Planar pentacoordinate carbons


Carbon centres in typical organic molecules have a coordination number that can reach a maximum of four, in which case the bonded atoms are situated at the vertices of a tetrahedron. Exceptions to those two structural rules have been posited and examined for decades, and planar tetracoordinate carbon (ptC) species are notable molecules that violate the second rule. There is continued interest in experimental and theoretical studies of ptCs, as well as emerging molecules that contain planar pentacoordinate carbon (ppC) and planar hexacoordinate carbon (phC) atoms, species that violate both structural rules. This Review describes recent progress in the theoretical prediction of viable entities that contain ppC centres. The first such molecule reported, the D5h-symmetric ppC species CAl5+, was followed by a series of predicted ppC species that could be obtained by substituting the Al centres for other heteroatoms. More complicated ppC systems have also been suggested, including metallocene-stabilized ppCs and quasi-ppCs embedded within cage structures or 2D materials. To date, computational studies have identified at least 65 local and 39 global minimum energy structures that contain ppCs or quasi-ppCs. The general design principles for ptC-centred candidate structures include delocalization of the central C 2pz lone electron pair, ensuring an 18 valence electron count and allowing for strong electron delocalization. These principles have been extended to ppC systems with some success. It is hard to predict the extent to which the coordination number of planar C can be increased because it depends not only on the valence and size of C but also on the size of the atoms bonded to it and the mode of bonding. Although a few energetically low-lying planar hexacoordinate and heptacoordinate C species have been identified computationally, none have been observed experimentally.

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Figure 1: The structures of two prominent planar tetracoordinate carbon-containing molecules.
Figure 2: Charge-neutral compounds featuring planar pentacoordinate or pseudopentacoordinate C bound to Li or B.
Figure 3: Structures featuring one or more planar pentacoordinate carbon centres surrounded by metal or metalloid atoms.
Figure 4: Many local minimum boron–carbon systems featuring one or two planar pentacoordinate carbons can be designed.
Figure 5: Monomeric and dimeric hexanuclear motifs are useful arrangements to stabilize planar pentacoordinate carbons.
Figure 6: Hexanuclear and heptanuclear planar pentacoordinate carbon anions have been found to be global minima.
Figure 7: Binding metal or halogen cations to the periphery of a planar pentacoordinate carbon can afford stable global minimum energy structures.
Figure 8: Predicted structures of planar pentacoordinate carbon-containing fullerenes, 2D materials and metallocenes.


  1. 1

    Tal’roze, V. L. & Ljubimova, A. K. Secondary processes in the ion source of a mass spectrometer. J. Mass Spectrom. 33, 502–504 (1998).

    Article  Google Scholar 

  2. 2

    Bartlett, P. D. Nonclassical Ions (WA Benjamin, New York, 1965).

    Google Scholar 

  3. 3

    Brown, H. C. The Nonclassical-Ion Problem (Plenum, New York, 1977).

    Google Scholar 

  4. 4

    Olah, G. A., Prakash, G. K. S. & Saunders, M. Conclusion of the classical–nonclassical ion controversy based on the structural study of the 2-norbornyl cation. Acc. Chem. Res. 16, 440–448 (1983).

    CAS  Article  Google Scholar 

  5. 5

    Minkin, V. I., Minyaev, R. M. & Hoffmann, R. Nonclassical structures of organic compounds: non-standard stereochemistry and hypercoordination. Russ. Chem. Rev. 71, 869–892 (2002).

    CAS  Article  Google Scholar 

  6. 6

    Olah, G. A. My search for carbocations and their role in chemistry (Nobel lecture). Angew. Chem. Int. Ed. 34, 1393–1405 (1995).

    CAS  Article  Google Scholar 

  7. 7

    Monkhorst, H. J. Activation energy for interconversion of enantiomers containing an asymmetric carbon atom without breaking bonds. Chem. Commun. 1111–1112 (1968).

