Symmetry and Steric Effects on Spin States in Transition Metal Complexes

Abstract

Changes in the magnetic characteristics of transition metal complexes, particularly, their metal spin states, are most commonly achieved by varying the electron donor/acceptor properties of coordinated ligands. In some systems, it is possible to increase the steric bulk of ligands so that intramolecular crowding leads to changes in metal‐ligand bond distances; longer bonds produce weaker ligand fields and stabilize high‐spin or spin‐crossover complexes. Steric effects are also often associated, albeit indirectly, with a mechanism for influencing spin states that relies on modifying the symmetry‐based interactions of metal and ligand orbitals. Particular ligand orientations, as in bis(indenyl) compounds [M(R 9 C 7 ) 2 ], can be changed as the result of interligand steric pressure or crystal packing effects; such differences can alter metal d‐orbital energy levels and hence the stability of spin states. Both steric and symmetry effects add to the repertoire of techniques that can be used to control magnetic properties in transition metal complexes.

Development of Accurate DFT Methods for Computing Redox Potentials of Transition Metal Complexes: Results for Model Complexes and Application to Cytochrome P450

“…As seen with both B3LYP and B3LYP with the DBLOC corrections, many of the metallocenes in our database were found to be spin-crossover complexes, which is in agreement with experimental results 82 as well as with the behavior observed in other complexes that have metals with relatively low oxidation states bound to ligands that are relatively far to the right of the spectrochemical series, for example, the [Fe(II)(terpy) 2 ] +2 complex. All reduced complexes of this series are spin-crossover complexes, with the exception of the manganocene complex, where the high spin d 5 configuration provides extra stability.…”

Natural Products for Materials Protection: Mechanism of Corrosion Inhibition of Mild Steel by Acid Extracts of Piper guineense

“…Physical-induced spin-crossover could be caused by temperature change, light irradiation; magnetic fields etc., whereas chemical induced spin-crossover can result from solvation, bond breaking, ligand exchange, or isomerization . According to Meier and Hanusa, steric effects arising from certain ligand orientations can also influence the metal–ligand bond distances, with longer bonds creating weaker ligand fields, which stabilize high-spin or spin-crossover complexes. This reasoning is consistent with our computational results, i.e., significant distortion of the piperine molecule and subsequent cleavage of the Fe–O bond induced the spin transition.…”

“…Interestingly, Huse et al using time-resolved soft X-ray spectroscopy, observed significant reduction in orbital overlap between the 3d orbital of Fe and the ligand 2p orbital during low to high spin-crossover, which corroborate the resultant increase in bond length. Detailed discussion of the spin-crossover phenomenon is beyond the intended scope of this paper, but can be found in refs − and the references therein. The theoretically predicted low spin to high spin transition for the Fe–piperine couple, which has not been widely reported in the literature, could provide useful information in understanding the mechanisms of corrosion inhibition and thus requires further investigation and adequate experimental validation.…”

A B3LYP-DBLOC empirical correction scheme for ligand removal enthalpies of transition metal complexes: parameterization against experimental and CCSD(T)-F12 heats of formation

“…Spin-splitting energies of all of the complexes were calculated and only two of them were found to be spincrossover complexes, namely Fe(CO) 4 , involving a singlet to triplet transition, and Fe(C 5 H 5 ) 2 , involving a singlet to quintet transition, both of which are in qualitative agreement with experiment. [29][30][31] Based on the low temperature gas phase thermochemistry experiments employed to obtain the atomization energies of the complexes, the singlet forms of these complexes have been used in the following calculations.…”