Structure and decomposition of [HFe(CO)4(B2H5)], a revised behavior of an old uncharacterized complex

The protonation reaction of the anionic complex [Fe(CO)4B2H5]− gives a product which decomposes easily at −120 °C yielding [Fe(CO)5] and B2H6 as reported by Coffy and Shore (J. Organomet. Chem. C27 (1990) 7394). In this work we have theoretically revised the understanding of this protonation using B3PW91/6-311+G** and B3P86/6-311+G** levels of theory. We found an unusual mode of coordination of B2H6 to the iron centre. The most stable structure exhibits a Fe–(μ-H)–B–(μ-H)–B five-membered ring which undergoes an isomerisation leading to the intermediate which displays an unusual interaction between [Fe(CO)4] and B2H6 via (μ3-H) binding. Atom-in-molecule (AIM) theory data indicate the presence of weak interactions between iron and boron atoms which are partially responsible of the facile decomposition of the formed complex. We consider that the decomposition pathway can occur in two steps; the first one gives [Fe(CO)4] and B2H6 (the aim of this work) followed by second one where [Fe(CO)4] undergoes a transformation to give [Fe(CO)5], this step remaining ambiguous up to today. Parameters such as solvent effect, solvent interaction and spin split were considered to provide with reliable results about decomposition mechanism.

Graphical abstract[HFe(CO)4(B2H5)] complex exhibits an unusual borane coordination. This coordination mode is reproduced in Ru and Os congeners. The decomposition to Fe(CO)5 and B2H6 involves a three-centres-bridged-hydrogen structure. The spin multiplicity of Fe(CO)4 seems to take part of this decomposition. A clear structural study is proposed.Figure optionsDownload full-size imageDownload as PowerPoint slide