  8. 8

    Hoffmann, R., Alder, R. W. & Wilcox, C. F. Planar tetracoordinate carbon. J. Am. Chem. Soc. 92, 4992–4993 (1970).

    CAS  Article  Google Scholar 

  9. 9

    Pepper, M. J. M. et al. Is the stereomutation of methane possible? J. Comput. Chem. 16, 207–225 (1995).

    CAS  Article  Google Scholar 

  10. 10

    Gordon, M. S. & Schmidt, M. W. Does methane invert through square planar? J. Am. Chem. Soc. 115, 7486–7492 (1993).

    CAS  Article  Google Scholar 

  11. 11

    Sorger, K. & Schleyer, P. v. R. Planar and inherently non-tetrahedral tetracoordinate carbon: a status report. J. Mol. Struct. 338, 317–346 (1995).

    Article  Google Scholar 

  12. 12

    Keese, R. Carbon flatland: planar tetracoordinate carbon and fenestranes. Chem. Rev. 106, 4787–4808 (2006).

    CAS  Article  Google Scholar 

  13. 13

    Collins, J. B. et al. Stabilization of planar tetracoordinate carbon. J. Am. Chem. Soc. 98, 5419–5427 (1976).

    CAS  Article  Google Scholar 

  14. 14

    Cotton, F. A. & Millar, M. The probable existence of a triple bond between two vanadium atoms. J. Am. Chem. Soc. 99, 7886–7891 (1977).

    CAS  Article  Google Scholar 

  15. 15

    Schleyer, P. v. R. & Boldyrev, A. I. A new, general strategy for achieving planar tetracoordinate geometries for carbon and other second row periodic elements. J. Chem. Soc., Chem. Commun. 1536–1538 (1991).

  16. 16

    Boldyrev, A. I. & Simons, J. Tetracoordinated planar carbon in pentaatomic molecules. J. Am. Chem. Soc. 120, 7967–7972 (1998).

    CAS  Article  Google Scholar 

  17. 17

    Cui, Z.-H., Shao, C.-B., Gao, S.-M. & Ding, Y.-H. Pentaatomic planar tetracoordinate carbon molecules [XCAl3]q [(X. q) = (B, −2), (C, −1), (N, 0)] with C–X multiple bonding. Phys. Chem. Chem. Phys. 12, 13637–13645 (2010).

    CAS  Article  Google Scholar 

  18. 18

    Castro, A. C. et al. Planar tetracoordinate carbon in CE42− (E = Al–Ti) clusters. Chem. Phys. Lett. 519–520, 29–33 (2012).

    Article  Google Scholar 

  19. 19

    Cui, Z.-H. et al. Planar tetracoordinate carbons with a double bond in CAl3E clusters. Phys. Chem. Chem. Phys. 17, 8769–8775 (2015).

    CAS  Article  Google Scholar 

  20. 20

    Li, X., Wang, L.-S., Boldyrev, A. I. & Simons, J. Tetracoordinated planar carbon in the Al4C anion. A combined photoelectron spectroscopy and ab initio study. J. Am. Chem. Soc. 121, 6033–6038 (1999).

    CAS  Article  Google Scholar 

  21. 21

    Wang, L.-S., Boldyrev, A. I., Li, X. & Simons, J. Experimental observation of pentaatomic tetracoordinate planar carbon-containing molecules. J. Am. Chem. Soc. 122, 7681–7687 (2000).

    CAS  Article  Google Scholar 

  22. 22

    Albrecht, M., Erker, G. & Kruger, C. The synthesis of stable, isolable planar-tetracoordinate carbon compounds. Synlett 441–448 (1993).

  23. 23

    Evans, W. J., Keyer, R. A. & Ziller, J. W. Investigation of organolanthanide-based carbon–carbon bond formation: synthesis, structure, and coupling reactivity of organolanthanide alkynide complexes, including the unusual structures of the trienediyl complex [(C5Me5)2Sm]2[μ-η22-Ph(CH2)2C=C=C=C-(CH2)2Ph] and the unsolvated alkynide [(C5Me5)2Sm(C≡CCMe3)]2 . Organometallics 12, 2618–2633 (1993).

