Iron‐Iron Bond Lengths and Bond Orders in Diiron Lantern‐Type Complexes with High Spin Ground States

AbstractDiiron paddlewheel‐ or lantern‐type complexes comprise an interesting subclass of binuclear iron complexes, existing with digonal, trigonal, and tetragonal ligand arrays. Experimentally known members show Fe−Fe bonds of lengths ranging from 2.13 to 2.73 Å, with Fe−Fe bond orders ranging from 0.5 to 2. Truncated models for 30 experimentally characterized diiron paddlewheel‐type complexes have been studied by DFT using the M06‐L functional, reproducing the Fe−Fe bond lengths quite well. In addition, we use DFT to treat three series of model diiron complexes Fe2Lx (x=2, 3, 4) in various low‐lying spin states, L being the unsubstituted formamidinate, guanidinate, and formate ligands (along with a series of axially ligated formate complexes) in order to predict ground state spin multiplicities, Fe−Fe bond lengths, and features of the ligand arrays. The ground states all have high spin multiplicities (septets, octets, and nonets). Formal bond order (fBO) values are suggested for the Fe−Fe bonds in these 61 complexes using an electron bookkeeping procedure, in addition to the Fe−Fe bond orders obtained by metal‐metal MO analysis for ground state species. Fe−Fe bond orders up to 3 are noted in some excited states. Deviations from D3h and D4h symmetry are noted for many trigonal and tetragonal complexes, being attributed to inherent Jahn‐Teller distortions. The formamidinate and guanidinate series show many similarities, while the formate series differs from these two in several aspects. From these results, ranges are derived for Fe−Fe bond lengths of orders 0.5, 1.0, 1.5, 2.0, 2.5 and 3.0. The Fe−Fe bond length ranges for these non‐carbonyl lantern complexes are found to be appreciably lower than the corresponding ranges for diiron complexes with carbonyl ligands as compiled earlier from computational and experimental results.