18 electrons and counting

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Science  31 Aug 2018:
Vol. 361, Issue 6405, pp. 849-850
DOI: 10.1126/science.aau6622

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The “octet rule” is based on the stability afforded to species with closed-shell electron configurations like the noble gases. Simple “second-row” compounds like methane, ammonia, and water have eight electrons surrounding the central atom, as do their third-row analogs (silane, phosphine, and hydrogen sulfide). For atoms in the fourth row of the periodic table and beyond (principal quantum number designated by n), the single ns, three np, and five (n − 1)d orbitals must be filled with two electrons per orbital, resulting in an analogous “18-electron” rule for a closed shell. This simple electron counting guides inorganic chemists working with transition metals in predicting stable compounds, just as the octet rule guides organic chemists working with carbon. For example, the stable transition metal carbonyls Cr(CO)6, Fe(CO)5, and Ni(CO)4, as well as heavier homologs, can be formed, indicating that CO is a two-electron donor. Metals with an odd number of valence electrons must double up with a metal-metal bond, so Mn2(CO)10 and Co2(CO)8 form in order to satisfy the 18-electron rule. On page 912 of this issue, Wu et al. (1) demonstrate that the 18-electron guiding principle is not only limited to transition metals but can also be extended to nearby elements, the alkaline earths.