Cr(I)–Cr(I) Terphenyl Bridged Complexes: A Broken-Symmetry DFT and Multideterminant CASSCF/NEVPT2 Handshake

In this article, a Cr^I-Cr^I-based system stabilized by two monovalent terphenyl ligands Ar'CrCrAr' (Ar' = C₆H₂-2,6-(C₆H₃-2,6-Pr₂ ⁱ)₂-4-SiMe₃) is revisited. It is shown that multideterminant <i>ab initio</i> (CASSCF, NEVPT2) and DFT (BLYP, B3LYP as well as ωB97X-D, M06-2X, and B3LYP-GD3) calculations are capable of addressing this system in a consistent way: the spin state ordering, the presence of antiferromagnetic interaction within the Cr^I-Cr^I moiety, and the optimized geometry. Singlet is found to be the ground state by all methods, the broken-symmetry (BS) one in the case of DFT. The singlet CASSCF effective bond order of 3.36 is well resolved by BS singlet BLYP delocalization index (3.15) and Mayer bond order (3.19). The antiferromagnetic interaction is found non-negligible in the CASSCF wave function, with the weight of two open-shell singlet determinants being 31%. In the case of geometry optimizations, singlet NEVPT2 and BS singlet BLYP are able to assess the "experimental" geometry (<i>d</i> _Cr-Cr ∼ 1.7 Å) while singlet CASSCF and BS singlet B3LYP lead to a longer Cr^I-Cr^I bond length (<i>d</i> _Cr-Cr > 2.5 Å). The restricted singlet B3LYP and BLYP calculations are energetically disfavored, unstable, and tend to a larger covalency (bond order of four), but address the geometry of the system well, with <i>d</i> _Cr-Cr values of 1.65 and 1.60 Å, respectively.