Are there no d-electrons in calcium?

For calcium, the electron configuration is $\mathrm{1s^2\:2s^2\:2p^6\:3s^2\:3p^6\:4s^2}$. Does it mean there's no electron in d sub-shell? Also, how would one write the electron configuration for a calcium ion?




An isolated calcium atom has no $d$ electrons, but calcium in at least one compound has been found to incorporate $3d$ orbitals into its bonding. [Krieck et al.[1]](https://doi.org/10.1021/ja808524y),[[2]](https://doi.org/10.1021/ja105534w) describe the inverse sandwich complex $\ce{[(thf)3Ca(μ-C6H3}\text{-}\ce{1,3,5}\text{-}\ce{Ph3)Ca(thf)3]}$, a dicalcium complex in which what are ordinarily antibonding orbitals in the organic ligand overlap appropriately symmetric calcium $3d$ orbitals to become bonding between calcium and carbon. Note especially the oxidation state of calcium, $+1$. Calcium(II) would more likely show the classical predominantly ionic bonding in which the metal valence electrons play no role except the $4s$ electrons being largely transferred to the nonmetal atoms.


**Reference**


1. Sven Krieck, Helmar Görls, Lian Yu, Markus Reiher, and Matthias Westerhausen (2009). "Stable “Inverse” Sandwich Complex with Unprecedented Organocalcium(I): Crystal Structures of [(thf)2Mg(Br)-C6H2-2,4,6-Ph3] and [(thf)3Ca{μ-C6H3-1,3,5-Ph3}Ca(thf)3]". *J. Am. Chem. Soc.* **131**, 8, 2977–2985.
<https://doi.org/10.1021/ja808524y>.
2. Krieck, Sven; Görls, Helmar; Westerhausen, Matthias (2010). "Mechanistic Elucidation of the Formation of the Inverse Ca(I) Sandwich Complex [(thf)3Ca(μ-C6H3-1,3,5-Ph3)Ca(thf)3] and Stability of Aryl-Substituted Phenylcalcium Complexes". *Journal of the American Chemical Society*. **132** (35): 12492–12501. <https://doi.org/10.1021/ja105534w>. PMID 20718434.


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