Indoles from Alkynes and Aryl Azides: Scope and Theoretical Assessment of Ruthenium Porphyrin-Catalyzed Reactions.

A symbiotic experimental/computational study analyzed the Ru(TPP)(NAr)2 -catalyzed one-pot formation of indoles from alkynes and aryl azides. Thirty different C3 -substituted indoles were synthesized and the best performance, in term of yields and regioselectivities, was observed when reacting ArC≡CH alkynes with 3,5-(EWG)2 C6 H3 N3 azides, whereas the reaction was less efficient when using electron-rich aryl azides. A DFT analysis describes the reaction mechanism in terms of the energy costs and orbital/electronic evolutions; the limited reactivity of electron-rich azides was also justified. In summary, PhC≡CH alkyne interacts with one NAr imido ligand of Ru(TPP)(NAr)2 to give a residually dangling C(Ph) group, which, by coupling with a C(H) unit of the N-aryl substituent, forms a 5+6 bicyclic molecule. In the process, two subsequent spin changes allow inverting the conformation of the sp2 C(Ph) atom and its consequent electrophilic-like attack to the aromatic ring. The bicycle isomerizes to indole via a two-step outer sphere H-migration. Eventually, a 'Ru(TPP)(NAr)' mono-imido active catalyst is reformed after each azide/alkyne reaction.

Modelling strategies for the covalent functionalization of 2D phosphorene.

This paper is a comparative outline of the potential acid-base adducts formed by an unsaturated main group or transition metal species and P atoms of phosphorene (Pn), which derives from black phosphorus exfoliation. Various possibilities of attaining a realistic covalent functionalization of the 2D material have been examined via DFT solid state calculations. The distribution of neighbor P atoms at one side of the sheet and the reciprocal directionalities of their lone pairs must be clearly understood to foreshadow the best possible acceptor reactants. Amongst the latter, the main group BH3 or I2 species have been examined for their intrinsic acidity, which favors the periodic mono-hapto anchoring at Pn atoms. The corresponding adducts are systematically compared with other molecular P donors from a phosphine to white phosphorus, P4. Significant variations emerge from the comparison of the band gaps in the adducts and the naked phosphorene with a possible electronic interpretation being offered. Then, the Pn covalent functionalization has been analyzed in relation to unsaturated metal fragments, which, by carrying one, two or three vacant σ hybrids, may interact with a different number of adjacent P atoms. For the modelling, the concept of isolobal analogy is important for predicting the possible sets of external coligands at the metal, which may allow the anchoring at phosphorene with a variety of hapticities. Structural, electronic, spectroscopic and energy parameters underline the most relevant pros and cons of some new products at the 2D framework, which have never been experimentally characterized but appear to be reasonably stable.

The atomic level mechanism of white phosphorous demolition by di-iodine.

A detailed mechanism of the I2-induced transformation of white phosphorus into PI3 emerges from a DFT analysis. This multi-step process implies that at any stage one P-P and two I-I bonds cleavages, associated with the formation of two P-I bonds plus an in situ generated brand new I2 molecule. Significant electron transfer between the atoms is observed at any step, but the reactions are better defined as concerted rather than redox. Along the steepest descent to the product, no significant barrier is encountered except for the very first P4 activation, which costs +14.6 kcal mol-1. At the atomic level, one first I2 molecule, a typical mild oxidant, is first involved in a linear halogen bonding interaction (XB) with one P donor, while its terminal I atom is engaged in an additional XB adduct with a second I2. Significant electron transfer through the combined diatomics allows the external I atom of the dangling I3 grouping to convey electrons into the σ* level of one P-P bond with its consequent cleavage. This implies at some point the appearance of a six-membered ring, which alternatively switches its bonding and no-bonding interactions. The final transformation of the P2I4 diphosphine into two PI3 phosphines is enlightening also for the specific role of the I substituents. In fact, it is proved that an organo-diphosphine analogue hardly undergoes the separation of two phosphines, as reported in the literature. This is attributable to the particularly high donor power of the carbo-substituted P atoms, which prevents the concertedness of the reaction but favors charge separation in an unreactive ion pair.

