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Observation of alkaline earth complexes M(CO)8 (M = Ca, Sr, or Ba) that mimic transition metals

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Science  31 Aug 2018:
Vol. 361, Issue 6405, pp. 912-916
DOI: 10.1126/science.aau0839

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Carbonyls in the s block

Conventional wisdom in chemistry distinguishes transition metals from other elements by their use of d orbitals in bonding. Wu et al. now report that alkaline earth metals can slide their electrons from s- to d-orbital bonding motifs as well (see the Perspective by Armentrout). Calcium, strontium, and barium all form coordination complexes with a cubic arrangement of eight carbonyl ligands and an 18-electron valence shell. The compounds were characterized in frozen neon matrices by vibrational spectroscopy and in gas phase by mass spectrometry.

Science, this issue p. 912; see also p. 849

Abstract

The alkaline earth metals calcium (Ca), strontium (Sr), and barium (Ba) typically engage in chemical bonding as classical main-group elements through their ns and np valence orbitals, where n is the principal quantum number. Here we report the isolation and spectroscopic characterization of eight-coordinate carbonyl complexes M(CO)8 (where M = Ca, Sr, or Ba) in a low-temperature neon matrix. Analysis of the electronic structure of these cubic Oh-symmetric complexes reveals that the metal–carbon monoxide (CO) bonds arise mainly from [M(dπ)] → (CO)8 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.

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