completely flattened.In the molecular orbital picture for the resulting square planar geometry of methane, two of a total of three sp2-hybridized carbon atomic orbitals form regular bonds with two of the hydrogen atoms as in a planar alkene.