In this paper, we divide allotropes into two categories,  one is the zero-oxidation state allotrope and the other is the ionic form allotrope.  So far, many main group complexes L→Mn←L( L, Lewis base  ligands; M, main group element) have been found, which can be  regarded as zero-oxidation state allotrope of the main group elements stabilized by Lewis base  ligands. However, few compounds L→Mn+Mn-←L containing ionic form allotropes of the main group elements have been reported. This class of compounds L→Mn+Mn-←L is found only in  s-block alkalide salts.  In this work, we have theoretically confirmed that this class of compound L→Mn+Mn-←L can be extended to P-block boron. Rigorous thermodynamic calculations based on  Born–Haber cycle confirm that (NHC)3→B+ and (CO)2→B-are a compatible pair of positive and negative ions, which can form stable [(NHC)3→B+][(CO)2→B-] crystalline salt. More importantly, the bonding analysis of (NHC)3→B+ and (CO)2→B- based on frontier orbital analysis and NBO show that (NHC)3→B+ and (CO)2→B- can be regarded as positive monovalent boron B(1) and negative monovalent boron B(-1) complexes stabilized by Lewis base, respectively. So [(NHC)3→B+][(CO)2→B-] crystalline salt can be viewed as“B+B-”ionic salts stabilized by Lewis base ligands: an allotrope of boron in ionic form through asymmetric coordination of Lewis base.

Here we propose an antipodal conjecture, which is based on the synthetic scheme of alkalide [K(C222)]+K- , to prepare K+[K(CO)8]-. By introducing the concept of antipodal salts of alkalides, K+[K(CO)8]- is defined as antipodal salt of [K(C222)]+K-. As Group 1 elements of the Periodic Table, the alkali metal K is conventionally considered to form chemical bonding through its 4s and 4p valence orbitals. Our theoretical results show that the 3d orbitals of K element in >[K(CO)8]- participate in chemical bonding of >[K(CO)8]-. This makes the alkali metal K shows typical transition metal chemical properties, which also provides the possibility for the synthesis of K+[K(CO)8]-. Based on the Born-Haber thermodynamic cycle, the upper-limit stable temperature of K+[K(CO)8]- is estimated to be 24.426 K . Our theoretical results indicate that antipodal salts of alkalides will be a new family of compounds with good synthetic prospects.

The existence and stability of MNg42+(Sb2F11−1)2 (Ng=Ar,Ne,He,M=Au, Ag, Cu) salt compounds are theoretically investigated in this study. This undertaking is carried out to address the following challenges: (1) synthesizing a bulk salt compound containing a noble gas lighter than krypton and (2) synthesizing the congeners of AuXe42+(Sb2F11−1)2 containing noble gases other than Xe. The reliability of our calculations on the MNg42+(Sb2F11−1)2 (Ng=Ar,Ne,He,M=Au, Ag, Cu) systems is assessed by benchmark calculations of the well-known AuXe42+(Sb2F11−1)2 salt. In the benchmark calculations, a two-pronged evaluation strategy, including direct and indirect evaluation methods, is used to theoretically investigate the spectroscopic constants of AuXe42+and the existence and stability of the AuXe42+(Sb2F11−1)2 salt. The validity of the theoretical calculation methods in the benchmark calculations of AuXe42+(Sb2F11−1)2 allows us to adopt a similar methodology to effectively predict the existence and stability of MNg42+(Sb2F11−1)2 (Ng=Ar,Ne,He,M=Au, Ag, Cu) salt compounds. Calculations based on the Born–Haber cycle using estimated lattice energies and some necessary ancillary thermochemical data show that MAr42+(Sb2F11−1)2 (M=Au, Ag, Cu) salt compounds can be synthesized. The upper-limit stable temperatures are estimated to be −224.43, −146.21, and −80.39 °C. The CuAr42+(Sb2F11−1)2salt compound is a promising candidate. Our calculations also show that the MNg42+(Sb2F11−1)2 (Ng=Ne,He,M=Au, Ag, Cu) salt compounds cannot be stabilized.