Synthesis of [Cp′Fe(η 3 ‐BH 4 )] and Its Conversion to [Cp′FeBH 2 ] 3

Salt metathesis of [Cp′FeI] 2 [ 1 , Cp′ = η 5 ‐1,2,4‐(Me 3 C) 3 C 5 H 2 ] and NaBH 4 (2 equiv.) in tetrahydrofuran yields the low‐spin iron complex [Cp′Fe(η 3 ‐BH 4 )] ( 2 ), which is thermally unstable and degrades slowly to the trimeric complex [Cp′FeBH 2 ] 3 ( 3 ) and H 2 . Both molecules have been fully characterized by various spectroscopic techniques. An X‐ray structure analysis of 3 confirms its nature and reveals rather long B–B distances of 1.883(9) Å. DFT methods in combination with three complementary approaches [Wiberg bond indices (WBI), atoms in molecules (AIM) and compliance constants] have been used to explain the bonding in these iron(II) complexes. All methods predict a weak bonding interaction between the boron atoms in 3 .