Electronic Structure and Spectroscopic Properties of Group-7 Tri-Oxo-Halides MO3X (M = Mn-Bh, X = F-Ts).

The 24 trioxide halide molecules MO3X of the manganese group (M = Mn-Bh; X = F-Ts), which are iso-valence-electronic with the famous MnO4- ion, have been quantum-chemically investigated by quasi-relativistic density-functional and ab initio correlated approaches. Geometric and electronic structures, valence and oxidation numbers, vibrational and electronic spectral properties, energetic stabilities of the monomers in the gas phase, and the decay mode of MnO3F have been investigated. The light Mn-3d species are most strongly electron-correlated, indicating that the concept of a closed-shell Lewis-type single-configurational structure [Mn+7(d0) O-2(p6)3 F-(p6)] reaches its limits. The concept of real-valued spin orbitals φ(r)·α and φ(r)·ß breaks down for the heavy Bh-6d, At-6p and Ts-7p elements because of the dominating spin-orbit coupling. The vigorous decomposition of MnO3F at ambient conditions starts by the autocatalyzed release of n O2 and the formation of MnmO3m-2nFm clusters, triggered by the electron-depleted "oxylic" character of the oxide ligands in MnO3X.

Permanganyl Fluoride: A Brief History of the Molecule MnO3 F and of Those Who Cared For It.

Permanganyl fluoride's existence at the stability threshold in the series of oxides and oxide fluorides of the late 3d transition metals is reflected by its experimentally challenging properties and by the difficulties posed in the theoretical description of its bonding characteristics. The history of this molecule is reviewed from early qualitative observations and the growing scattered information on its chemical and physical properties to the accurate determination and interpretation of its molecular structure and spectral features. The still problematic theoretical models for MnO4 - and MnO3 F are briefly presented in the broader context of the chemistry of elements in high oxidation states. Short biographies of the scientists engaged in these studies are offered. Related technetium and rhenium compounds are briefly considered for comparison.

Innovative Molecular Design Strategies in Materials Science Following the Aurophilicity Concept.

The past decade has seen a diverse range of breakthrough inventions that are derived from gold complexes, including the application of aurophilic interactions in the preparation of stimuli-responsive materials. Examples of these gold-based materials include aurophilicity-induced metallogelators, mechanochromic, thermochromic, vapochromic, and solvatochromic luminescent compounds, as well as sensory materials for the detection of metal ions. Sophisticated properties of gold complexes with Au···Au contacts have been explored at the edge of several disciplines including chemistry, crystallography, molecular engineering and advanced materials. As science paves its way to innovation, cross-disciplinary research moves from being a luxury to becoming a necessity. Development of the concept of aurophilicity and its use in designing novel materials is a true example of innovation on a multidisciplinary platform. As miniaturization continues to influence the next generation of technological advancement, using the properties of molecules as chemical tools to enable such developments becomes extremely important. In this Review, recent examples of gold complexes which exhibit a response to external stimuli have been collected and some of their potential applications discussed for selected cases.

Excimer and Exciplex Formation in Gold(I) Complexes Preconditioned by Aurophilic Interactions.

Excimers and exciplexes are defined as assemblies of atoms or molecules A/A' where interatomic/intermolecular bonding appears only in excited states such as [A2 ]* (for excimers) and [AA']* (for exciplexes). Their formation has become widely known because of their role in gas-phase laser technologies, but their significance in general chemistry terms has been given little attention. Recent investigations in gold chemistry have opened up a new field of excimer and exciplex chemistry that relies largely on the preorganization of gold(I) compounds (electronic configuration AuI (5d10 )) through aurophilic contacts. In the corresponding excimers, a new type of Au⋅⋅⋅Au bonding arises, with bond energies and lengths approaching those of ground-state Au-Au bonds between metal atoms in the Au0 (5d10 6s1 ) and AuII (5d9 ) configurations. Excimer formation gives rise to a broad range of photophysical effects, for which some of the relaxation dynamics have recently been clarified. Excimers have also been shown to play an important role in photoredox binuclear gold catalysis.

