Paramagnetic Organometallic ComplexesUpdate based on the original article by Daniel B. Leznoff and Garry Mund,Encyclopedia of Inorganic ChemistrySecond Edition © 2005, John Wiley & Sons, Ltd
Authors: Leznoff Daniel B. , Hayes Cassandra E. , Mund Garry 
Published: December 2012

The synthesis, characterization, and reactivity of paramagnetic (or open‐shell) organometallic species are described. Many stable complexes featuring transition metals, lanthanides, and actinides have been reported. These systems, which are exceptions to the 18‐electron rule (or 16‐electron rule), are increasingly being used in the catalytic realm, and also have different reactivity patterns compared to their diamagnetic counterparts due to the presence of unpaired electrons. The formation and subsequent stability of paramagnetic organometallic complexes in violation of the 18‐electron rule can be explained via molecular orbital (MO) theory in terms of (i) the use of π‐donor ligands (as opposed to the ubiquitous π‐acceptor ligands usually found in organometallic chemistry); (ii) kinetic stabilization with sterically demanding ligands; and (iii) partially filled MOs being non‐bonding or slightly antibonding/bonding. Although metallocene‐based complexes are probably the most well studied of all paramagnetic organometallic complexes, homoleptic σ‐alkyl and σ‐aryl complexes (particularly with bulky groups such as CH2SiMe3andmeta‐terphenyl respectively) and other non‐Cp systems have also been explored. Such non‐Cp ancillary ligands include amido‐, alkoxo‐, and thiolato‐ligands, β‐diketiminate ligands, tris(pyrazolylborate) andN‐heterocyclic carbene ligands. f‐Electron organometallic systems also utilize a similar range of ligands, but because of their larger size, they can accommodate more sterically congested coordination spheres; for example, Cp*3M complexes can be prepared, and they show unusual reactivity pathways.

Open‐shell organometallic molecules are known to span a range of electron counts from 8 to 20. Nineteen‐electron organometallic complexes are of particular interest with respect to the location of the extra electron residing primarily on the metal or on the ligand (in this case, the complex is referred to as an 18 + δ system). Redox‐active ligands in general are now recognized to play a significant role in organometallic chemistry and catalysis and are discussed briefly.

The discussion of short‐lived paramagnetic organometallic systems revolves around 17‐ and 19‐electron radicals, and their use in catalytic processes and as “super reducing agents.” Many reactivity pathways for such organometallic radicals (or “metalloradicals”) have been identified.

Characterization tools for paramagnetic organometallic complexes, such as nuclear magnetic resonance (NMR) and electron spin resonance spectroscopy, density functional theory calculations, and magnetic measurements are presented. Variable‐temperature NMR studies and2H NMR can be of significant use in interpreting paramagnetic NMR spectra. Solid‐state NMR spectroscopy of paramagnetic organometallic systems is still in its infancy. The applications of paramagnetic organometallic complexes span many areas including catalysis (e.g., olefin polymerization), synthetic organic chemistry, and the development of molecule‐based magnetic materials.