    CAS  Article  Google Scholar 

  24. 24

    Rö ttger, D. & Erker, G. Compounds containing planar-tetracoordinate carbon. Angew. Chem. Int. Ed. 36, 812–827 (1997).

    Article  Google Scholar 

  25. 25

    Choukroun, R. & Cassoux, P. Planar tetracoordination of carbon in groups 4 and 5 organometallic chemistry. Acc. Chem. Res. 32, 494–502 (1999).

    CAS  Article  Google Scholar 

  26. 26

    Choukroun, R. & Lorber, C. Adventures in vanadocene chemistry. Eur. J. Inorg. Chem. 4683–4692 (2005).

  27. 27

    Su, M.-D. Theoretical designs for planar tetracoordinated carbon in Cu, Ag, and Au organometallic chemistry: a new target for synthesis. Inorg. Chem. 44, 4829–4833 (2005).

    CAS  Article  Google Scholar 

  28. 28

    Suresh, C. H. & Frenking, G. Direct 1–3 metal–carbon bonding and planar tetracoordinated carbon in group 6 metallacyclobutadienes. Organometallics 29, 4766–4769 (2010).

    CAS  Article  Google Scholar 

  29. 29

    Jia, X. F. & Zhang, C. J. Structure, bonding and reactivity of coinage metal complexes TML2 and TML2+ (TM = Cu, Ag, Au) with planar tetracoordinate carbon, a theoretical investigation. Comput. Theor. Chem. 1075, 47–53 (2016).

    CAS  Article  Google Scholar 

  30. 30

    Merino, G., Mendez-Rojas, M. A. & Vela, A. (C5M2−n)n− (M = Li, Na, K, and n = 0, 1, 2). A new family of molecules containing planar tetracoordinate carbons. J. Am. Chem. Soc. 125, 6026–6027 (2003).

    CAS  Article  Google Scholar 

  31. 31

    Pérez-Peralta, N. et al. Stabilizing carbon–lithium stars. Phys. Chem. Chem. Phys. 13, 12975–12980 (2011).

    Article  Google Scholar 

  32. 32

    Perez, N. et al. Planar tetracoordinate carbons in cyclic hydrocarbons. Org. Lett. 7, 1509–1512 (2005).

    CAS  Article  Google Scholar 

  33. 33

    Merino, G., Méndez-Rojas, M. A., Vela, A. & Heine, T. Recent advances in planar tetracoordinate carbon chemistry. J. Comput. Chem. 28, 362–372 (2007).

    CAS  Article  Google Scholar 

  34. 34

    Perez-Peralta, N. et al. Planar tetracoordinate carbons in cyclic semisaturated hydrocarbons. J. Org. Chem. 73, 7037–7044 (2008).

    CAS  Article  Google Scholar 

  35. 35

    Cui, Z.-H., Contreras, M., Ding, Y.-H. & Merino, G. Planar tetracoordinate carbon versus planar tetracoordinate boron: the case of CB4 and its cation. J. Am. Chem. Soc. 133, 13228–13231 (2011).

    CAS  Article  Google Scholar 

  36. 36

    Wu, Y.-B. et al. Starlike aluminum–carbon aromatic species. Chem. Eur. J. 17, 714–719 (2011).

    CAS  Article  Google Scholar 

  37. 37

    Crigger, C., Wittmaack, B. K., Tawfik, M., Merino, G. & Donald, K. J. Plane and simple: planar tetracoordinate carbon centers in small molecules. Phys. Chem. Chem. Phys. 14, 14775–14783 (2012).

    CAS  Article  Google Scholar 

  38. 38

    Pan, S. et al. C5Li7+ and O2Li5+ as noble-gas-trapping agents. Chem. Eur. J. 19, 2322–2329 (2013).

