Dimethyl(aminomethyl)phosphine oxide (DMAO) reacts with the zinc halides, giving complexes of different compositions, depending on the reagent ratio, on the halide, and on the order of the addition of the reagents. All three halides give compounds of the composition ZnX(2)(DMAO) (X = Cl, Br, I) when a 1:1 reagent ratio is used. An X-ray diffraction study of ZnBr2(DMAO) shows that this compound is polymeric in the solid state, each zinc ion being coordinated to two bromide ions, the oxygen atom of one DMAO, and the nitrogen atom of another DMAO molecule. The coordination at zinc is distorted tetrahedral. The crystals belong to the space group P2(1)/c with Z = 4, a = 8.211(4) Angstrom, b = 12.626(5) Angstrom, c = 9.776(4) Angstrom, beta = 107.92(4)degrees, and V = 964(1) Angstrom(3). The structure was refined to R = 0.029 for 1125 reflections. The corresponding chloro and iodo complexes are likely to be isostructural. Acetonitrile solution of the compounds ZnX(2)(DMAO) contain mononuclear species which are likely to consist of tetrahedrally coordinated zinc cations bonded to a chelating DMAO molecule and the two anions. Complexes of the compositions (ZnCl2)(3)(DMAO)(4) and ZnX(2)(DMAO)(2) (X = Br, I) are obtained using higher ligand:zinc halide ratios. An X-ray structure determination of the iodo compound shows that the crystal contains polymeric {ZnI(DMAO)(2)}(+) cations and iodide anions. The pseudotetrahedral coordination at zinc is made up of one iodide, the nitrogen atom of a monodentate DMAO, the nitrogen atom of a second DMAO, the latter bridging two zinc ions, and the oxygen atom of a third DMAO molecule, which also bridges two metal ions. The crystals belong to the space group P2(1)/c with Z = 4, a = 12.667(1) Angstrom, b = 11.810(2) Angstrom, c = 11.799(1) Angstrom, beta = 109.91(1)degrees, and V = 1658.7(4) Angstrom(3). The structure was refined to R = 0.028 for 2604 reflections. Acetonitrile solutions of the compounds ZnX(2)(DMAO)(2) (X = Br, I) are likely to contain mononuclear four-coordinate species, in which the zinc atoms are bonded to one halide and two DMAO molecules, one of them being monodentate and the other acting as a chelating ligand. The two forms of coordinated DMAO are in rapid equilibrium on the NMR time scale. The preparations of compounds of the compositions Ni(SCN)(2)(DMAO)(2) and PdCl2(DMAO)(2) are also reported.