CpFe(CO)₂ Radical Generated from Dinuclear [CpFe(CO)₂]₂ and Mononuclear (Cp)(CO)₂Fe(H): Density Functional Theory Is Accurate for One, But Not Both

Density functional theory (DFT) methods remain the most practical approach to calculating properties and reaction mechanisms of transition metal complexes. While the accuracy of DFT methods has been evaluated for some properties of mononuclear organometallic complexes there has been a general lack of evaluation for dinuclear organometallic complexes, in particular bonding changes related to reaction mechanisms. This work evaluated DFT and coupled cluster methods for the accuracy of calculating the CpFe(CO)₂ radical (Fp•) generated from dinuclear [CpFe(CO)₂]₂ (Fp₂) and mononuclear [(Cp)(CO)₂Fe(H)] (Fp-H). This transition metal radical fragment was evaluated because dinuclear complexes built with it have recently shown a variety of unique reactions but has proven challenging to accurately calculate with DFT methods. Here we show that DFT methods provide a surprising wide range of fragmentation energies for Fp₂ and lower and mid rung DFT methods as well as DLPNO-CCSD(T) perform well for this dissociation energy. The highest rung double-hybrid methods have a large range in the Fp₂ dissociation energy, and the energy greatly depends on the amount of MP2 correlation energy included. For generating Fp• from Fp-H the lower and mid rung methods that worked well for Fp₂ showed significant error. Double-hybrid methods unfortunately are only accurate for the Fe-H bond if they are very inaccurate for the Fp₂ dissociation energy. While DLPNO-CCSD(T) is not perfect, and not close to chemically accurate for the Fe-H bond, it does provide reasonable accuracy for both Fp₂ and Fp-H dissociation energies.