Orbital entanglement and CASSCF analysis of the Ru-NO bond in a Ruthenium nitrosyl complex.

Leon Freitag,Leticia González,Sebastian F. Keller,Stefan Knecht,Roland Lindh,Markus Reiher,Thomas Bondo Pedersen,Francesco Aquilante,Mickaël G. Delcey

doi:10.1039/c4cp05278a

Abstract

Complete active space self-consistent field (CASSCF) wavefunctions and an orbital entanglement analysis obtained from a density-matrix renormalisation group (DMRG) calculation are used to understand the electronic structure, and, in particular, the Ru-NO bond of a Ru nitrosyl complex. Based on the configurations and orbital occupation numbers obtained for the CASSCF wavefunction and on the orbital entropy measurements evaluated for the DMRG wavefunction, we unravel electron correlation effects in the Ru coordination sphere of the complex. It is shown that Ru-NO π bonds show static and dynamic correlation, while other Ru-ligand bonds feature predominantly dynamic correlation. The presence of static correlation requires the use of multiconfigurational methods to describe the Ru-NO bond. Subsequently, the CASSCF wavefunction is analysed in terms of configuration state functions based on localised orbitals. The analysis of the wavefunctions in the electronic singlet ground state and the first triplet state provides a picture of the Ru-NO moiety beyond the standard representation based on formal oxidation states. A distinct description of the Ru and NO fragments is advocated. The electron configuration of Ru is an equally weighted superposition of Ru(II) and Ru(III) configurations, with the Ru(III) configuration originating from charge donation mostly from Cl ligands. However, and contrary to what is typically assumed, the electronic configuration of the NO ligand is best described as electroneutral.

Highlights

Nitric oxide (NO) is a well-known non-innocent ligand in coordination chemistry,[7] which leads to an intricate and ambiguous electronic structure of the transition metal nitrosyls
In this work we investigate the electronic structure of the {RuNO}6 moiety in the trans-[RuCl4(NO)(1H-indazole)]-complex (RuHIndNO),[45] which is closely related to the anti-cancer drug KP1019
To shed more light on the electronic structure of the {RuNO}6 complex, we transform the Complete active space self-consistent field (CASSCF) wavefunctions of the S0 and T1 states into a localised orbital basis and analyse the Ru–NO bond and the Ru coordination sphere in terms of configuration state functions (CSFs) based on localised orbitals; we compare the results obtained from the localised orbital analysis to the Mulliken population of Ru d orbitals based on both singleconfigurational density functional theory (DFT) and the CASSCF wavefunction

Summary

Introduction

NO is a well-known non-innocent ligand in coordination chemistry,[7] which leads to an intricate and ambiguous electronic structure of the transition metal nitrosyls. The majority of computational studies on transition metal complexes employ density functional theory (DFT).[17] many structures belong to the class of the so-called strongly correlated systems which cannot be described by Kohn–Sham DFT18 due to its single Slater determinant approximation. To shed more light on the electronic structure of the {RuNO}6 complex, we transform the CASSCF wavefunctions of the S0 and T1 states into a localised orbital basis and analyse the Ru–NO bond and the Ru coordination sphere in terms of configuration state functions (CSFs) based on localised orbitals; we compare the results obtained from the localised orbital analysis to the Mulliken population of Ru d orbitals based on both singleconfigurational DFT and the CASSCF wavefunction. The resulting orbitals and corresponding natural orbital occupation numbers of the optimised S0 and T1 geometries are collected in Fig. 1a and b, respectively

Computational details

Results and discussion

Conclusion