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

TL;DR

This paper studies a chromium complex using advanced computational methods to understand its electronic structure and bonding.

Contribution

The study demonstrates a consistent approach using DFT and CASSCF/NEVPT2 to analyze a Cr(I)–Cr(I) complex.

Findings

Singlet is the ground state for all methods, with DFT using broken-symmetry approach.

CASSCF and DFT methods agree on the bond order and antiferromagnetic interaction.

Geometry optimization results vary with method, affecting Cr–Cr bond lengths.

Abstract

In this article, a CrI–CrI-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 ab initio (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 CrI–CrI 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 (d
Cr–Cr ∼ 1.7 Å)
while singlet CASSCF and BS singlet B3LYP lead to a longer CrI–CrI bond length (d
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 d
Cr–Cr values of 1.65 and 1.60 Å, respectively.

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