Main Group Elements as Transition Metals: Alkaline Earth Octacarbonyls with 18-electrons
Generally, main group elements prefer obeying the 8-electron rule; while the transition metal elements tend to follow the 18-electron rule. Recently, scientists reported the isolation and spectroscopic identification of the eight-coordinated alkaline earth carbonyl complexes M(CO)8 (M = Ca, Sr, Ba) in a low-temperature matrix, which unexpectedly obey the 18-electron rule.
In their recently published Science paper, Xuan Wu et al. used scalar relativistic ADF calculations with EDA-NOCV analysis to show that the metal-CO bonds are mainly due to [M(dπ)]→(CO)8 π backdonation, which explains the strong red-shift of the C-O stretching frequencies. The complexes M(CO)8 exhibit typical features of transition metal complexes obeying the 18-electron rule.
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