Chemical intuition yields the second low-lying isomer after AUTOMATON found the global minimum and few other low-lying isomers.

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.

Considering the recent findings of linear doublet ( 2 Σ + ) MgC n H isomers ( n = 2, 4, and 6) in the evolved carbon star IRC+10216, various structural isomers of MgC 3 H and MgC 3 H + are theoretically investigated here. For MgC 3 H, 11 doublet and 8 quartet stationary points ranging from 0.0 to 71.8 and 0.0 to 110.1 kcal mol –1 , respectively, have been identified initially at the UωB97XD/6-311++G(2d,2p) level. To get accurate relative energies, further energy evaluations are carried out for all isomers with coupled cluster methods and thermochemical modules such as G3//B3LYP, G4MP2, and CBS-QB3 methods. Unlike the even series, where the global minima are linear molecules with a Mg atom at one end, in the case of MgC 3 H, the global minimum geometry turns out to be a cyclic isomer, 2-magnesabicyclo[1.1.0]but-1,3,4-triyl ( 1 , C 2 v , 2 A 1 ). In addition, five low-lying isomers, magnesium-substituted cyclopropenylidene ( 2 , C s , 2 A ′), 1-magnesabut-2,3-dien-1-yl-4-ylidene ( 3 , C s , 2 A ″), 1-magnesabut-2-yn-1-yl-4-ylidene ( 4 , C s , 2 A ″), 2λ 3 -magnesabicyclo[1.1.0]but-1,3-diyl-4-ylidene ( 5 , C 2 v ;, 2 A 1 ), and 1-magnesabut-2,3-dien-2-yl-4-ylidene ( 6 , C ∞v , 2 Σ + ), were also identified. The doublet linear isomer of MgC 3 H, 1-magnesabutatrienyl ( 10 , C ∞v , 2 Σ + ) turns out to be a minimum but lies 54.1 kcal mol –1 above 1 at the ROCCSD(T)/cc-pVTZ level. The quartet ( 4 Σ + ) electronic state of 10 was also found to be a minimum, but it lies 8.0 kcal mol –1 above 1 at the same level. Among quartets, isomer 10 is the most stable molecule. The next quartet electronic state (of isomer 11 ) is 34.4 kcal mol –1 above 10 , and all other quartet electronic states of other isomers are not energetically close to low-lying ...

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