Hierarchy of Supramolecular Arrangements and Building Blocks: Inverted Paradigm of Crystal Engineering in the Unprecedented Metal Coordination of Methylene Blue.

The aromatic methylene blue cation (MB+) shows unprecedented ligand behavior in the X-ray structures of the trigonal-planar (TP) complexes MBMCl2 (M = CuI, AgI). The two isostructural compounds were exclusively synthesized by grinding together methylene blue chloride and MCl solids. Only in the case of AuCl did the technique lead to a different, yet isoformular, AuI derivative with separated MB+ and AuCl2- counterions and no direct N-Au linkage. While the density functional theory (DFT) molecular modeling failed in reproducing the isolated Cu and Ag complexes, the solid-state program CRYSTAL satisfactorily provided for Cu the correct TP building block associated with a highly compact π stacking of the MB+ ligands. In this respect, the dispersion interactions, evaluated with the DFT functional, provide to the system an extra energy, which likely supports the unprecedented metal coordination of the MB+ cation. The feature seems governed by subtle chemical factors, such as, for instance, the selected metal ion of the coinage triad. Thus, the electronically consistent AuI ion does not form the analogous TP building block because of a looser supramolecular arrangement. In conclusion, while a given crystalline design is generally fixed by the nature of the building block, a peculiarly efficient supramolecular packing may stabilize an otherwise unattainable metal complex.

From Widely Accepted Concepts in Coordination Chemistry to Inverted Ligand Fields.

We begin with a brief historical review of the development of our understanding of the normal ordering of nd orbitals of a transition metal interacting with ligands, the most common cases being three below two in an octahedral environment, two below three in tetrahedral coordination, and four below one in a square-planar environment. From the molecular orbital construction of these ligand field splittings evolves a strategy for inverting the normal order: the obvious way to achieve this is to raise the ligand levels above the metal d's; that is, make the ligands better Lewis bases. However, things are not so simple, for such metal/ligand level placement may lead to redox processes. For 18-electron octahedral complexes one can create the inverted situation, but it manifests itself in the makeup of valence orbitals (are they mainly on metal or ligands?) rather than energy. One can also see the effect, in small ways, in tetrahedral Zn(II) complexes. We construct several examples of inverted ligand field systems with a hypothetical but not unrealistic AlCH3 ligand and sketch the consequences of inversion on reactivity. Special attention is paid to the square-planar case, exemplified by [Cu(CF3)4](-), in which Snyder had the foresight to see a case of an inverted field, with the empty valence orbital being primarily ligand centered, the dx2-y2 orbital heavily occupied, in what would normally be called a Cu(III) complex. For [Cu(CF3)4](-) we provide theoretical evidence from electron distributions, geometry of the ligands, thermochemistry of molecule formation, and the energetics of abstraction of a CF3 ligand by a base, all consistent with oxidation of the ligands in this molecule. In [Cu(CF3)4](-), and perhaps more complexes on the right side of the transition series than one has imagined, some ligands are σ-noninnocent. Exploration of inverted ligand fields helps us see the continuous, borderless transition from transition metal to main group bonding. We also give voice to a friendly disagreement on oxidation states in these remarkable molecules.

Intriguing I2 Reduction in the Iodide for Chloride Ligand Substitution at a Ru(II) Complex: Role of Mixed Trihalides in the Redox Mechanism.

The compound [Ru(CN(t)Bu)4(Cl)2], 1, reacts with I2, yielding the halogen-bonded (XB) 1D species {[Ru(CN(t)Bu)4(I)2]·I2}n, (2·I2)n, whose building block contains I(-) ligands in place of Cl(-) ligands, even though no suitable redox agent is present in solution. Some isolated solid-state intermediates, such as {[Ru(CN(t)Bu)4(Cl)2]·2I2}n, (1·2I2)n, and {[Ru(CN(t)Bu)4(Cl)(I)]·3I2}n, (3·3I2)n, indicate the stepwise substitution of the two trans-halide ligands in 1, showing that end-on-coordinated trihalides play a key role in the process. In particular, the formation of ClI2(-) triggers electron transfer, possibly followed by an inverted coordination of the triatomic species through the external iodine atom. This allows I-Cl separation, as corroborated by Raman spectra. The process through XB intermediates corresponds to reduction of one iodine atom combined with the oxidation of one coordinated chloride ligand to give the corresponding zerovalent atom of I-Cl. This redox process, explored by density functional theory calculations (B97D/6-31+G(d,p)/SDD (for I and Ru atoms)), is apparently counterintuitive with respect to the known behavior of the corresponding free halogen systems, which favor iodide oxidation by Cl2. On the other hand, similar energy barriers are found for the metal-assisted process and require a supply of energy to be passed. In this respect, the control of the temperature is fundamental in combination with the favorable crystallizations of the various solid-state products. As an important conclusion, trihalogens, as XB adducts, are not static in nature but are able to undergo dynamic inner electron transfers consistently with implicit redox chemistry.

