Experimental charge density in an oxidized trinuclear iron complex using 15 K synchrotron and 100 K conventional single-crystal X-ray diffraction

The experimental electron density distribution in a crystal consisting of the simplest conceivable trinuclear carboxylate-bridged iron-μ³- oxo dianion with two α-picolinium cations has been determined using both synchrotron (15 K) and conventional (100 K) X-ray diffraction data. The constituent trinuclear oxo-centered molecule consists of six μ₂-bridging formate groups between the iron pairs, while the axial ligand for all iron atoms is another formate group. The compound {[Fe ₃O(HCOO)₆(HCOO)₃]^2-·H ₂O·2(α-CH₃NC₅H₅) ⁺}, (1) crystallizes in the monoclinic space group P2₁/m with charge assisted hydrogen bonds linking the α-picolinium cations to the trinuclear groups. The chemical bonding in the weakly asymmetric Fe ₃O-core of 1 has been examined through the use of the quantum theory of atoms in molecules, and in combination with experimental d-orbital populations, a significant electron sharing is observed between the Fe atoms and the central oxygen. The central oxygen exhibits clear sp² hybridization, and the iron atoms have valence shell charge concentrations in all metal-ligand bond directions. The relative bond strengths are evaluated based upon the charge density distribution and found to be in accordance with the geometrical results. Integrated group charges follow expectations from formal chemical valences.