Low-coordinate iron(I) and manganese(I) dimers: Kinetic stabilization of an exceptionally short fe-fe multiple bond

The fundamental and applied chemistry of metal-metal bonded complexes has rapidly expanded since Cotton s landmark report of metal-metal quadruple bonding in the dianion, [Re2Cl8]2 , nearly 50 years ago.[1,2] Many of the recent advances in the field have centered on the stabilization of reactive low oxidation state/low coordination number M-M bonded complexes using sterically imposing ligand systems. Representative examples include the singly bonded zinc(I) and magnesium(I) dimers, [Cp*ZnZnCp*] (Cp*=C5Me5)[3] and [(DipNacnac)MgMg(DipNacnac)] (DipNacnac=[(DipNCMe) 2CH], Dip=2,6-iPr2C6H3),[4] and the quintuply bonded chromium(I) dimer, [Ar CrCrAr (Ar =2,6-(Dip)2C6H3).[5] The unprecedented bonding in the latter initiated a number of studies aimed at preparing complexes with ever shorter Cr Cr bonds.[6] These culminated in the isolation of quintuply bonded amidinate and guanidinate bridged complexes, for example, [Cr2 m-N,N -(DipN)2CR 2] (1; R=H, Me, or NMe2), which exhibit the shortest known metal-metal bonds (ca. 1.74 ).[7] As these bonds are essentially derived from the filling of five metal-based bonding molecular orbitals (MOs), the compounds are diamagnetic.