# Superatom

A superatom is any cluster of atoms that seem to exhibit some of the properties of elemental atoms.

Sodium atoms, when cooled from vapor, naturally condense into clusters, preferentially containing a magic number of atoms (2, 8, 20, 40, 58, etc.). The first two of these can be recognized as the numbers of electrons needed to fill the first and second shells, respectively. The superatom suggestion is that free electrons in the cluster occupy a new set of orbitals that are defined by the entire group of atoms, i.e. cluster, rather than each individual atom separately (non-spherical or doped clusters show deviations in the number of electrons that form a closed shell as the potential is defined by the shape of the positive nuclei.) Superatoms tend to behave chemically in a way that will allow them to have a closed shell of electrons, in this new counting scheme. Therefore, a superatom with one more electron than a full shell should give up that electron very easily, similar to an alkali metal, and a cluster with one electron short of full shell should have a large electron affinity, such as a halogen.

## Aluminium clusters

Certain aluminium clusters have superatom properties. These aluminium clusters are generated as anions (Aln with n = 1, 2, 3, … ) in helium gas and reacted with a gas containing iodine. When analyzed by mass spectrometry one main reaction product turns out to be Al13I.[1] These clusters of 13 aluminium atoms with an extra electron added do not appear to react with oxygen when it is introduced in the same gas stream. Assuming each atom liberates its 3 valence electrons, this means that there are 40 electrons present, which is one of the magic numbers noted above for sodium, and implies that these numbers are a reflection of the noble gases. Calculations show that the additional electron is located in the aluminium cluster at the location directly opposite from the iodine atom. The cluster must therefore have a higher electron affinity for the electron than iodine and therefore the aluminium cluster is called a superhalogen. The cluster component in Al13I ion is similar to an iodide ion or better still a bromide ion. The related Al13I2 cluster is expected to behave chemically like the triiodide ion.

Similarly it has been noted that Al14 clusters with 42 electrons (2 more than the magic numbers) appear to exhibit the properties of an alkaline earth metal which typically adopt +2 valence states. This is only known to occur when there are at least 3 iodine atoms attached to an Al14 cluster, Al14I3. The anionic cluster has a total of 43 itinerant electrons, but the three iodine atoms each remove one of the itinerant electrons to leave 40 electrons in the jellium shell.[2][3]

It is particularly easy and reliable to study atomic clusters of inert gas atoms by computer simulation because interaction between two atoms can be approximated very well by the Lennard-Jones potential. Other methods are readily available and it has been established that the magic numbers are 13, 19, 23, 26, 29, 32, 34, 43, 46, 49, 55, etc.[4]

• Al7 = the property is similar to germanium atoms.
• Al13 = the property is similar to halogen atoms, more specifically, chlorine.
• Al13Ix, where x = 1–13.[5]
• Al14 = the property is similar to alkaline earth metals.
• Al14Ix, where x = 1–14.[5]
• Al23
• Al37

## Other clusters

• Li(HF)3Li = the (HF)3 interior causes 2 valence electrons from the Li to orbit the entire molecule as if it were an atom's nucleus.[6]
• VSi16F = has ionic bonding.[7]
• A cluster of 13 platinum becomes paramagnetic.[8]
• A cluster of 2000 rubidium atoms.[9]

## Superatom complexes

Superatom complexes are a special group of superatoms that incorporate a metal core which is stabilized by organic ligands. In thiolate-protected gold cluster complexes a simple electron counting rule can be used to determine the total number of electrons (ne) which correspond to a magic number via,

${\displaystyle n_{e}=N\nu _{A}-M-z}$

where N is the number of metal atoms (A) in the core, v is the atomic valence, M is the number of electron withdrawing ligands, and z is the overall charge on the complex.[10] For example the Au102(p-MBA)44 has 58 electrons and corresponds to a closed shell magic number.[11]

### Gold superatom complexes

• Au25(SMe)18 [12]
• Au102(p-MBA)44
• Au144(SR)60 [13]

### Other superatom complexes

• Ga23(N(Si(CH3)3)2)11[14]
• Al50(C5(CH3)5)12[15]

## References

1. ^ Formation of Al13I: Evidence for the Superhalogen Character of Al13 D. E. Bergeron, A.W. Castleman Jr., T. Morisato, S. N. Khanna Science, Vol 304, Issue 5667, 84–87 , 2 April 2004 Abstract MS spectra
2. ^ Philip Ball, "A New Kind of Alchemy", New Scientist Issue dated 2005-04-16.
3. ^ Al Cluster Superatoms as Halogens in Polyhalides and as Alkaline Earths in Iodide Salts D. E. Bergeron, P. J. Roach, A.W. Castleman Jr., N.O. Jones, S. N. Khanna Science, Vol 307, Issue 5707, 231–235 , 14 January 2005 Abstract MS spectrum
4. ^ I. A. Harris et al. Phys. Rev. Lett. Vol. 53, 2390–94 (1984).
5. ^ a b Naiche Owen Jones, 2006.
6. ^ Extraordinary superatom containing double shell nucleus: Li(HF)3Li connected mainly by intermolecular interactions, Sun, Xiao-Ying, Li, Zhi-Ru, Wu, Di, & Sun, Chia-Chung, 2007.
7. ^ Electronic and geometric stabilities of clusters with transition metal encapsulated by silicon, Kiichirou Koyasu et al.
8. ^ Platinum nanoclusters go magnetic, nanotechweb.org, 2007
9. ^ Ultra Cold Trap Yields Superatom, NIST, 1995
10. ^ M. Walter, J. Akola, O. Lopez-Acevedo, P. D. Jadzinsky, G. Calero, C. J. Ackerson, R. L. Whetten, H. Grönbeck, H. Häkkinen, Gold Superatom Complexes"A unified view of ligand-protected gold clusters as superatom complexes ", PNAS 105, 9157 (2008)
11. ^ P.D. Jadzinsky, G. Calero, C.J. Ackerson, D.A. Bushnell, R.D. Kornberg, Gold Superatom Complexes Structure of a thiol monolayer-protected gold nanoparticle at 1.1 Å resolution" Science 318, 430–433 (2007)
12. ^ J. Akola, M. Walter, R.L. Whetten, H. Häkkinen and H. Grönbeck, "On the structure of thiolate-protected Au25", JACS 130, 3756–3757 (2008)
13. ^ O. Lopez-Acevedo, J. Akola, R.L. Whetten, H. Grönbeck, H. Häkkinen, "Structure and Bonding in the Ubiquitous Icosahedral Metallic Gold Cluster Au144(SR)60", JPCC 130, 3756–3757 (2009)
14. ^ J. Hartig, A. Stösser, H. Schnöckel, "A metalloid (Ga23{N(SiMe3)2}11) cluster: The jellium model put to test" Angew. Chemie. Int. Ed. 46, 1658–1662 (2007).
15. ^ P.A. Clayborne, O. Lopez-Acevedo, R.L. Whetten, H. Grönbeck and H. Häkkinen, “Al50Cp*12 Cluster: A 138-electron (L=6) Superatom”, Eur. J. Inorg. Chem. 2011.