    CAS  Article  Google Scholar 

  39. 39

    Bomble, L., Steinmann, S., Perez-Peralta, N., Merino, G. & Corminboeuf, C. Bonding analysis of planar hypercoordinate atoms via the generalized BLW-LOL analysis. J. Comput. Chem. 34, 2242–2248 (2013).

    CAS  Article  Google Scholar 

  40. 40

    Bolton, E. E., Laidig, W. D., Schleyer, P. v. R. & Schaefer, H. F. Does singlet 1,1-dilithioethene really prefer a perpendicular structure? J. Phys. Chem. 99, 17551–17557 (1995).

    CAS  Article  Google Scholar 

  41. 41

    Wang, Z.-X. & Schleyer, P. v. R. Construction principles of “hyparenes”: families of molecules with planar pentacoordinate carbons. Science 292, 2465–2469 (2001).

    CAS  Article  Google Scholar 

  42. 42

    Wang, Z.-X. & Schleyer, P. v. R. Planar hypercoordinate carbons joined: wheel-shaped molecules with C–C axles. Angew. Chem. Int. Ed. 41, 4082–4085 (2002).

    CAS  Article  Google Scholar 

  43. 43

    Li, S.-D., Miao, C.-Q. & Ren, G.-M. D5 Cu5H5X: pentagonal hydrocopper Cu5H5 containing pentacoordinate planar nonmetal centers (X = B, C, N, O). Eur. J. Inorg. Chem. 2232–2234 (2004).

  44. 44

    Li, S.-D., Guo, Q.-L., Miao, C.-Q. & Ren, G.-M. Investigation on transition-metal hydrometal complexes MnHnC with planar coordinate carbon centers by density functional theory. Acta Phys. Chim. Sin. 23, 743–745 (2007).

    Google Scholar 

  45. 45

    Erhardt, S., Frenking, G. & Chen, Z. F. & Schleyer, P. v. R. Aromatic boron wheels with more than one carbon atom in the center: C2B8, C3B93+, and C5B11+. Angew. Chem. Int. Ed. 44, 1078–1082 (2005).

    CAS  Article  Google Scholar 

  46. 46

    Luo, Q. Theoretical observation of hexaatomic molecules containing pentacoordinate planar carbon. Sci. China Ser. B Chem. 51, 1030–1035 (2008).

    CAS  Article  Google Scholar 

  47. 47

    Pei, Y., An, W., Ito, K., Schleyer, P. v. R. & Zeng, X. C. Planar pentacoordinate carbon in CAl5+: a global minimum. J. Am. Chem. Soc. 130, 10394–10400 (2008).

    CAS  Article  Google Scholar 

  48. 48

    Pei, Y. & Zeng, X. C. Probing the planar tetra-, penta-, and hexacoordinate carbon in carbon–boron mixed clusters. J. Am. Chem. Soc. 130, 2580–2592 (2008).

    CAS  Article  Google Scholar 

  49. 49

    Averkiev, B. B. et al. Carbon avoids hypercoordination in CB6, CB62−, and C2B5 planar carbon–boron clusters. J. Am. Chem. Soc. 130, 9248–9250 (2008).

    CAS  Article  Google Scholar 

  50. 50

    Liang, J.-X., Jia, W.-H., Zhang, C.-J. & Cao, Z.-X. Unusual boron–carbon compounds containing planar tetracoordinate and pentacoordinate carbons. Acta Phys. Chim. Sin. 25, 1847–1852 (2009).

    CAS  Google Scholar 

  51. 51

    Zdetsis, A. D. Success and pitfalls of the Sin − 2C2H2–C2Bn−2Hn isolobal analogy: depth and breadth of the boron connection. J. Chem. Phys. 130, 064303 (2009).

    Article  Google Scholar 

  52. 52

    Zdetsis, A. D. Novel pentagonal silicon rings and nanowheels stabilized by flat pentacoordinate carbon(s). J. Chem. Phys. 134, 094312 (2011).