Proof of Concept for Hydrogen Bonding to Gold, Au⋠⋠⋠H-X.

Convincing and consistent evidence for the existence of hydrogen bonding to gold has been obtained. An ammonium or pyridinium group has been shown to be an efficient hydrogen bond donor unit for gold(I) coordination centers, and the assembly leads to the structural pattern typical for standard hydrogen bonds. This constitutes the first rigorous, scrutinizing, and comprehensive study of hydrogen bonds to a metal atom, with gold being an ideal model element because of relativistic effects.

Influence of Relativistic Effects on Bonding Modes in M(II) Dinuclear Complexes (M = Au, Ag, and Cu).

The stability and bonding in dinuclear group 11 metal complexes (M = Au, Ag, and Cu) in their +2 oxidation state has been investigated by quantum chemical methods. Two model complexes were selected as representatives of different bonding situations in the dinuclear M(II) complexes, a direct metal-metal bond between two ligand stabilized monomers and ligand-mediated bridged dimer system, making them interesting for a direct comparison and to study the influence of relativistic effects. Relativity substantially stabilizes the direct metal-metal bonded system obtaining the sequence in M-M bond stability Au > Ag > Cu. In the ligand-bridged structure, an asymmetric bonding situation is obtained for gold, resulting in two stronger/covalent and two weaker/ionic bonds per gold atom. Here we observe the opposite trend in stability Cu > Ag > Au. Our analysis nicely corroborates with what is known from experimental observation.

Argentophilic interactions.

The decade 1990-2000 saw a growing interest in aurophilic interactions in gold chemistry. These interactions were found to influence significantly a variety of structural and other physical characteristics of gold(I) compounds. The attention paid to this unusual and counterintuitive type of intra- and intermolecular bonding between seemingly closed-shell metal centers has rapidly been extended to also include silver chemistry. Hundreds of experimental and computational studies have since been dedicated to the argentophilicity phenomenon. The results of this development are reviewed herein focusing on molecular systems where two or more silver(I) centers are in close contact leading to specific structural characteristics and a variety of novel physical properties. These include strongly modified ligand-to-metal charge-transfer processes observed in absorption and emission spectroscopy, but also colossal positive and negative thermal expansion on the one hand and unprecedented negative linear compressibility of crystal parameters on the other.

The gold-hydrogen bond, Au-H, and the hydrogen bond to gold, Au∙∙∙H-X.

In the first part of this review, the characteristics of Au-H bonds in gold hydrides are reviewed including the data of recently prepared stable organometallic complexes with gold(I) and gold(III) centers. In the second part, the reports are summarized where authors have tried to provide evidence for hydrogen bonds to gold of the type Au∙∙∙H-X. Such interactions have been proposed for gold atoms in the Au(-I), Au(0), Au(I), and Au(III) oxidation states as hydrogen bonding acceptors and H-X units with X = O, N, C as donors, based on both experimental and quantum chemistry studies. To complement these findings, the literature was screened for examples with similar molecular geometries, for which such bonding has not yet been considered. In the discussion of the results, the recently issued IUPAC definitions of hydrogen bonding and the currently accepted description of agostic interactions have been used as guidelines to rank the Au∙∙∙H-X interactions in this broad range of weak chemical bonding. From the available data it appears that all the intra- and intermolecular Au∙∙∙H-X contacts are associated with very low binding energies and non-specific directionality. To date, the energetics have not been estimated, because there are no thermochemical and very limited IR/Raman and temperature-dependent NMR data that can be used as reliable references. Where conspicuous structural or spectroscopic effects have been observed, explanations other than hydrogen bonding Au∙∙∙H-X can also be advanced in most cases. Although numerous examples of short Au∙∙∙H-X contacts exist in the literature, it seems, at this stage, that these probably make only very minor contributions to the energy of a given system and have only a marginal influence on molecular conformations which so far have most often attracted researchers to this topic. Further, more dedicated investigations will be necessary before well founded conclusions can be drawn.

