The reaction of mer-Tc(PMe2Ph)3Cl3 with t-butylisonitrile (TBI) in ethanol solution produced, depending on the experimental conditions, two compounds which were identified as trans-[Tc(PMe2Ph)2(TBI)4]+ and [Tc(PMe2Ph)(TBI)5]+. The proton NMR spectra of the two compounds showed that the complexes are quite pure. The two compounds were studied by X-ray diffraction methods. Trans-[Tc(PMe2Ph)2(TBI)4]PF6 (I) is monoclinic, space group C2/c with a = 17.933(9), b = 9.878(6), c = 25.675(18) Å and β = 105.29(5)°. The Tc atom is located on a center of inversion, while P (of PF6−) is located on a two-fold axis. [Tc(PMe2Ph)(TBI)5]PF6 (II) belongs to the tetragonal P4/n space group with a = 29.561(11), c = 10.614(5) Å. The geometry around the Tc atom is octahedral with TcC bonds from 2.036(15) to 2.045(13) Å for I and from 1.996(19) to 2.064(18) Å for II. The TcP bond distances are quite short for I (2.389(3) Å) and slightly longer in crystal II (2.403(6) Å). The short TcP bonds are caused by the presence of multiple bonding which increases as the oxidation state of Tc decreases, while the slightly longer TcP bond distance observed for crystal II is ascribed to a large trans influence of the isonitrile ligands, greater than the one of phosphines.