Chem. Rev. 2005, 105, 3716−3757 3716 All-Metal Aromaticity and Antiaromaticity Alexander I. Boldyrev* Department of Chemistry and Biochemistry, Utah State University, Logan, Utah 84322-0300 Lai-Sheng Wang* Department of Physics, Washington State University, 2710 University Drive, Richland, Washington 99354, and W. R. Wiley Environmental Molecular Sciences Laboratory and Chemical Sciences Division, Pacific Northwest National Laboratory, MS K8-88, P.O. Box 999, Richland, Washington 99352 Received December 2, 2004 - Contents 7.5. Is Li3Al4 Antiaromatic or Aromatic? The 3744 Controversy over the Net Antiaromaticity of - 1. Introduction 3716 Li3Al4 2. What Is Aromaticity? 3718 7.6. More Recent Works on the Net 3744 - 3. Experimental Generation and Photoelectron 3719 Antiaromaticity of Li3Al4 and Related Spectroscopy (PES) Characterization of All-Metal Species Aromatic/Antiaromatic Clusters 8. All-Metal Aromatic Clusters as Building Blocks of 3747 Molecules and Solids 4. Photoelectron Spectroscopic and Theoretical 3720 - + 2- 2 Study of M [Al4 ](M) Li, Na, Cu) 8.1. Robinson’s R3Ga3 Aromatic Organometallic 3747 + 2- ) Compounds 4.1. Photoelectron Spectra of M [Al4 ](M Li, 3720 2- Na, Cu) 8.2. Power’s R2Ga4 Aromatic Organometallic 3747 Compound with π Aromaticity and σ 4.2. Searching for Global Minimum Structures of 3720 Antiaromaticity M+[Al 2-](M) Li, Na, Cu) 4 8.3. Tetraatomic Aromatic Species of Group V 3748 4.3. Calculated Vertical Detachment Energies and 3721 2- and VI Elements: M4 (M ) N, P, As, Sb, Comparison with Experimental Data 2+ Bi) and M4 (M ) O, S, Se, Te) 2- 5. Multiple Aromaticity in the Al4 Cluster 3722 8.4. Pentaatomic Aromatic Species of Group IV 3749 - 5.1. Geometric Structure 3722 and V Elements: M5 (M ) N, P, As, Sb, 6- 5.2. Molecular Orbital Analyses and Multiple 3723 Bi) and M5 (Ge, Sn, Pb) 6- Aromaticity 8.5. Aromatic Hg4 Cluster in the Na3Hg2 3752 Amalgam 5.3. Electron Density Analyses 3724 9. Challenges in the Future 3753 5.4. Resonance Energy Evaluations 3725 2- 10. Acknowledgments 3754 5.5. Valence Bond Treatment of Al4 3727 11. Abbreviations 3754 5.6. Ring Current and Magnetic Criteria 3728 12. References 3755 6. Other Multiply Aromatic Clusters 3729 2- 2- 6.1. Valence Isoelectronic Species of Al4 :X4 3729 (X ) B, Ga, In, Tl) - 1. Introduction 6.2. Mixed Valence Isoelectronic Species: MAl3 3730 (M ) C, Si, Ge, Sn, Pb), M′Al3 (M′ ) P, As, Since its introduction by August Kekule´ in 1865,1 ′′ ′′ ) Bi), and Si2M 2 (M B, Al, Ga) the concept of aromaticity has been continuously - 6.3. X3 and NaX3 (X ) B, Al, Ga) Clusters 3733 conquering new territories in chemistry (refs 2-20 6.4. Pure σ Aromatic/Antiaromatic Metal Clusters 3735 are only a small fraction of the published articles, 6.5. Aromatic Metal Clusters with Only π Bond 3738 reviews, books, and conference proceedings on this 6.6. Sandwich Complexes Based on All-Metal 3740 subject). Initially, aromaticity was developed for the Aromatic Clusters following classes of organic compounds: (i) monocy- 7. All-Metal Clusters with Conflicting Aromaticity: 3740 clic planar conjugate hydrocarbons and their ions - + Antiaromaticity in Li3Al4 with (4n 2) π electrons; (ii) polycyclic conjugate 7.1. General Considerations 3740 hydrocarbonssbenzenoid hydrocarbons built of fused 7.2. Search for the Global Minimum Structures 3741 benzene rings; (iii) polycyclic conjugated carbocyclic - hydrocarbons based on nonbenzenoid systems such 7.3. Photoelectron Spectroscopy of Li3Al4 and 3741 Comparison between Experiment and Theory as azulene and other conjugate hydrocarbons with 2,4-17 7.4. Molecular Orbital Analyses and 3742 four-, five-, seven-, and eight-membered rings. 4- Today, aromaticity has been extended to a highly Antiaromaticity in Al4 diverse number of heterocyclic compounds,3,8,13,15,17,21-23 as well as three-dimensional (3D) compounds, such - * E-mail (A.I.B.) [email protected]; (L.-S.W.) [email protected]. as ferrocenes and related sandwich systems,24 29 10.1021/cr030091t CCC: $53.50 © 2005 American Chemical Society Published on Web 09/28/2005 All-Metal Aromaticity and Antiaromaticity Chemical Reviews, 2005, Vol. 105, No. 10 3717 from the initial proposal of Thorn and Hoffmann, approximately 25 metallobenzenes have been isolated and characterized.36 The first example of a stable, isolabile metallobenzenesosmabenzeneswas reported by Elliot et al. in 1982.37 A large family of metallo- benzenessthe iridabenzenesswas synthesized by Bleeke and co-workers,38-40 whereas a series of dimetallobenzenes with two metal atoms incorpo- rated into the benzene ring was synthesized and characterized by Rothwell et al.41,42 Recent advances in metallobenzenes have been reviewed by Bleeke.36 The term “metalloaromaticity” was introduced by Bursten and Fenske43 in 1979 