The systematic description of planarizing distortions of tetracoordina
te carbon is outlined and specifically discussed for C(C)(4) moieties
which form the central part of fenestranes. Recent synthetic developme
nts of the preparation of stereoisomeric [4.5.5.5]fenestranes and [5.5
.5.5]fenestranes are described. Transformations with the aim of increa
sed planarizing distortions are presented. The interaction of Fe-2(CO)
(9) with a fenestradiene led to three structurally well defined Fe-car
bonyl complexes.