d–d Dative Bonding Between Iron and the Alkaline‐Earth Metals Calcium, Strontium, and Barium

Double deprotonation of the diamine 1,1′‐(tBuCH(2)NH)‐ferrocene (1‐H(2)) by alkaline‐earth (Ae) or Eu(II) metal reagents gave the complexes 1‐Ae (Ae=Mg, Ca, Sr, Ba) and 1‐Eu. 1‐Mg crystallized as a monomer while the heavier complexes crystallized as dimers. The Fe⋅⋅⋅Mg distance in 1‐Mg is too long for a bonding interaction, but short Fe⋅⋅⋅Ae distances in 1‐Ca, 1‐Sr, and 1‐Ba clearly support intramolecular Fe⋅⋅⋅Ae bonding. Further evidence for interactions is provided by a tilting of the Cp rings and the related (1)H NMR chemical‐shift difference between the Cp α and β protons. While electrochemical studies are complicated by complex decomposition, UV/Vis spectral features of the complexes support Fe→Ae dative bonding. A comprehensive bonding analysis of all 1‐Ae complexes shows that the heavier species 1‐Ca, 1‐Sr, and 1‐Ba possess genuine Fe→Ae bonds which involve vacant d‐orbitals of the alkaline‐earth atoms and partially filled d‐orbitals on Fe. In 1‐Mg, a weak Fe→Mg donation into vacant p‐orbitals of the Mg atom is observed.