to describe metal complexes of cyclobutadienesaromatic compounds containing a metal atom coordinated to C H , which Alexander I. Boldyrev was born in Siberia, Russia (1951), and received 4 4 his B.S./M.S. (1974) in chemistry from Novosibirsk University, his Ph.D. is the simplest and archetypal Hu¨ ckel antiaromatic in physical chemistry from Moscow State University, and his Dr.Sci. in (4n π electrons) molecule. Interestingly, a coordinated chemical physics from Moscow Physico-Chemical Institute (1984). He is C4H4 often acts as if it were aromatic rather than currently a professor in the Department of Chemistry and Biochemistry antiaromatic. at Utah State University. His current scientific interest is the development The first organometallic compound containing an of chemical bonding models capable of predicting the structure, stability, chemical bonding, and other molecular properties of pure and mixed main aromatic cycle completely composed of metal atoms 44 group metal, nonmetal, and metalloid clusters, with the most recent accent was synthesized by Robinson et al. in 1995. That on transition metal clusters. aromatic compound, Na2(Mes2C6H3)Ga]3 (Mes ) 2,4,6- 2- Me3C6H2), contains a triangular aromatic Ga3 ring embedded in a large organometallic molecule. In the follow-up publications, Robinson, in collaboration with Schaefer and others, proved computationally 2- that the Ga3 unit indeed has two completely delo- calized π electrons similar to that in the hydrocarbon + 45-48 analogue C3H3 . The first π aromatic organome- tallic compound composed of four gallium atoms, in which an almost perfect square-planar gallium clus- ter Ga4 was embedded in an organometallic environ- ment in the K2[Ga4(C6H3-2,6-Trip2)2] (Trip ) C6H2- i 2,4,6- Pr3) compound, was prepared by Twamley and Power.49 Aromaticity in the Power compound was established computationally.50-52 It was shown that the planar -Ga4- unit does possess two completely delocalized π electrons and is hence π aromatic, Lai-Sheng Wang was born in Henan, China, in 1961 and received his 2+ B.S. in chemistry from Wuhan University, Wuhan, China (1982). After a similar to that in the hydrocarbon analogue C4H4 . brief stay at the Institute of Chemistry, Chinese Academy of Sciences in A few solid compounds containing aromatic met- Beijing in 1982, he came to the University of California, Berkeley, and alloid and metal clusters have been characterized as obtained his Ph.D. in physical chemistry in 1990. He was a postdoctoral well. Among them, experimental characterization has research associate at Rice University from 1990 to 1992. In 1993, he been reported on aromatic square clusters, such as took a joint position between Washington State University and the Pacific 2+ 2+ 53-56 2- 2- 57-59 Northwest National Laboratory, where he conducts his research in the Se4 ,Te4 , Sb4 , and Bi4 , which are W. R. Wiley Environmental Molecular Sciences Laboratory. He is currently valence isoelectronic to the prototypical aromatic 2- professor of physics and materials sciences at Washington State University hydrocarbon C4H4 , as well as planar pentagonal - 60,61 6- 6- 62 and holds an affiliate senior chief scientist position at the Chemical aromatic clusters, such as As5 , ,Sn5 , and Pb5, , Sciences Division, Pacific Northwest National Laboratory. His research which are valence isoelectronic to the prototypical interests include the study of size-dependent properties of atomic clusters aromatic hydrocarbons C H -. using photoelectron spectroscopy, discovery of new chemical bonds in 5 5 clusters, and cluster-assembled materials. His group has also developed King and Rouvray30,63a and Aihara63b,c advanced experimental techniques to investigate multiply charged anions in the gas aromaticity in deltahedral boranes and in related phase and is currently involved with the study of the microsolvation of compounds including deltahedral metal carbonyls, complex anions and solution phase species in the gas phase. He has such as the octahedral clusters Rh6(CO)16,Ru6C- received a Sloan Research Fellowship and is a Fellow of the American 2- 30 (CO)17, and Os6(CO)18 . King extended this treat- Physical Society. He is currently holding a John Simon Guggenheim - Fellowship. ment to Rh Ni carbonyl clusters with structures related to deltahedral 3D aromatic systems such as pyramidal hydrocarbons,8 boron hydrides, carbo- and boranes.64 The deltahedral structure of the Rh-Ni heteroboranes,30,31 and fullerenes.32-34 carbonyl clusters can be explained on the basis of Compounds containing metal atoms have been Wade’s rules.65 King also considered a variety of found to be aromatic, too. In 1979, Thorn and Zintl-like ions composed of metals, which are isolobal Hoffmann35 predicted that some hypothetical metal- and