Mono- and binuclear aryldiazenido complexes [Fe(ArN2)(CO)(2)P-2]BPh4 (1-4) and [{Fe(CO)(2)P-2}(2)(mu-N2Ar-ArN2)]BPh4)(2) (5-8) [P = P(OEt)(3), PPh(OEt)(2), PPh2OEt, P(OPh)(3); Ar = C6H5, 2-CH3C6H4, 4-CH3C6H4; Ar-Ar = 4,4'-C6H4-C6H4, 4,4'-(2-CH3)C6H3-C6H3(2-CH3), 4,4'-C6H4-CH2-C6H4] were prepared by allowing hydride species FeH2(CO)(2)P-2 to react with an excess of mono-(ArN2)(BF4) or bis-aryldiazonium (N2Ar-ArN2)(BF4)(2) salts, respectively, at low temperature. A reaction path involving a hydride-aryldiazene intermediate [FeH(ArN=NH)(CO)(2)P-2](+), which, through the loss of H-2, affords the final aryldiazenido complexes 1-8, is proposed. The compounds were characterized by H-1 and P-31{H-1} NMR spectroscopy (including N-15 isotopic substitution) and X-ray crystal structure determination. The complex [Fe(CO)(2){P(OEt)(3)}(2){mu-4,4'-N-2(2-CH3)C6H3-C6H3(2-CH3)N-2}](BPh4)(2) (5b) crystallizes in the space group P (1) over bar with a = 15.008(4) Angstrom, b = 17.094(5) Angstrom, c = 10.553(3) Angstrom, alpha = 99.56(1)degrees, beta = 102.80(1)degrees, gamma = 65.30(1)degrees, and Z = 1. The structure is centrosymmetric and consists of binuclear cations with the two iron atoms in a quite regular trigonal bipyramidal environment, with the two CO in the equatorial and the two phosphites in the apical position, respectively. Aryldiazenido complexes 1-8 react with strong acids HX (X = Cl, CF3SO3, CF3CO2) to give the corresponding aryldiazene derivatives, according to the equilibrium [Fe(ArN2)(CO)(2)P-2](+) HX reversible arrow [FeX(ArN=NH)(CO)(2)P-2](+). Electrochemical studies of both mono- (1-4) and binuclear (5-8) compounds were undertaken, and a mechanism for oxidation and reduction processes is proposed.