A series of [M(CO)₅L][M = Cr, Mo or W) complexes, where L is a phosphonium-substituted phenyl isocyanide ligand of the type o-R₃PCH₂C₆H₄NC [PR₃= PMe₃(L¹), PPh₂(CH₂Ph)(L²) or PPh₃(L³)], have been prepared by reaction of [NEt₄][M(CO)₅I] with the isocyanides in the presence of AgBF₄ in acetone and they have been characterized by IR, ¹H and ³¹P NMR spectroscopies. The co-ordinated ligands L react in tetrahydrofuran at 0 °C with Na[N(SiMe₃)₂] to give in most cases a mixture of different cyclization products whose structures have been assigned, on the basis of spectroscopic and electrochemical data, as the carbene-type [M(CO)₅{o-[graphic omitted](PR₃)}]A, indole-type [M(CO)₅{o-[graphic omitted]H₄)}]B or protonated indole-type species [M(CO)₅{o-[graphic omitted]H₄)}]C. The relative abundance of the final products is strictly related to the nature of the metal and the type of R group in the phosphonium moiety: as a general trend the [W(CO)₅L] complexes yield mostly type A derivatives, while those of Cr and Mo give primarily types B and C. A possible mechanism for the formation of these different species is discussed. These complexes, in aprotic media, exhibit upon cyclic voltammetry, a single-electron (reversible or irreversible) anodic wave, the oxidation potential of which has been applied to estimate the ligand parameters P_L and E_L for the isocyanide and derived ligands. According to their net electron-donor ability, the ligands can be ordered as follows: indoles > carbenes > protonated carbenes or protonated indoles > isocyanides. For the ligating metal centres M(CO)₅(M = Mo or W) the electrochemical parameter β(polarizability) has also been estimated.