Contributions to the Chemistry of Phosphorus, 2101.2). – Pentaorganyltricyclononaphosphanes with a Structure Analogous to Brexane,b‐P9R5(R = Me, Et,iPr)The reaction of RPCl2, P4, and magnesium provides two constitutionally isomeric pentaorganyltricyclononaphosphanes. Besides the isomer with a P9skeleton analogous to that of noradamantane (a‐P9R5), which was already identified, a further isomer of lower relative abundance (b‐P9R5) has now been found. In the case of R = Et (2) it has been isolated as a pale yellow, oily liquid and, in the cases of R = Me (1) andiPr (3), has been highly enriched. According to their31P{1H}‐NMR spectra,1—3have a constitution analogous to that of the hydrocarbon brexane, thus being 2,4,5,8,9‐pentaorganyltricyclo[4.3.0.03.7]nonaphosphanes. They are each formed as mixtures of several configurational isomers, which differ in their relative arrangements of the o