Phi bond explained

Phi bond should not be confused with Pi bond.

In chemistry, phi bonds (φ bonds) are covalent chemical bonds, where six lobes of one involved atomic orbital overlap six lobes of the other involved atomic orbital. This overlap leads to the formation of a bonding molecular orbital with three nodal planes which contain the internuclear axis and go through both atoms.

The Greek letter φ in their name refers to f orbitals, since the orbital symmetry of the φ bond is the same as that of the usual (6-lobed) type of f orbital when seen down the bond axis.

There was one possible candidate known in 2005 of a molecule with phi bonding (a U−U bond, in the molecule U₂).^[1] However, later studies that accounted for spin orbit interactions found that the bonding was only of fourth order.^{[2] [3] [4]} Experimental evidence for phi bonding between a thorium atom and cyclooctatetraene in thorocene has been supported by computational analysis, though this mixed-orbital bond has strong ionic character and is not a traditional covalent phi bond.^[5]

Notes and References

1. Gagliardi. Laura. Roos. Björn O.. Quantum chemical calculations show that the uranium molecule U₂ has a quintuple bond. Nature. 2005. 433. 7028. 848–851. 10.1038/nature03249. 2005Natur.433..848G. 15729337. 421380.
2. 10.1039/c7ra07400j . RSC Adv. . Introducing DDEC6 atomic population analysis: part 3. Comprehensive method to compute bond orders . T. A. Manz . 2017 . 7 . 72 . 45552–45581. 2017RSCAd...745552M . free.
3. Web site: The diuranium molecule has a quadruple bond. chab.ethz.ch. en. 2020-03-21.
4. Knecht. Stefan. Jensen. Hans Jørgen Aa. Saue. Trond. January 2019. Relativistic quantum chemical calculations show that the uranium molecule U 2 has a quadruple bond. Nature Chemistry. en. 11. 1. 40–44. 10.1038/s41557-018-0158-9. 30374039. 53112083. 1755-4349.
5. Minasian . Stefan G. . Keith . Jason M. . Batista . Enrique R. . Boland . Kevin S. . Clark . David L. . Kozimor . Stosh A. . Martin . Richard L. . Shuh . David K. . Tyliszczak . Tolek . 2014 . New evidence for 5f covalency in actinocenes determined from carbon K-edge XAS and electronic structure theory . Chem. Sci. . en . 5 . 1 . 351–359 . 10.1039/C3SC52030G . 2041-6520.