Electronic Structure of Paramagnetic Iron and Manganese Cluster Compounds: Historical Developments and Current Understanding

Abstract

This article presents current understanding of the electronic structure of magnetic multinuclear transition metal ion clusters, with an emphasis on manganese and iron heterocubane-type systems, but uses electronic structure descriptions applicable to a wide array of cluster systems, and accessible to the inorganic chemist. A primer on bonding, magnetism, and electronic structure considerations relevant to magnetic clusters precedes a more detailed discussion of electronic structure and bonding descriptions of synthetic and natural cluster systems. A goal of the article is to present the unique and sometimes enigmatic properties of magnetic metal clusters, and explain them using simple inorganic theoretical approaches familiar to coordination chemists. An overarching theme is that understanding of metal cluster electronic structure requires going beyond simple ferro- and anti-ferromagnetic coupling models, and frequently requires consideration of metal-metal covalent interactions. Such interactions result in averaging of oxidation state character of individual elements via double exchange, and apparent non-Hund spin states for individual metal ions. Such seemingly enigmatic phenomena are best elucidated using rigorous theoretical approaches, yet may be explained and understood simply using qualitative molecular orbital arguments.