A multiconfigurational approach to the electronic structure of trichromium extended metal atom chains

Abstract

Density functional theory, Complete Active Space Self-Consistent Field (CASSCF) and perturbation theory (CASPT2) methodologies have been used to explore the electronic structure of a series of trichromium Extended Metal Atom Chains (EMACS) with different capping ligands. The study is motivated by the very different structural properties of these systems observed in X-ray experiments: the CN-capped example has a symmetric Cr unit while for the NO-capped analogue the same unit has two very different Cr-Cr bond lengths. Density functional theory fails to locate an unsymmetric minimum for any of the systems studied, although the surface corresponding to the asymmetric stretch is very flat. CASPT2, in contrast, does locate an unsymmetric minimum only for the NO-capped system, although again the surface is very flat. We use the Generalized active space (GASSCF) technique and effective Hamiltonian theory to interrogate the multi-configurational wavefunctions of these systems, and show that the increase in the σ-σ* separation as the chain becomes unsymmetric plays a defining role in the stability of the ground state and its expansion in terms of configuration state functions.

Solution structure of a pentachromium(ii) single molecule magnet from DFT calculations, isotopic labelling and multinuclear NMR spectroscopy

“…DFT calculations have shown that in trichromium(II) EMACs (except those containing very weak axial ligands) the asymmetric stretching of the Cr 3 unit is a very soft mode and that the potential energy surface features a single minimum corresponding to the symmetric structure. 23,24 The symmetry emerging from NMR spectra thus reflects the actual ground symmetry of 3, although a large asymmetric stretching amplitude is expected.…”

“…NO 3 − ). 24 However, the central metal ion lies in a very shallow potential energy surface and distortion of the symmetric structure is an energetically facile process, thereby explaining the structural versatility of trichromium(II) strings. 18,25,26 In pentachromium(II) EMACs, as well as in their Cr 7 and Cr 9 congeners, 27,28 application of the "splitatom" model results in a much more pronounced structural asymmetry, with d < = 1.86-2.03 Å, d > = 2.58-2.66 Å and Δd ∼ 0.7 Å in solvatomorphs of 2 at −60°C.…”

Electronic Structure of Cubane-Like Vanadium–Nitrogen Cationic Clusters [V4N4]+ and [V6N6]+

“…The inclusion of exact Hartree-Fock exchange in the B3LYP functional, for example, causes a very substantial weakening of the V-V bond, which appears inconsistent with the available experimental data. In recent work, we identified a number of other cases where the inclusion of exact exchange in the functional provides a description of metal-metal bonding that is qualitatively different from the picture derived from GGA or meta-GGA functionals [28]. Here, we report results obtained using a range of different density functionals and compare these to the electronic structure picture that emerges from a parallel series of calculations performed with the CASSCF methodology.…”

Trends in the Bond Multiplicity of Cr2, Cr3, and Cr2M (M = Zn, Ni, Fe, Mn) Complexes Extracted from Multiconfigurational Wave Functions

“…EMACs offer the opportunity to study multimetallic complexes with various degrees of metal–metal interaction. − In trimetallic chromium-based EMACs the Cr 2 unit discussed above is part of a larger structure, coupled with at least one other transition metal (M). − In such cases, the Cr–Cr distance varies from short to long depending on both M and the axial ligand X, the homometallic Cr 3 case being indeed one of the most challenging ones because of the dramatic influence of the axial ligands on the structure and properties of the molecule. We recently studied the structural effects of different axial ligands on Cr 3 compounds and proposed a strategy to make explicit the differences in the inherently very complex multiconfigurational wave functions . We now extend the study of the electronic structure of chromium complexes with the complete active space self-consistent field (CASSCF) method focusing on the metal–metal bonding multiplicity and the σ, π, and δ contributions to it, connecting the results with the nature of the axial ligands.…”