Complexes of the type [Hg{Fe(CO)2(NO)L}2][L = R3P, (RO)3P, R3As, and R3Sb; R = alkyl or aryl] have been prepared. They react with mercury(II) halides to give [XHgFe(CO)2(NO)L] derivatives, some of which have been isolated (L = CO and PhEt2As; X = Br and l). The i.r. spectra of both series of compounds indicate that they possess linear L–Fe–Hg–X systems [X = Cl, Br, I, and Fe(CO)2(NO)L] with trigonal bipyramidal co-ordination about the iron atoms. The parent compound, [Hg{Fe(CO)3NO}2], dissociates completely in dimethyl sulphoxide solution to give the [Fe(CO)3NO]–ion. The substituted derivatives do not ionize.