Highly Coordinated Heteronuclear Calcium–Iron Carbonyl Cation Complexes [CaFe(CO)(n)](+) (n=5–12) with d−d Bonding

Heteronuclear calcium–iron carbonyl cation complexes in the form of [CaFe(CO)(n)](+) (n=5–12) are produced in the gas phase. Infrared photodissociation spectroscopy in conjunction with quantum chemical calculations confirm that the n=10 complex is the coordination saturated ion where a Fe(CO)(4) fragment is bonded with a Ca(CO)(6) fragment through two side‐on bridging carbonyl ligands. Bonding analysis indicates that it is best described by the bonding interactions between a [Ca(CO)(6)](2+) dication and an [Fe(CO)(4)](−) anion forming a Fe→Ca d−d dative bond in the [(CO)(6)Ca–Fe(CO)(4)](+) structure, which enriches the pool of experimentally observed complexes of calcium that mimic transition metal compounds. The molecule is the first example of a heteronuclear carbonyl complex featuring a d−d bond between calcium and a transition metal.