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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Xuan Wu, Lili Zhao, Jiaye Jin, Sudip Pan, Wei Li, Xiaoyang Jin, Guanjun Wang, Mingfei Zhou, Gernot Frenking, Observation of alkaline earth complexes M(CO)8 (M = Ca, Sr, or Ba) that mimic transition metals, Science 321, 912-916 (2018)
See also these books:
The Chemical Bond. Fundamental Aspects of Chemical Bonding. G. Frenking and S. Shaik (Eds), Wiley-VCH, Weinheim, 2014.
The Chemical Bond. Chemical Bonding Across the Periodic Table. G. Frenking and S. Shaik (Eds), Wiley-VCH, Weinheim, 2014.
And also check out the chemical bonding analysis page on our website.