The cationic metallabenzenes [Ir(C5H4{SMe-1))(kappa(2)-S2CNEt2)(PPh3)(2)]PF6 (1) and [Os(C5H4{SMe-1})(CO)(2)(PPh3)(2)][CF3SO3] (2) undergo regioselective nucleophilic aromatic substitution of hydrogen at the metallabenzene ring position gamma to the metal in a two-step process that first involves treatment with appropriate nucleophiles and then oxidation. Thus, reaction between compound 1 and NaBH4, MeLi, or NaOEt gives the corresponding neutral iridacyclohexa-1,4-diene complexes Ir(C5H3{SMe-1}{H-3}{Nu-3})(kappa(2)-S2CNEt2)-(PPh3)(2) (Nu = H (3), Me (4), OEt (5)). Similarly, reaction between 2 and NaBH4 or MeLi gives the corresponding osmacyclohexa-1,4-diene complexes Os(C5H3(SMe-1}{H-3}(Nu-3))(CO)(2)(PPh3)(2) (Nu = H (8), Me (9)). The metallacyclohexa-1,4-diene rings in all these compounds are rearomatized on treatment with the oxidizing agent O-2, CuCl2, or 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). Accordingly, the cationic metallabenzene 1 or 2 is returned after reaction between 3 and DDQ/NEt4PF6 or between 8 and DDQ/NaO3SCF3, respectively. The substituted cationic iridabenzene [Ir(C5H3{SMe-1}{Me-3})(kappa(2)-S2CNEt2)(PPh3)(2)]PF6 (6) or [Ir(C5H4{SMe-1}{OEt-3})(kappa(2)-S2CNEt2)(PPh3)(2)PF6 (7) is produced in a similar manner through reaction between 4 or 5, respectively, and DDQ/NEt4PF6, and the substituted cationic osmabenzene [Os(C5H3(SMe-1}(Me-3})(CO)(2)(PPh3)(2)]Cl (10) is formed in good yield on treatment of 9 with CuCl2. The starting cationic iridabenzene 1 is conveniently prepared by treatment of the neutral iridabenzene Ir(C5H4{SMe-1))Cl-2(PPh3)(2) with NaS2CNEt2 and NEt4PF6, and the related starting cationic osmabenzene 2 is obtained by treatment of Os(C5H4{S-1})(CO)(PPh3)(2) with CF3SO3CH3 and CO. The stepwise transformations of 1 into 6 or 7 as well as 2 into 10 provide the first examples in metallabenzene chemistry of regioselective nucleophilic aromatic substitutions of hydrogen by external nucleophiles. DFT calculations have been used to rationalize the preferred sites for nucleophilic attack at the metallabenzene rings of 1 and 2. The crystal structures of 1, 3, 6, and 7 have been obtained.