Structural Characterization and Bonding Analysis of [Hg{Fe(CO)₅}₂]²⁺ [SbF₆]⁻₂

Abstract

The non-classical carbonyl complex [Hg­{Fe­(CO)5}2]2+ [SbF6]− 2 is prepared by reaction of Hg­(SbF6)2 and excess Fe­(CO)5 in anhydrous HF. The single-crystal X-ray structure reveals a linear Fe–Hg–Fe moiety as well as an eclipsed conformation of the eight basal CO ligands. Interestingly, the Hg–Fe bond length of 2.5745(7) Å is relatively similar to the corresponding Hg–Fe bonds in literature-known [Hg­{Fe­(CO)4}2]2– dianions (2.52–2.55 Å), which intrigued us to analyze the bonding situation in both the dications and dianions with the energy decomposition analysis with natural orbitals for chemical valence (EDA-NOCV) method. Both species are best described as Hg(0) compounds, which are also confirmed by the shape of the HOMO-4 and HOMO-5 of the dication and dianion, respectively, in which the electron pair is located mainly at the Hg. Furthermore, for the dication and the dianion, the σ back-donation from Hg into the [Fe­(CO)5]2 2+ or the [Fe­(CO)4]2 2– fragment is the most dominant orbital interaction and surprisingly these interaction energies are also very similar even in absolute values. The fact that both iron-based fragments are missing two electrons explains their prominent σ-acceptor properties.

Homoleptic Transition Metal Carbonyl Cations: Synthetic Approaches, Characterization and Follow-Up Chemistry

“…This complex can be assigned as a metal-only Lewis pair (MOLP): the silver­(I) cation acts as a Lewis acid, and the zerovalent metal atom in Fe­(CO) 5 acts as a Lewis base. Later, others prepared MOLP-type 2:1 adducts of Fe­(CO) 5 including the coinage and post TMs Cu + , Au + , Cd 2+ and Hg 2+ . − …”