Dear Colleagues,

Since the first description of a quadruple bond between rhenium atoms in Re₂Cl₈^2- by Cotton in 1964, there has continued to be tremendous interest in the structural, electronic, and magnetic properties, and reactivity of metal–metal bonded complexes. These complexes are exemplified by the tetragonal M₂(L-L)₄ and M₂L₈ complexes possessing supported and unsupported metal–metal bonds. Much of the original interest in these complexes derived from their potential for the metal–metal bond to act as an electron reservoir for multi-electronic transformations. Indeed, many complexes can carry out a variety of catalytic transformations. Others have found that metal–metal bonded complexes possess potential chemotherapeutic applications. As the field continued to expand, so did the number of metals involved in the bonding. The development of bridging ligands capable of spanning three or more metal centers has led to the synthesis of so-called “extended metal atom chains”, or EMACs. The ultimate objective is to develop extended chains of metal ions capable of functioning as 1-dimensional nanowires. By acting as three-dimensional supports, metal-bonded complexes and clusters have been valuable in the development of metal organic frameworks (MOFs) and polymeric one, two, and three-dimensional materials. The use of very bulky ligands to limit metal–ligand interactions led to the isolation of the first complexes to possess a quintuple bond between two atoms. The steric bulk of the ligands limited the coordination environment of metals, freeing up additional orbitals and electrons to participate in the metal–metal bonding manifold. Additional novel ligand sets have been used to engender the facile formation of heterobimetallic complexes, as well as trigonal coordination environments, further pushing the frontiers of metal–metal bond reactivity. Despite the field’s seeming maturity, metal–metal bond chemistry continues to expand in new and dramatic ways.

This Special Issue on New Directions in Metal–Metal Bond Chemistry seeks to expand on these frontiers and explore unique applications of metal–metal bonded complexes. Submissions of previously unpublished work detailing the synthesis, structure, spectroscopic properties, and reactivity of metal–metal bonded complexes are welcome.

Prof. Dr. Steve C. Haefner
Guest Editor

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• Metal–metal bonding
• Polymetallic complexes
• Extended metal atom chains
• Synthesis
• Spectroscopy
• Electronic structure
• Crystal structure
• Bonding
• Magnetism
• Small-molecule activation