Salt metathesis of [Cp′FeI]2 [1, Cp′ = η5-1,2,4-(Me3C)3C5H2] and NaBH4 (2 equiv.) in tetrahydrofuran yields the low-spin iron complex [Cp′Fe(η3-BH4)] (2), which is thermally unstable and degrades slowly to the trimeric complex [Cp′FeBH2]3 (3) and H2. 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.