Compounds containing the [Mo(NO)]3+ group are redox-active. Incorporation into tris(3,5-dimethylpyrazolyl)borato (L*) systems, e.g. [MoL*(NO)XY] where X, Y = halide, alkoxide, aryloxide, amide, anilide, thiolate, etc. affords a variety of species which undergo one-electron reduction. The factors influencing this behaviour are described. Attachment of the molybdenum redox centre to macrocyclic polyethers and the effect of added cation is discussed. The group {MoL*(NO)X} can be attached to the periphery of tetra-aryl porphyrins, and the electrochemistry and photochemistry of the extra-ring metallated species is reported. A series of ferrocenyl compounds, [(η5-C5H5)Fe(η5-C5H4) C6H4E{ML*(NO)X}] and trans-[(η5-C5H5)Fe(η5-C5H4)C6H4ZZC6H4E{ML*(NO)X}] (where Z = CH or H, E = O or NH, M = Mo or W, X = Cl or I) have been prepared, their electrochemistry investigated and several compounds identified as having non-linear optical (Secondary Harmonic Generation) properties.