Bis-isocyanide complexes of Re(IV) and Ru(II) trans-/cis-[ReCl ₄(CNMe) ₂] and trans-/cis-[RuCl ₂(PH ₃) ₂(CNMe) ₂] and their cycloaddition reactions with nitrone CH ₂N(Me)O were extensively studied by theoretical (DFT) methods. The reactivity of these complexes is interpreted in terms of both kinetic and thermodynamic factors as well as of orbital and charge arguments. The ligation of CNMe to Re(IV) significantly activates the isocyanide towards the cycloaddition (by 10.8 kcal/mol in CH ₂Cl ₂ solution), and such an activation effect is comparable with that provided by Pt(II) and Pd(II). In contrast, the coordination of CNMe to Ru(II) does not change significantly the reactivity of isocyanide and even makes the reactions more endoergonic. The mechanism of the reactions is concerted asynchronous. The calculations predict an inhibition of cycloaddition of the second nitrone molecule to the bis-isocyanide complexes and an easy decomposition of the derived N,N-substituted oxadiazolines via the NO bond rupture. The nature of the coordination bond in bis-and mono-isocyanide species and transition states was investigated in detail using the AIM, NBO and CDA analyses.