    Article  Google Scholar 

  53. 53

    Yamaguchi, W. δ and σ versus π conflicting aromatic pentagonal ring of tungsten with a planar pentacoordinate carbon at the ring center. Int. J. Quantum Chem. 110, 1086–1091 (2010).

    CAS  Article  Google Scholar 

  54. 54

    Merino, G. & Solà, M. Celebrating the 150th anniversary of the Kekulé benzene structure. Phys. Chem. Chem. Phys. 18, 11587–11588 (2016).

    Article  Google Scholar 

  55. 55

    Hoffmann, R. The many guises of aromaticity. Am. Sci. 103, 18 (2015).

    Article  Google Scholar 

  56. 56

    Li, X., Kuznetsov, A. E., Zhang, H.-F., Boldyrev, A. I. & Wang, L.-S. Observation of all-metal aromatic molecules. Science 291, 859–861 (2001).

    CAS  Article  Google Scholar 

  57. 57

    Kuznetsov, A. E. et al. All-metal antiaromatic molecule: rectangular Al44− in the Li3Al4 anion. Science 300, 622–625 (2003).

    CAS  Article  Google Scholar 

  58. 58

    Islas, R., Heine, T. & Merino, G. Structure and electron delocalization in Al42− and Al44−. J. Chem. Theory Comput. 3, 775–781 (2007).

    CAS  Article  Google Scholar 

  59. 59

    Jimenez-Halla, J. O. C. et al. CAl4Be and CAl3Be2: global minima with a planar pentacoordinate carbon atom. Chem. Commun. 46, 8776–8778 (2010).

    CAS  Article  Google Scholar 

  60. 60

    Castro, A. C. et al. CBe5E (E = Al, Ga, In, Tl): planar pentacoordinate carbon in heptaatomic clusters. Phys. Chem. Chem. Phys. 14, 14764–14768 (2012).

    CAS  Article  Google Scholar 

  61. 61

    Wu, Y.-B., Duan, Y., Lu, H.-G. & Li, S.-D. CAl2Be32− and its salt complex LiCAl2Be3: anionic global minima with planar pentacoordinate carbon. J. Phys. Chem. A 116, 3290–3294 (2012).

    CAS  Article  Google Scholar 

  62. 62

    Zhang, X.-Y. & Ding, Y.-H. CAl4Ga+. Comput. Theor. Chem. 1048, 18–24 (2014).

    CAS  Article  Google Scholar 

  63. 63

    Grande-Aztatzi, R. et al. Planar pentacoordinate carbons in CBe54− derivatives. Phys. Chem. Chem. Phys. 17, 4620–4624 (2015).

    CAS  Article  Google Scholar 

  64. 64

    Guo, J.-C. et al. CBe5Hnn−4 (n = 2–5): hydrogen-stabilized CBe5 pentagons containing planar or quasi-planar pentacoordinate carbons. J. Phys. Chem. A 119, 13101–13106 (2015).

    CAS  Article  Google Scholar 

  65. 65

    Guo, J.-C. et al. Star-like superalkali cations featuring planar pentacoordinate carbon. J. Chem. Phys. 144, 244303 (2016).

    Article  Google Scholar 

  66. 66

    Cui, Z.-H., Sui, J.-J. & Ding, Y.-H. How can carbon favor planar multi-coordination in boron-based clusters? Global structures of CBxEy2− (E = Al, Ga. x + y = 4). Phys. Chem. Chem. Phys. 17, 32016–32022 (2015).

    CAS  Article  Google Scholar 

  67. 67

    Hou, J. H. et al. Exploring the geometrical structures of X©BnHnm [(X. m) = (B, +1), (C, +2) for n = 5; (X, m) = (Be, 0), (B, +1) for n = 6] by an electronic method. New J. Chem. 39, 8630–8637 (2015).

    CAS  Article  Google Scholar 

  68. 68

    Gribanova, T. N., Minyaev, R. M. & Minkin, V. I. Structure and stability of the C-doped boron fullerenes B60C12 and B80C12 with quasi-planar pentacoordinated cage carbon atoms: a quantum-chemical study. Mendeleev Commun. 26, 485–487 (2016).