Tracing hydrogen bonding Au···H-C at gold atoms: a case study.

It has been shown in preceding experimental work that cyclometalated 6-benzylpyridines with gold(III) centers in the metallacycle (1) adopt a boat conformation reminiscent of the structure of 9,10-dihydroanthracene. There is a conspicuously short Au···H-C contact with a pseudoaxial methyl group suggesting a significant interaction which may be the prototype for Au(III)···H-C hydrogen bonding. Quantum chemical calculations on the B3LYP/def2-TZVP level have now shown that the ground state structures and conformations adopted by the homologues with two hydrogen atoms, two methyl groups, and a hydrogen atom and a methyl group at the carbon atom bridging the two (hetero)arene rings can be explained on the basis of simple conformation rules. There is no evidence that the Au(III)···H-C contact leads to an attractive interaction. The results are discussed in the context of literature data for Pt(II) analogues. Examples for potential Au(III)···H-X interactions presented in other references appear to be of a similar character.

Implications of the crystal structure of the ammonia solvate [Au(NH3)2]Cl·4NH3.

Crystals of diammine gold(I) chloride ammonia (1/4), [Au(NH(3))(2)]Cl·4NH(3), have been grown from solutions of AuCl in liquid ammonia. The X-ray diffraction analysis (at 123 K) has shown that the crystals feature an extensive network of hydrogen bonds between the [H(3)N-Au-NH(3)](+) cations (with C(i) symmetry) and the Cl(-) anions, including also the ammonia molecules. There is no evidence for an emerging increase of the coordination number of the gold atom by adopting another ammonia molecule or by approaching a chloride anion. Moreover, the geometry of two distant and angular N-H···Au contacts is not a strong support of hydrogen bonds recently amply discussed in the literature.

Coordination chemistry at carbon: the patchwork family comprising (Ph3P)2C, (Ph3P)C(C2H4), and (C2H4)2C.

The revitalized concept of "coordination at carbon" allows relationships between seemingly unrelated families of carbon-centered compounds to be discovered generating fascinating patchwork families of compounds. It is shown how olefins and cyclopropanes can be regarded as donors for carbon acceptors C(1), C(2), and C(3). Through this approach, hydrocarbons such as spiropentane and dicyclopropylidene are found to be counterparts of the bis-ylidic carbodiphosphoranes and the corresponding mixed mono-ylidic systems.

Pyrophosphate complexation of tin(II) in aqueous solutions as applied in electrolytes for the deposition of tin and tin alloys such as white bronze.

Electrodeposition of tin and tin alloys from electrolytes containing tin(II) and pyrophosphates is an important process in metal finishing, but the nature of the tin pyrophosphate complexes present in these solutions in various pH regions has remained unknown. Through solubility and pH studies, IR and (31)P and (119)Sn NMR spectroscopic investigations of solutions obtained by dissolving Sn(2)P(2)O(7) in equimolar quantities of either Na(4)P(2)O(7)·10H(2)O or K(4)P(2)O(7) the formation of anionic 1:1 complexes {[Sn(P(2)O(7))]}(n)(2n-) has now been verified and the molecular structures of the monomer (n = 1) and the dimer (n = 2) have been calculated by density functional theory (DFT) methods. Whereas the alkali pyrophosphates Na/K(4)P(2)O(7) give strongly alkaline aqueous solutions (pH ∼13), because of partial protonation of the [P(2)O(7)](4-) anion, the [Sn(P(2)O(7))](2-) anion is not protonated and the solutions of Na/K(2)[Sn(P(2)O(7))] are almost neutral (pH ∼8). The monomeric dianion appears to have a ground state with C(2v) symmetry with the Sn atom in a square pyramidal coordination and the lone pair of electrons in the apical position, while the dimer approaches C(2) symmetry with the Sn atoms in a rhombic pyramidal coordination, also with a sterically active lone pair. A comparison of experimental and calculated IR details favors the monomer as the most abundant species in solution. With an excess of pyrophosphate, 3:2 and 2:1 complexes (P(2)O(7)):(Sn) are first formed, which, in the presence of more pyrophosphate, undergo rapid ligand exchange on the NMR time scale. The structure of the 2:1 complex [Sn(P(2)O(7))(2)](6-) was calculated to have a pyramidal complexation by two 1,5-chelating pyrophosphate ligands. Neutralization of these alkaline solutions by sulfuric or sulfonic acids (H(2)SO(4), MeSO(3)H), as also practiced in electroplating, appears to afford the tin(II) hydrogen pyrophosphates [Sn(P(2)O(7)H)](-) and [Sn(H(2)P(2)O(7))](0). The molecular structures of the mononuclear model units have also been calculated and were shown to have an unsymmetrical complexation and to feature trigonal pyramidal (pseudotetrahedral) coordination. NMR observations have shown that, contrary to the results obtained for Sn(II) compounds, Sn(IV) as present in K(2)SnO(3) or its hydrated form (K(2)Sn(OH)(6)) does not form a pyrophosphate complex in aqueous solution near pH 7. There is also no interference of sulfite.

