View source for Metal aromaticity

{{Short description|Concept of aromaticity extended to metals}}
{{Use dmy dates|date=December 2014}}
'''Metal aromaticity''' or '''metalloaromaticity''' is the concept of [[aromaticity]], found in many [[organic compound]]s, extended to [[metals]] and metal-containing compounds.<ref>{{Cite journal |last1=Feixas |first1=Ferran |last2=Matito |first2=Eduard |last3=Poater |first3=Jordi |last4=Solà |first4=Miquel |date=2012-09-13 |title=Metalloaromaticity |url=https://onlinelibrary.wiley.com/doi/10.1002/wcms.1115 |journal=Wiley Interdisciplinary Reviews: Computational Molecular Science |language=en |volume=3 |issue=2 |pages=105–122 |doi=10.1002/wcms.1115|s2cid=222199114 |url-access=subscription }}</ref> The first experimental evidence for the existence of aromaticity in metals was found in aluminium [[cluster compound]]s of the type {{chem|MAl|4|−}} where M stands for [[lithium]], [[sodium]] or copper.<ref>''Observation of All-Metal Aromatic Molecules '' Xi Li, Aleksey E. Kuznetsov, Hai-Feng Zhang, Alexander I. Boldyrev, Lai-Sheng Wang [[Science (journal)|Science]] Vol. 291. p. 859 '''2001''' {{doi|10.1126/science.291.5505.859}}</ref> These [[anion]]s can be generated in a [[helium]] gas by [[laser vaporization]] of an aluminium / [[lithium carbonate]] composite or a copper or sodium / aluminium [[alloy]], separated and selected by [[mass spectrometry]] and analyzed by [[photoelectron spectroscopy]]. The evidence for aromaticity in these compounds is based on several considerations. [[Computational chemistry]] shows that these aluminium clusters consist of a tetranuclear {{chem|Al|4|2−}} plane and a counterion at the apex of a [[square pyramidal molecular geometry|square pyramid]].  The {{chem|Al|4|2−}} unit is perfectly planar and is not perturbed by the presence of the [[counterion]] or even the presence of two counterions in the neutral compound {{chem|M|2|Al|4}}. In addition its [[HOMO]] is calculated to be a doubly occupied delocalized pi system making it obey [[Hückel's rule]]. Finally a match exists between the calculated values and the experimental photoelectron values for the energy required to remove the first 4 valence electrons. The first fully metal aromatic compound was a cyclogallane with a Ga<sub>3</sub><sup>2-</sup> core discovered by Gregory Robinson in 1995.<ref name="chemworld">{{cite web |last1=Krämer |first1=Katrina |title=The search for the grand unification of aromaticity |url=https://www.chemistryworld.com/features/the-search-for-the-grand-unification-of-aromaticity/4013915.article |website=Chemistry World}}</ref>

'''D-orbital aromaticity''' is found in trinuclear [[tungsten]] {{chem|W|3|O|9|−}} and [[molybdenum]] {{chem|Mo|3|O|9|−}} [[metal cluster]]s generated by laser vaporization of the pure metals in the presence of [[oxygen]] in a helium stream.<ref>''Observation of d-Orbital Aromaticity '' Xin Huang, Hua-Jin Zhai, Boggavarapu Kiran, Lai-Sheng Wang, [[Angewandte Chemie International Edition]] Volume 44, Issue 44, Pages 7251–54 '''2005''' {{doi|10.1002/anie.200502678}}</ref> In these clusters the three metal centers are bridged by oxygen and each metal has two terminal oxygen atoms. The first signal in the photoelectron spectrum corresponds to the removal of the valence electron with the lowest energy in the anion to the neutral {{chem|M|3|O|9}} compound. This energy turns out to be comparable to that of bulk [[tungsten trioxide]] and [[molybdenum trioxide]]. The photoelectric signal is also broad which suggests a large difference in conformation between the anion and the neutral species. [[Computational chemistry]] shows that the {{chem|M|3|O|9|-}} anions and {{chem|M|3|O|9|2-}} dianions are ideal hexagons with identical metal-to-metal [[bond length]]s. Tritantalum oxide clusters (Ta<sub>3</sub>O<sub>3</sub><sup>−</sup>) also are observed to exhibit possible D-orbital aromaticity.<ref name="chemworld"/>

The molecules discussed thus far only exist diluted in the gas phase. A study exploring the properties of a compound formed in water from [[sodium molybdate]] ({{chem|Na|2|MoO|4|·2H|2|O}}) and [[iminodiacetic acid]] also revealed evidence of aromaticity, but this compound has actually been isolated. [[X-ray crystallography]] showed that the sodium atoms are arranged in layers of hexagonal clusters akin to [[pentacene]]s. The sodium-to-sodium [[bond length]]s are unusually short (327 [[picometer|pm]] versus 380 pm in elemental sodium) and, like benzene, the ring is planar. In this compound each sodium atom has a distorted [[octahedral molecular geometry]] with coordination to molybdenum atoms and water molecules.<ref>''Synthesis and structure of 1-D Na6 cluster chain with short Na–Na distance: Organic like aromaticity in inorganic metal cluster'' Snehadrinarayan Khatua, Debesh R. Roy, Pratim K. Chattaraj and Manish Bhattacharjee [[Chem. Commun.]], '''2007''', 135–37, {{doi|10.1039/b611693k}}</ref> The experimental evidence is supported by computed [[NICS aromaticity]] values.

==See also==
* {{annotated link|Metal cluster compound}}
* {{annotated link|Catenation}}

==References==
{{Reflist}}

{{chemical bonds}}

[[Category:Cluster chemistry]]
[[Category:Chemical bonding]]