The diphenoxy bicyclic tetraphosphapentazane derivatives (EtN)₅P₄(OPh)₂ 2 and its monoxide (EtN)₅P₄(O)(OPh)₂ 3 have been prepared. Both 2 and 3 exist as a mixture of two isomers. One isomer of (EtN)₅P₄(O)(OPh)₂ 3a has been isolated and its reaction with tetrachloro-1,2-benzoquinone yielded (EtN)₅P₄(O)(OPh)₂(O₂C₆Cl₄) 5 in which the junction phosphorus atom becomes five-co-ordinated. Treatment of 2 or 3a with [Mo(CO)₄(nbd)] (nbd = norbornadiene, bicyclo[2.2.1]hepta-2,5-diene), on the other hand, yielded the chelate complex [Mo(CO)₄{(EtN)₅P₄(O)_n(OPh)₂}] (n = 0 or 1; 6 or 7) in which the peripheral phosphorus atoms are bonded to the metal. The structures of 3a and 5–7 have been confirmed by single-crystal X-ray diffraction studies. The two P₃N₃ rings in 3a and 5 adopt twist/twist and irregular/twist conformations respectively; the phenoxy substituents occupy the ‘pseudo axial’ positions. However, an ideal chair conformation is observed for the P₃N₃ rings in 6 and 7 with the phenoxy substituents taking up the ‘pseudo equatorial’ positions. The NMR spectroscopic data for the compounds are discussed.