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 Cr3 unit while for the NO3--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 NO3--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.
Highlights
The chemistry of multiple bonds between transition metal ions is well established in the inorganic literature, more than 50 years on from Cotton’s discussion of the bonding in [Re2Cl8]2−.1 The quadruple bond is known for a variety of elements from the middle of the transition series and over the last decade the chemistry of the transbent quintuple bonds of Cr and Mo has been developed.[1]
The problem is compounded by the complex structural chemistry of this sub-class of Extended Metal Atom Chains (EMACs), where the symmetry of the Cr3 unit appears to be strongly dependent on the identity
We have employed the Complete Active Space Self-Consistent Field (CASSCF), General Active Space selfconsistent field (GASSCF) and CASPT2 methodologies to interrogate the electronic structure of a series of trichromium extended metal atom chains, Cr3(dpa)4X2
Summary
A multiconfigurational approach to the electronic structure of trichromium extended metal atom chains†. 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. Density functional theory fails to locate an unsymmetric minimum for any of the systems studied, the surface corresponding to the asymmetric stretch is very flat. CASPT2, in contrast, does locate an unsymmetric minimum only for the NO3−-capped system, 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
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