    CAS  Article  Google Scholar 

  69. 69

    Wang, Y., Li, F., Li, Y. F. & Chen, Z. F. Semi-metallic Be5C2 monolayer global minimum with quasi-planar pentacoordinate carbons and negative Poisson's ratio. Nat. Commun. 7, 11488 (2016).

    CAS  Article  Google Scholar 

  70. 70

    Li, J.-J. et al. Zigzag double-chain C–Be nanoribbon featuring planar pentacoordinate carbons and ribbon aromaticity. J. Mater. Chem. C 5, 408–414 (2017).

    CAS  Article  Google Scholar 

  71. 71

    Cui, Z.-H. et al. Planar pentacoordinate carbon atoms embedded in a metallocene framework. Chem. Commun. 53, 138–141 (2017).

    CAS  Article  Google Scholar 

  72. 72

    Exner, K. & Schleyer, P. v. R. Planar hexacoordinate carbon: a viable possibility. Science 290, 1937–1940 (2000).

    CAS  Article  Google Scholar 

  73. 73

    Islas, R., Heine, T., Ito, K., Schleyer, P. v. R. & Merino, G. Boron rings enclosing planar hypercoordinate group 14 elements. J. Am. Chem. Soc. 129, 14767–14774 (2007).

    CAS  Article  Google Scholar 

  74. 74

    Wang, L.-M., Huang, W., Averkiev, B. B., Boldyrev, A. I. & Wang, L.-S. CB7: experimental and theoretical evidence against hypercoordinate planar carbon. Angew. Chem. Int. Ed. 46, 4550–4553 (2007).

    CAS  Article  Google Scholar 

  75. 75

    Wu, Y.-B. et al. D3h CN3Be3+ and CO3Li3+: viable planar hexacoordinate carbon prototypes. Phys. Chem. Chem. Phys. 14, 14760–14763 (2012).

    CAS  Article  Google Scholar 

  76. 76

    Zhang, C.-F., Han, S.-J., Wu, Y.-B., Lu, H.-G. & Lu, G. Thermodynamic stability versus kinetic stability: is the planar hexacoordiante carbon species D3h CN3Mg3+ viable? J. Phys. Chem. A 118, 3319–3325 (2014).

    CAS  Article  Google Scholar 

  77. 77

    Li, Y., Liao, Y. & Chen, Z. Be2C monolayer with quasi-planar hexacoordinate carbons: a global minimum structure. Angew. Chem. Int. Ed. 53, 7248–7252 (2014).

    CAS  Article  Google Scholar 

  78. 78

    Heine, T. & Merino, G. What is the maximum coordination number in a planar structure? Angew. Chem. Int. Ed. 51, 4275–4276 (2012).

    CAS  Article  Google Scholar 

  79. 79

    Zhao, T., Wang, Q. & Jena, P. Cluster-inspired design of high-capacity anode for Li-ion batteries. ACS Energy Lett. 1, 202–208 (2016).

    CAS  Article  Google Scholar 

  80. 80

    Dai, J., Wu, X., Yang, J. & Zeng, X. C. AlxC monolayer sheets: two-dimensional networks with planar tetracoordinate carbon and potential applications as donor material in solar cell. J. Phys. Chem. Lett. 5, 2058–2065 (2014).

    CAS  Article  Google Scholar 

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The authors acknowledge the generous support from Conacyt (Grand CB-2015-252356). V.V.-G. thanks Conacyt for the awarding of a fellowship. K.J.D. was supported in Richmond by the National Science Foundation (NSF-CAREER Award CHE-1056430 and the Henry Dreyfus Teacher-Scholar Awards Program. S.P. thanks Nanjing Tech University for his postdoctoral fellowship.

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Correspondence to Gabriel Merino.

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Vassilev-Galindo, V., Pan, S., Donald, K. et al. Planar pentacoordinate carbons. Nat Rev Chem 2, 0114 (2018).

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