Forty-one isomers of Al2C4H2 that lie within 50 kcal mol−1 are theoretically identified in this work using density functional theory. Among these, isomers 3 and 14 contain a planar tetracoordinate carbon (ptC) atom that lies at 3.3 and 16.9 kcal mol−1, respectively, and are above the global minimum geometry 1 at the ωB97XD/6-311++G(2d,2p) level of theory. The other ten isomers that also contain unique bonding features are isomers 4, 18, 20, 21, 22, 27, 28, 31, 34, and 40. Out of these isomers, 4, 18, 20, 22, 27, 28, and 34 contain planar tetracoordinate aluminum (ptAl) whereas isomers 31 and 40 contain both ptC and ptAl atoms. Chemical bonding characteristic features are thoroughly analyzed for all these eleven isomers with various bonding and topological quantum chemical tools, such as NBO, AdNDP, WBI, and ELF, except isomer 27 due to the observed elongated Al-Al bond length. The current results indicate that ptC isomer 3 is more stable than other isomers because electron delocalization is more prevalent and it also has double aromaticity as observed from the ELF, NICS, and AdNDP analysis. Further, the structural stability of these isomers is investigated through ab initio molecular dynamics (AIMD) simulation. Isomer 21 shows the planar pentacoordinate aluminum but it is observed as a kinetically unstable geometry from AIMD and, further, one could notice that it isomerizes to isomer 12.

Pentacoordinate carbon atoms are theoretically predicted here in a ferrocene dication derivative in the eclipsed-(1; C2v), gauche-(2; C2) and staggered-[Fe(Si2-η5-C5H2)2]2+(3; C2h) forms for the first time. Energetically, the relative energy gaps for 2 and 3 range from −3.06 to 16.74 and −2.78 to 40.34 kJ mol−1, respectively, when compared to the singlet electronic state of 1 at different levels. The planar tetracoordinate carbon (ptC) atom in the ligand Si2C5H2 becomes a pentacoordinate carbon upon complexation. The ligand with a ptC atom was predicted to be both a thermodynamically and kinetically stable molecule by some of us in our earlier theoretical works. Natural bond orbital and adaptive natural density partitioning analyses confirm the pentacoordinate nature of carbon in these three complexes (1–3). Although they are hypothetical at the moment, they support the idea of “hypercoordinate metallocenes” within organometallic chemistry. Moreover, ab initio molecular dynamics simulations carried out at 298 K temperature for 2000 fs suggest that these molecules are kinetically stable.