Among the fascinating planar tetracoordinate carbon (ptC) species, pentaatomic molecules belong to the smallest class, well-known as "pptC". It has been generally accepted that the planarity of pptC structure is realized via the "delocalization" of the p(z) lone pair at the central carbon and the ligand-ligand bonding interaction. Although "localization" is as key driving force in organic chemistry as "delocalization", the "localization" concept has not been applied to the design of pptC molecules, to the best of our knowledge. In this paper, we apply the "localization" strategy to design computationally a series of new pptC. It is shown that the central carbon atom and one "electronegative" ligand atom X (compared to the Al ligand) effectively form a highly localized C-X multiple bond, converting the lone pair at the central carbon to a two-center two-electron π-bond. At the aug-cc-pVTZ-B3LYP, MP2 and CCSD(T) levels, the designed 18-valence-electron pptC species [XCAl(3)](q); [(X,q) = (B,-2), (C,-1), (N,0)] are found to each possess a stable ptC structure bearing a C-X double bond, indicated by the structural, molecular orbital, Wiberg bonding, potential energy surface and Born-Oppenheimer molecular dynamics (BOMD) analysis. Moreover, our OVGF calculations showed that the presently disclosed (yet previously unconsidered) pptC structure of [C(2)Al(3)](-) could well account for the observed photoelectron spectrum (previously only ascribed to a close-energy fan-like structure). Therefore, [C(2)Al(3)](-) could be the first pptC that bears the highly localized C-X double bond that has been experimentally generated. Notably, the pptC structure is the respective global minimum point for [BCAl(3)](2-) and [NCAl(3)], and the counterion(s) would further stabilize [BCAl(3)](2-) and [C(2)Al(3)](-). Thus, these newly designed pptC species with interesting bonding structure should be viable for future experimental characterization. The presently applied "localization" approach complements well the previous "delocalization" one, indicating that the general "localization vs. delocalization" concept in organic chemistry can be effectively transplanted to exotic pptC chemistry.

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Molecules with hypercoordinate planar centers have continued to receive enthusiastic attention due to their violation of the traditional models of three-dimensional chemical bonding and maximum tetracoordination. These electronic exotic but structurally aesthetic species have been optimistically conceived as building blocks in cluster-assembly for bulky materials. Recently, the planar hexacoordinate nitrogen (phN) unit, NB(6)(-), has been theoretically incorporated into a series of sandwich-like transition-metal compounds. However, the intrinsic stability of NB(6)(-) in both gas-phase and assembly has not been tackled, though it is the key factor for predicting the viability of any molecules. In this paper, at the CCSD(T)/6-311+G(2df)//B3LYP/6-311+G(d)+ZPVE level, we investigate for the first time the thermodynamic and kinetic stability of the phN unit, NB(6)(-), in both free and assembled ([NB(6)](2)Fe) forms. The calculated least barrier height of phN towards conversion is 9.2 and 4.4 kcal mol(-1) in free and assembled forms, respectively. Most importantly, the phN structure is thermodynamically rather unstable, by 102.8 and 162.1 kcal mol(-1) higher than the respective lower-lying conversion isomers. Therefore, in view of the combined thermodynamic and kinetic consideration, we propose that isolation of the phN structure of NB(6)(-) in either gas phase or assembly is unlikely. The present results manifest that for predicting any viable molecule with exotic structures, investigation of its "intrinsic stability" is highly necessary. The maintenance of the phN-NB(6)(-) is discussed at the 6-311+G(d)-B3LYP, MP2, CCSD and CCSD(T) optimization levels in comparison with the isoelectronic and milestone phC-CB(6)(2-).