The first uncoordinated 7 lambda(3)-phosphanorbornadiene 10 is reported. This missing member of the class of 7-heteranorbornadienes was prepared by decomplexation of its pentacarbonyltungsten complex 6 with iodine and N-methylimidazole It is moderately stable at room temperature toward loss of the phosphinidene bridge (PhP:) because the aromatic fragment, [5]metacyclophane (5), is a highly strained compound. For this reason, 10 may be considered to be a hyperstable compound. An analysis of the energetics of the system by density functional calculations supports this rationalization.