MICROREVIEW DOI:10.1002/ejic.201402340 CLUSTER ISSUE A Decade of Dinuclear Technetium Complexes with Multiple Metal–Metal Bonds Frederic Poineau,* [a] Paul M. Forster, [a] Tanya K. Todorova, [b] Erik V. Johnstone, [a] William M. Kerlin, [a] Laura Gagliardi, [c] Kenneth R. Czerwinski, [a] and Alfred P. Sattelberger [d] Keywords: Technetium / Metal–metal interactions / Structure elucidation / Electronic structure Transition metal complexes with multiple metal–metal bonds exhibit interesting catalytic and biological properties. One el- ement whose metal–metal bond chemistry has been poorly studied is technetium. Currently, only 25 technetium com- plexes with multiple metal–metal bonds have been structur- ally characterized. The nature of metal–metal interactions in these complexes, as well as the influence of ligands on the bonding in the Tc 2 n+ unit (n = 6, 5, 4) are not well understood. In order to better understand the influence of ligands on the Tc–Tc bonding, a study of the solid-state and electronic struc- ture of dinuclear complexes with the Tc 2 n+ unit (n = 6, 5, 4) has been performed. Dinuclear technetium complexes (nBu 4 N) 2 Tc 2 X 8 , Tc 2 (O 2 CCH 3 ) 4 X 2 , Tc 2 (O 2 CCH 3 ) 2 Cl 4 , cesium salts of Tc 2 X 8 3– , and Tc 2 X 4 (PMe 3 ) 4 (X = Cl, Br) were synthe- sized; their molecular and electronic structures, as well as their electronic absorption spectra, were studied by a number of physical and computational techniques. The structure and bonding in these systems have been investigated by using multiconfigurational quantum calculations. For all these complexes, the calculated geometries are in very good agree- 1. Introduction Transition metal compounds with multiple metal–metal bonds play an important role in inorganic, materials, bio- inorganic, and organometallic chemistry. [1a–1d] The study of their crystallographic and electronic structures is essential to understand their catalytic and biological properties. At the fundamental level, the study of their electronic struc- tures permits a better insight into the nature of metal–metal interactions in metal–metal bonded compounds. Transition metal compounds with multiple metal–metal bonds can be [a] Department of Chemistry, University of Nevada Las Vegas, Las Vegas, NV 89154, USA E-mail: poineauf@unlv.nevada.edu radchem.nevada.edu [b] Department of Physical Chemistry, University of Geneva, 1211 Geneva, Switzerland [c] Department of Chemistry and Supercomputing Institute, University of Minnesota, Minneapolis, MN 55455, USA [d] Energy Engineering and Systems Analysis Directorate, Argonne National Laboratory, Argonne, IL 60439, USA Eur. J. Inorg. Chem. 2014, 4484–4495 © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 4484 ment with those determined experimentally. Bond order analysis demonstrates that all these complexes exhibit a total bond order of approximately 3. Analysis of individual effec- tive bond order (EBO) components shows that these com- plexes have similar σ components, while the strength of their π components follows the order Tc 2 X 4 (PMe 3 ) 4  Tc 2 X 8 3–  Tc 2 (O 2 CCH 3 ) 2 Cl 4  Tc 2 X 8 2– . Calculations indicate that the δ components are the weakest bond in Tc 2 X 8 n– (n = 2, 3) and Tc 2 (O 2 CCH 3 ) 2 Cl 4 . Further analysis of Tc 2 X 8 3– and Tc 2 X 4 (PMe 3 ) 4 (X = Cl, Br) indicates that the electronic struc- ture of the Tc 2 5+ and Tc 2 4+ units is insensitive to the nature of the coordinating ligands. The electronic absorption spectra of Tc 2 X 8 n– (n = 2, 3), Tc 2 (O 2 CCH 3 ) 2 Cl 4 , and Tc 2 X 4 (PMe 3 ) 4 (X = Cl, Br) were studied in solution, and assignment of the tran- sitions was performed by multiconfigurational quantum chemical calculations. For the Tc 2 X 8 n– (n = 2, 3; X = Cl, Br) anions and Tc 2 (O 2 CCH 3 ) 2 Cl 4 , the lowest-energy band is at- tributed to the δδ* transition. For Tc 2 X 4 (PMe 3 ) 4 , the assign- ment of the transitions follow the following order in energy: δ*σ*  δ*π*  δσ*  δπ*. found in various dimensionalities including molecular clus- ters (i.e., di-, tri-, tetranuclear species, etc.) and solids with extended structures (i.e., binary halides). [2a–2d] More than 4000 dinuclear complexes and several binary halides with multiple metal–metal bonds have been characterized. One element whose metal–metal bond chemistry is not well developed is technetium, the lighter radioactive conge- ner of rhenium. As of 2005, the number of complexes with multiple Tc–Tc bonds was limited. Only 25 dinuclear spe- cies, 4 hexanuclear, and 6 octanuclear halide clusters had been structurally characterized. [3] Prior to 2005, no binary technetium halides with multiple Tc–Tc bonds or complexes with a Tc 3 9+ core had been reported. In this context, we have focused on expanding the chemistry of dinuclear com- plexes and identifying new binary halides of technetium. For the binary halides, seven new phases have been re- ported, and their chemistry has recently been reviewed. [4] Dinuclear technetium complexes consist of Tc 2 n+ units (n = 4, 5, 6) coordinated to ligands. In these complexes, the Tc 2 n+ units can exhibit bond orders (BO) between three and four. The bond order is defined as (n b – n a )/2, where n b and n a