Observations and descriptions versus explanations--an example: does nature, does theory know about steric hindrance?

Quantum chemical investigations in the literature sometimes underpin traditional chemical concepts (such as those of different primary and secondary types of chemical bonding or of steric repulsions) but sometimes also seem to undermine them. Conceptual pitfalls in recent discussions in this and other leading journals, when combining empirically and quantum chemically based reasoning, can be avoided when taking four points into account: 1) The construction of theoretical concepts should be guided by both, physical rigor and the aim of recovering the notions that have been found chemically fruitful. 2) Partitioning of a chemical phenomenon into elementary physical steps should not only consider the stationary initial and final states but also include non-stationary intermediate states that exhibit the physically characteristic internal relaxation mechanism. 3) Even if sufficiently accurate, the partitioning into large contributions of opposite sign is less valuable for a qualitative chemical understanding, as long as the trend of one big decisive term or sum of terms cannot be made obvious (for instance on the basis of the variation principle--or by chemical intuition). 4) Causal explanatory notions, even if more vague or fuzzy than some more rigorous and unique descriptive concepts, have to be advanced towards a more fruitful chemical theory.

Aurophilic interactions as a subject of current research: an up-date.

Recently accumulated experimental evidence for aurophilic interactions in and between molecular gold(I) compounds and the results of pertinent theoretical calculations are reviewed for the period from 2007 to mid-2011. The influence of the intra- and intermolecular bonding contacts between the closed-shell metal centres, Au-Au, on the molecular and crystal structures, and the consequences of these effects for the chemical and physical properties of gold compounds are summarized for the various classes of mono- and polynuclear systems. The literature survey builds on the contents of previous reviews and relates new experimental and theoretical findings to earlier observations (353 references).

Gold--an introductory perspective.

We introduce the collection of reviews in this thematic issue of Chemical Society Reviews that demonstrate and discuss the current cutting edge research in the field of gold chemistry and materials science as it stands today. We also highlight achievements in the fields of gold catalysis, gold nanoparticles and the preparative, structural and theoretical chemistry of gold, and discuss the remaining challenges and opportunities. Our aim is to inspire further discovery in these new and deeply fascinating fields.

A briefing on aurophilicity.

There is now compelling experimental evidence for the existence of specific intra- and intermolecular bonding between seemingly closed-shell gold(I) centers (5d10) which manifests itself in all areas of gold chemistry. This "aurophilic interaction", which had not been predicted by conventional valence theory, was found to be associated with binding energies in some cases exceeding even those of strong hydrogen bonds and therefore to be highly significant in co-determining molecular structure and dynamics. In high-level theoretical treatments the attraction is rationalized as a "super van der Waals bonding" based on particularly strong relativistic, dispersion and correlation effects (critical review, 265 references).

