Rhenium: Der Zauberer

Despite a belated seating at the Periodic Table, rhenium cemented its reputation in coordination chemistry with two landmark discoveries: molecular trigonal prismatic geometry and metal-metal multiple bonds. The former arrived with the crystallographic characterization of the tris(dithiolene) complex, [Re(S2C2Ph2)3]. Recent interest in ligand redox activity has stimulated a re-examination of such complexes, and to that end we correctly interpret its truly amazing EPR spectrum that unambiguously defines its electronic structure.
The recognition of the quadruple metal-metal bond in [Re2Cl8]2- was the defining moment for multicenter coordination transition metal chemistry. More than half a century later, most of the periodic table has been implicated in metal-metal bonding, and this new domain of chemistry has seen prodigious growth. The reaction of [Re2Cl8]2– with Na2mnt (mnt2– = maleonitriledithiolate) gives a Class 1 mixed valent, edge-sharing bioctahedron, [Re2(mnt)5]3–. Interestingly, it gives an unexpected EPR signal for a compound with multiple metal bonds. This is contrasted with isoelectronic [Re2(NCS)10]3– – a compound with crystallographic inversion symmetry yet the unpaired electron is magically localized.