[Co2 (CO)6 (Alkynyl)] Complexes of Dibenzosuberyl and Dibenzosuberenyl Carbocations: Dibenzotropylium or Dibenzoheptafulvene?

The reaction of dibenzo[a,d]cycloheptan-5-one (dibenzosuberone) and dibenzo[a,d]cyclohept-10-en-5-one (dibenzosuberenone) with aryl- or trimethylsilylacetylides led to the formation of the corresponding alkynyldibenzosuberols and alkynyldibenzosuberenols. Treatment with dicobalt octacarbonyl and then with bis(diphenylphosphino)methane (dppm) furnished the corresponding [Co2 (CO)4 (dppm)(alkynol)] clusters 25 and 29. Upon protonation with HBF4 at 203 K to generate the relevant cobalt-stabilised cations, the dibenzosuberyl system 30 exhibited fluxionality such that the cation migrated between cobalt centres. Variable-temperature 31 P NMR spectroscopy revealed a barrier of approximately 12.5 kcal mol-1 . In contrast, in the supposedly aromatic [Co2 (CO)4 (dppm)(dibenzosuberenyl)]+ cation (31), which would be expected to have less need of cobalt stabilisation, the barrier was too high to be measured experimentally, but is certainly in excess of 16 kcal mol-1 . These data were rationalised by DFT calculations on the structures and energies of the relevant ground states and transition states, which suggested that the nonplanar alkynyldibenzosuberenyl moiety in 31 is better regarded as a neutral dibenzoheptafulvene coordinated to a cationic alkynyl-dicobalt cluster. The question of the bonding of both aromatic and antiaromatic cations to alkyne-dicobalt clusters is considered, and it is proposed that their stabilities, when complexed, parallel the inversion of (4n+2) π and 4n π systems seen under photochemical conditions.

CNN pincer ruthenium catalysts for hydrogenation and transfer hydrogenation of ketones: experimental and computational studies.

Reaction of [RuCl(CNN)(dppb)] (1-Cl) (HCNN=2-aminomethyl-6-(4-methylphenyl)pyridine; dppb=Ph2 P(CH2 )4 PPh2 ) with NaOCH2 CF3 leads to the amine-alkoxide [Ru(CNN)(OCH2 CF3 )(dppb)] (1-OCH2 CF3 ), whose neutron diffraction study reveals a short RuO⋅⋅⋅HN bond length. Treatment of 1-Cl with NaOEt and EtOH affords the alkoxide [Ru(CNN)(OEt)(dppb)]⋅(EtOH)n (1-OEt⋅n EtOH), which equilibrates with the hydride [RuH(CNN)(dppb)] (1-H) and acetaldehyde. Compound 1-OEt⋅n EtOH reacts reversibly with H2 leading to 1-H and EtOH through dihydrogen splitting. NMR spectroscopic studies on 1-OEt⋅n EtOH and 1-H reveal hydrogen bond interactions and exchange processes. The chloride 1-Cl catalyzes the hydrogenation (5 atm of H2 ) of ketones to alcohols (turnover frequency (TOF) up to 6.5×10(4) h(-1) , 40 °C). DFT calculations were performed on the reaction of [RuH(CNN')(dmpb)] (2-H) (HCNN'=2-aminomethyl-6-(phenyl)pyridine; dmpb=Me2 P(CH2 )4 PMe2 ) with acetone and with one molecule of 2-propanol, in alcohol, with the alkoxide complex being the most stable species. In the first step, the Ru-hydride transfers one hydrogen atom to the carbon of the ketone, whereas the second hydrogen transfer from NH2 is mediated by the alcohol and leads to the key "amide" intermediate. Regeneration of the hydride complex may occur by reaction with 2-propanol or with H2 ; both pathways have low barriers and are alcohol assisted.

