The phosphanes P(NH2)Hal2 and P(NHMe)2Cl are stabilised as ligands in (CO)5MoP(NH2)Hal2 [Hal = Cl (2), Br (3), I (4)] and (CO)5MoP(NHMe)2Cl (10) by reactions of (CO)5MoP(NH2)3 (1) with HHal or of (CO)5MoP(NHMe)3 (8) with HCl, respectively. 4 decomposes at room temperature and reacts with t-BuNH2 to give, (CO)5MOP(NH-t-BU)2NH2 (6). 1 and HI form (CO)5MoPI3. 3 can also be prepared from 2 with HBr, 4 from 2 and 3 with HBr and HI, respectively. The reactions of 2-4 with NR3 produce polymers, whereas the reaction of 3 with pyridine leads to (CO)5MoNC5H5. The mixtures (CO)5MoP(NHMe)Br2/(CO)5MoPBr3 and (CO)5MoP(NH-i-Pr)2Cl/(CO)5MoP(NH-i-Pr)Cl2 are isolated from reactions of 8 with HBr and of 9 with HI, and 9 reacts with HBr to give (CO)5MoPBr3. (CO)5MoP(NHMe)I2 is formed in small amounts by the reaction of 8 with HI, the main product is an insoluble solid, which is probably the impure intermediate [(CO)5MoP(NHMe)(NH2Me)2]2+2I-. Reaction of 10 with NEt3 leads to the formation of the dinuclear complex 12 in which the new diazadiphosphetidine [MeNHP-NMe]2 functions as a bridging ligand between two (CO)5Mo units.