The reaction between λ3-diphosphazane [EtN(PCl 2)2] and the sodium salts of substituted phenols affords sterically encumbered diphosphazanes [EtN{P(OR)2}2] (R = -C6H3iPr2-2,6 (1), -C 6H3Me2-2,6 (2) and -C6H 2Me3-2,4,6 (3)). When the same reaction was carried out with bulky sodium 2,4-di-tert-butyl-4-methylphenoxide, only a monosubstitution takes place to result in the formation of [EtN{PCl(OR)}2] (R = -C6H2tBu2-2,6-Me-4) (4). Further reaction of 2 with [Mo(CO)4(NBD)] produces cis-[(EtN{P(OC 6H3Me2-2,6)2}2)Mo(CO) 4] (5). Diphosphazanes 1-4 and the metal derivative 5 have been characterized by means of their analytical data and EI-MS, IR and multinuclear NMR (1H and 31P) spectral data. The solid-state structure of the diphosphazanes 1, 2 and 4, and the molybdenum complex 5 have been determined by X-ray diffraction studies. Irrespective of the size of substituent, the bulky groups on the phosphorus and nitrogen are on the same side of the P-N-P skeleton with a local C2v symmetry. The central nitrogen remains almost trigonal planar in all the compounds. © The Royal Society of Chemistry 2006.