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)₆Ca–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.