The experimental gas-phase structures of 1,3,5-trisilylbenzene and hexasilylbenzene and the theoretical structures of all benzenes with three or more silyl substituents.

The structures of 1,3,5-trisilylbenzene and hexasilylbenzene in the gas phase have been determined by electron diffraction, and that of 1,3,5-trisilylbenzene by X-ray crystallography. The structures of three trisilylbenzene isomers, three tetrasilylbenzenes, pentasilylbenzene and hexasilylbenzene have been computed, ab initio and using Density Functional Theory, at levels up to MP2/6-31G*. The primary effect of silyl substituents is to narrow the ring angle at the substituted carbon atoms. Steric interactions between silyl groups on neighbouring carbon atoms lead first to displacement of these groups away from one another, and then to displacement out of the ring plane, with alternate groups moving to opposite sides of the ring. In the extreme example, hexasilylbenzene, the SiCCSi dihedral angle is 17.8(8) degrees .

Bromination of (phosphine)gold(I) bromide complexes: stoichiometry and structure of products.

The course of the oxidative addition of elemental bromine to complexes of the type (L)AuBr is strongly influenced by the nature of the tertiary phosphine ligand L. Standard square planar gold(III) complexes (L)AuBr3 are obtained not only with L = PMe3 but also with P((I)Pro)3 for which the oxidative addition fails in the corresponding iodine system. Excess bromine is integrated into crystals of the products with the stoichiometry [(Me3P)AuBr3].(Br2) and {[(iPro)3P]AuBr3}.(Br2). Of the series of iodine analogues, an intercalate [(Me3P)AuI3]2.(I2) has been structurally characterized. [((t)Bu)3P]AuBr undergoes ligand redistribution upon treatment with bromine to give a complex reaction mixture, from which {[(tBu)3P]2Au}+(Br3)-.(Br2) could be crystallized. It contains polymeric anions [(Br5)-]n as zig-zag chains. [(o-Tol)3P]AuBr is readily brominated to give [(o-Tol)3P]AuBr3. Contrary to the situation in the gold(I) complex with its linear PAuBr unit, the square planar structure of the PAuBr3 unit causes steric hindering of the rotation of the tolyl groups about the P-C bonds as demonstrated by solution NMR studies. (The corresponding reaction with iodine is known to give only polyiodides with the oxidation state of the gold atom unchanged.) The even more severe congestion in [(Mes)3P]AuBr prevents oxidative addition not only of iodine but also of bromine. With the latter, P-Au cleavage occurs instead affording [(Mes)3PBr]+[AuBr4]-.

Isomeric mono- and bis[(phosphane)gold(I)] thiocyanate complexes.

The solid-state IR spectrum of Me(3)PAuSCN shows two signals in the range of the C-N stretching vibrations at 2075 and 2113 cm(-1). On the basis of thoroughly tested quantum chemical ab initio calculations (MP2 level of theory) these signals have been assigned to the two isomeric forms Me(3)PAuNCS and Me(3)PAuSCN. The molecular structures, the vibrational frequencies, and the relative energies of the two species have been calculated and the results compared with the experimental IR data. Treatment of Me(3)PAuSCN with equimolar quantities of [(Me(3)P)Au](+)[SbF(6)](-) in CH(2)Cl(2) at -78 degrees C gives the dinuclear reaction product [C(7)H(9)Au(2)NP(2)S](+)[SbF(6)](-) in high yields. A comparison of results of ab initio calculations and IR data suggest that at least three isomeric cationic species [(R(3)PAu)(2)NCS](+), [(R(3)PAu)(2)SCN](+) and [(R(3)PAu)SCN(AuPR(3))](+) are present, the second and third being the predominant components. The structures and vibrational frequencies of all three species have been calculated. The relative energies in the gas phase and in solution are discussed and compared with the corresponding data of the experimental IR spectra.