Observation of alkaline earth complexes M(CO)<sub>8</sub> (M = Ca, Sr, or Ba) that mimic transition metals.
basic_science · Level V
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- Also identified by DOI 10.1126/science.aau0839.
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Abstract
The alkaline earth metals calcium (Ca), strontium (Sr), and barium (Ba) typically engage in chemical bonding as classical main-group elements through their <i>n</i>s and <i>n</i>p valence orbitals, where <i>n</i> is the principal quantum number. Here we report the isolation and spectroscopic characterization of eight-coordinate carbonyl complexes M(CO)<sub>8</sub> (where M = Ca, Sr, or Ba) in a low-temperature neon matrix. Analysis of the electronic structure of these cubic <i>O<sub>h</sub></i> -symmetric complexes reveals that the metal-carbon monoxide (CO) bonds arise mainly from [M(d<sub>π</sub>)] → (CO)<sub>8</sub> π backdonation, which explains the strong observed red shift of the C-O stretching frequencies. The corresponding radical cation complexes were also prepared in gas phase and characterized by mass-selected infrared photodissociation spectroscopy, confirming adherence to the 18-electron rule more conventionally associated with transition metal chemistry.