Mono-silicon isoelectronic replacement in CAl4 : van't hoff/le bel carbon or not? 
In cluster studies, the isoelectronic replacement strategy has been successfully used to introduce new elements into a known structure while maintaining the desired topology.  The well-known penta-atomic 18 valence electron (ve) species C Al 4 2 - and its Al- /Si or Al/Si+ isoelectronically replaced clusters CAl3 Si- , CAl2 Si2 , C AlSi 3 - , and C Si 4 2 + , all possess the same anti-van't Hoff/Le Bel skeletons, that is, nontraditional planar tetracoordinate carbon (ptC) structure.  In this article, however, we found that such isoelectronic replacement between Si and Al does not work for the 16ve-CAl4 with the traditional van't Hoff/Le Bel tetrahedral carbon (thC) and its isoelectronic derivatives CAl3 X (X = Ga/In/Tl).  At the level of CCSD(T)/def2-QZVP//B3LYP/def2-QZVP, none of the global minima of the 16ve mono-Si-containing clusters CAl2 SiX+ (X = Al/Ga/In/Tl) maintains thC as the parent CAl4 does.  Instead, X = Al/Ga globally favors an unusual ptC structure that has one long C─X distance yet with significant bond index value, and X = In/Tl prefers the planar tricoordinate carbon.  The frustrated formation of thC in these clusters is ascribed to the CSi bonding that prefers a planar fashion.  Inclusion of chloride ion would further stabilize the ptC of CAl2 SiAl+ and CAl2 SiGa+ .  The unexpectedly disclosed CAl2 SiAl+ and CAl2 SiGa+ represent the first type of 16ve-cationic ptCs with multiple bonds.  © 2019 Wiley Periodicals, Inc.