Experimental and theoretical insights into the oxodiperoxomolybdenum-catalysed sulphide oxidation using hydrogen peroxide in ionic liquids.

The oxidation of organic sulphides with aqueous hydrogen peroxide in ionic liquids (ILs) catalysed by oxodiperoxomolybdenum complexes was investigated. The selective formation of several sulfones was achieved using the 1 : 3 ratio of sulphide : H2O2 in [C4mim][PF6] (C4mim = 1-butyl-3-methylimidazolium) in a reaction catalysed by the [Mo(O)(O2)2(H2O)n] complex. Conversely, sulfoxides were produced with good selectivities using a 1 : 1 ratio in the same solvent in a 1 h reaction with [Mo(O)(O2)2(Mepz)2] (Mepz = methylpyrazol). The use of [C4mim][PF6] as the solvent was advantageous for two reasons: (i) the improved performance of the H2O2-IL combination; (ii) recycling of the catalyst/IL mixture without a significant diminution of conversion or selectivity. A DFT analysis using the [Mo(O)(O2)2(L)] catalysts (L = Mepz, a; 3,5-dimethylpyrazole, dmpz, b; and H2O, c) indicated that a Sharpless-type outer-sphere mechanism is more probable than a Thiel-type one. The highest barrier of the catalytic profile was the oxo-transfer step, in which the nucleophilic attack of sulphide onto the peroxide ligand occurred with formation of dioxoperoxo species. In order to yield the sulfoxide and the starting catalyst, the oxidation of the resulting dioxoperoxo species with H2O2 was found to be the most favourable pathway. Subsequently, the sulfoxide to sulfone oxidation was performed through a similar mechanism involving the [Mo(O)(O2)2(L)] catalyst. The comparable energies found for the successive two oxo-transfer steps were in agreement with the experimental formation of sulfone in both the reaction with an excess of the oxidant and the stoichiometric reaction in the absence of the oxidant. In the latter case, diphenylsulfone was isolated as the major product in the 1 : 1 combination of diphenylsulphide and [Mo(O)(O2)2(Mepz)2] in the ionic liquid [C4mim][PF6]. Also, the compounds [HMepz]4[Mo8O26(Mepz)2]·2H2O, 1, [Hdmpz]4[Mo8O26(dmpz)2]·2dmpz, , and [Hpz]4[Mo8O22(O2)4(pz)2]·3H2O, 3, were obtained by treating in water, stoichiometrically, dimethylsulfoxide and the corresponding [Mo(O)(O2)2(L)2] complex (L = Mepz; 3,5-dimethylpyrazole, dmpz; pyrazol, pz). The crystal structures of octanuclear compounds 1-3 were indirect proof of the formation of the theoretically proposed intermediates.

Unprecedented tris-phosphido-bridged triangular clusters with 42 valence electrons. Chemical, electrochemical and computational studies of their formation and stability.

