Przemysław J. Malinowski

Developments in the chemistry of weakly coordinating anions enabled isolation of numerous unique metal complexes with unusual ligands. An important example is the family of metal‐Fe(CO) 5 complexes. In the current paper we present synthesis and thorough characterization of the first truly homoleptic {Cu[Fe(CO) 5 ] 2 } + cation obtained as a salt of weakly coordinating [Al(OR F ) 4 ] − (R F =C(CF 3 ) 3 ) anion. TGA/DSC/MS study show that its decomposition becomes noticeable only above 110 °C, thus it can be stored as powder in air‐free conditions for months. The crystal structure of {Cu[Fe(CO) 5 ] 2 } + shows strong asymmetry of the cation and very short Cu‐CO bonds in comparison to analogous {M[Fe(CO) 5 ] 2 } + where M=Ag or Au. Characterization is complemented with analysis of vibrational spectra and extensive DFT calculations which give insight into the energetics of Cu + ‐Fe(CO) 5 systems. Our results show that {Cu[Fe(CO) 5 ] 2 } + is homoleptic only as salt of [Al(OR F ) 4 ] − . Furthermore, in the presence of additional, even weakly basic ligands, the Cu + ‐Fe(CO) 5 bond strength decreases what may contribute to the complex's instability in liquid SO 2 or in the presence of [SbF 6 ] − . These conclusions point at the key role of selection of proper anion and solvent in stabilization of these types of complexes.