[en] Solutions of MoCl₅ in POCl₃ react with NOCl forming the nitrosyl compound Mo(NO)Cl₃.2POCl₃(I), which in CH₂Cl₂ cleaves off one solvate molecule, yielding the dimeric complex [Mo(NO)Cl₃.POCl₃]₂(II). Reaction with AsPh₄Cl in dichloro methane leads to the nitrosyl complexes AsPh₄ [Mo(NO)Cl₄].CH₂Cl₂(III) and [AsPh₄]₂[Mo(NO)Cl₅].2CH₂Cl₂(IV), respectively. The i.r. spectra are recorded and assigned. [Mo(NO)Cl₃.POCl₃]₂ crystallizes monoclinic in the space group P2₁/c with two dimeric units per unit cell. The crystal structure was determined by X-ray diffraction methods (R = 0.040; 1391 observed, independent reflexions). Complex II is linked by chlorine bridges, forming a dimeric, centrosymmetric molecule of symmetry Csub(i). The N-O bond of the nitrosyl ligand is extremely short (108 pm), the Mo-N bond (181 pm) corresponds to a double bond. In trans position to the NO ligand, which is coordinated in linear array, there is the O atom of the solvate molecule POCl₃. [AsPh₄]₂[Mo(NO)Cl₅].2CH₂Cl₂ crystallizes triclinic in the space group P1 with two units per unit cell (R = 0.039; 1967 observed, independent reflexions). The molybdenum atom is coordinated octahedrally by five Cl ligands and a nitrosyl group, as well coordinated in linear array (Mo-N-O 174⁰). The nitrosyl ligand exerts a significant trans-effect (r Mo-Clsub((trans)) = 247 pm, r MoClsub(4(eq))(average) = 239 pm). (author)