Trigonal bipyramidal or square planar? Density functional theory calculations of iron bis(dithiolene) N-heterocyclic carbene complexes†

Density functional theory (DFT) calculations of 57 iron bis(dithiolene)-N-heterocyclic carbene adducts were conducted to determine what parameters predict, and possibly influence, the coordination of these aforementioned adducts. The parameters considered herein include three different types of nuclear magnetic resonance (C-NMR, Se-NMR, and P-NMR) isotropic chemical shifts, the Tolman Electronic Parameter (TEP), the Huynh Electronic Parameter (HEP), and the percent buried volume (%V_bur) of the different N-heterocyclic carbenes (NHCs) calculated from DFT. These parameters were selected based upon prior literature connection to σ-donor ability, π-acidity, and steric effects. The computed values of the properties were compared via multivariable linear regression models to see which properties best predict the Addison τ parameter—a measure of whether the complex would assume the square pyramidal or trigonal bipyramidal geometry. It was determined that a combination of TEP and %V_bur are the best predictors of the τ value, for the parameters considered herein. The inclusion of additional parameters yields mild improvement to the statistical models for the prediction of the τ value.