Another Torture Track for Quantum Chemistry: Reinvestigation of the Benzaldehyde Amidation by Nitrogen-Atom Transfer from Platinum(II) and Palladium(II) Metallonitrenes

We showcase here a dramatic failure of CCSD(T) theory that originates from the pronounced multi-reference character of a key intermediate formed in the benzaldehyde amidation by N-atom transfer from Pd(II) and Pt(II) metallonitrenes studied recently in combined experimental and theoretical work. For detailed analysis we devised a minimal model system, for which we established reliable reference energies based on approximate full configuration interaction theory, to assess the performance of single-reference coupled-cluster theory up to the CCSDTQ(P) excitation level. While RHF-based CCSD(T) theory suffered dramatic errors, in one case exceeding 220 kcal mol(-1), we show that the use of broken-symmetry (BS) or Kohn-Sham (KS) orbital references yields substantially improved CCSD(T) results. Further, the EOM-SF-CCSD(T)(a)* approach met the reference data with excellent accuracy. We applied the KS-CCSD(T*)-F12b variant as high-level part of an ONIOM(KS-CC:DFT) scheme to reinvestigate the reactivity of the full Pt(II) and Pd(II) metallonitrenes. The revised reaction pathway energetics provide a detailed mechanistic rationale for the experimental observations.