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The composition and valence electrons of molecules usually have a great impact on the eventual topology. With a high tendency to form sp/sp2-hybridized multiple bonding of C2, the main-group dicarbides (C2Xn) usually adopt a non-closo (or open) topology. By contrast, naked Zintl ions (e.g., Si52−) usually feature deltahedral structures. In this paper, we report an unexpected example of a pentatomic carbon-silicon cluster C2Si32− which has the global minimum C2Si32−-01 featuring a closo-structure with all deltahedras. The global minimum nature of C2Si32−-01 was confirmed by various sophisticated methods including G3B3, G4, CBS-QB3 and W1BD as well as CCSD(T) extrapolated up to the complete basis set limit based on the CCSD(T)/aug-cc-pVTZ, CCSD(T)/aug-cc-pVQZ and CCSD(T)/aug-cc-pV5Z calculations. The AdNDP analysis revealed that C2Si32−-01 possesses a lone pair of electrons on each of the three silicon atoms, four 2c–2e bonds and four 3c–2e bonds, among which the two 2c–2e bonds between the two carbons indicate the existence of a multiply bonded C2 (1.320 A) that carries the most negative charges. With a total of 22 valence electrons, C2Si32−-01 formally resembles the known Wade–Mingos clusters with (n + 1) polyhedral skeletal electron pairs (PSEPs). Replacement of Si2 by the highly electron-withdrawing C2 does not break the deltahedral topology of the Zintl ion Si52−. To the best of our knowledge, C2Si32− represents the smallest deltahedral main-group dicarbide and also the first deltahedral main-group dicarbide with (n + 1) PSEPs. To direct its organometallic applications, we designed the hetero-deckered sandwich compounds CpMg(C2Si32−)MgCp, in which the C2Si32−-01 unit can be nicely maintained. |
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