This paper presents the synthesis and structural characterization of the unprecedented tris-phosphido-bridged compounds Pt3(µ-PBu(t)2)3X3 (X = Cl, Br, I), having only 42 valence electrons, while up to now analogous clusters typically have 44e(-). The new species were obtained by an apparent bielectronic oxidation of the 44e(-) monohalides Pt3(µ-PBu(t)2)3(CO)2X with the corresponding dihalogen X2. Their X-ray structures are close to the D3h symmetry, similarly to the 44e(-) analogues with three terminal carbonyl ligands. The products were also obtained by electrochemical oxidation of the same monohalides in the presence of the corresponding halide. In a detailed study on the formation of Pt3(µ-PBu(t)2)3I3, the redox potentials indicated that I2 can only perform the first monoelectronic oxidation but is unsuited for the second one. Accordingly, the 43e(-) intermediate [Pt3(µ-PBu(t)2)3(CO)2I](+) was ascertained to play a key role. Another piece of information is that, together with the fully oxidized product Pt3(µ-PBu(t)2)3I3, the transient 44e(-) species [Pt3(µ-PBu(t)2)3(CO)3](+) is formed in the early steps of the reaction. In order to extract detailed information on the formation pathway, involving both terminal ligand substitutions and electron transfer processes, a DFT investigation has been performed and all the possible intermediates have been defined together with their associated energy costs. The profile highlights many important aspects, such as the formation of an appropriate couple of 43e(-) intermediates having different sets of terminal coligands, and suitable redox potentials for the transfer of one electron. Optimizations of 45e(-) associative intermediates in the ligand substitution reactions indicate their possible involvement in the redox process with reduction of the overall energy cost. Finally, according to MO arguments, the unique stability of the 42e(-) phosphido-bridged Pt3 clusters can be attributed to the simultaneous presence of three terminal halides.

Electronic stabilization of trigonal bipyramidal clusters: the role of the Sn(II) ions in [Pt5(CO)5{Cl2Sn(µ-OR)SnCl2}3]3- (R = H, Me, Et, iPr).

The new [Pt(5)(CO)(5){Cl(2)Sn(µ-OR)SnCl(2)}(3)](3-) (R = H, Me, Et, (i)Pr; 1-4) clusters contain trigonal bipyramidal (TBP) Pt(5)(CO)(5) cores, as certified by the X-ray structures of [Na(CH(3)CN)(5)][NBu(4)](2)[1]·2CH(3)CN and [PPh(4)](3)[4]·3CH(3)COCH(3). The TBP geometry, which is rare for group 10 metals, is supported by an unprecedented interpenetration with a nonbonded trigonal prism of tin atoms. By capping all the Pt(3) faces, the Sn(II) lone pairs account for both Sn-Pt and Pt-Pt bonding, as indicated by DFT and topological wave function studies. In the TBP interactions, the metals use their vacant s and p orbitals using the electrons provided by Sn atoms, hence mimicking the electronic picture of main group analogues, which obey the Wade's rule. Other metal TBP clusters with the same total electron count (TEC) of 72 are different because the skeletal bonding is largely contributed by d-d interactions (e.g., [Os(5)(CO)(14)(PR(3))(µ-H)(n)](n-2), n = 0, 1, 2). In 1-4, fully occupied d shells at the Pt(ax) atoms exert a residual nucleophilicity toward the adjacent main group Sn(II) ions permitting their hypervalency through unsual metal donation.

Thiodiacetate-manganese chemistry with N ligands: unique control of the supramolecular arrangement over the metal coordination mode.

Compounds based on the Mn-tda unit (tda=S(CH(2)COO)(2)(-2) ) and N co-ligands have been analyzed in terms of structural, spectroscopic, magnetic properties and DFT calculations. The precursors [Mn(tda)(H(2)O)](n) (1) and [Mn(tda)(H(2)O)(3)]·H(2)O (2) have been characterized by powder and X-ray diffraction, respectively. Their derivatives with bipyridyl-type ligands have formulas [Mn(tda)(bipy)](n) (3), [{Mn(N-N)}(2)(µ-H(2)O)(µ-tda)(2)](n) (N-N=4,4'-Me(2)bipy (4), 5,5'-Me(2)bipy, (5)) and [Mn(tda){(MeO)(2)bipy}·2H(2)O](n) (6). Depending on the presence/position of substituents at bipy, the supramolecular arrangement can affect the metal coordination type. While all the complexes consist of 1D coordination polymers, only 3 has a copper-acetate core with local trigonal prismatic metal coordination. The presence of substituents in 4-6, together with water co-ligands, reduces the supramolecular interactions and typical octahedral Mn(II) ions are observed. The unicity of 3 is also supported by magnetic studies and by DFT calculations, which confirm that the unusual Mn coordination is a consequence of extended noncovalent interactions (π-π stacking) between bipy ligands. Moreover, 3 is an example of broken paradigm for supramolecular chemistry. In fact, the desired stereochemical properties are achieved by using rigid metal building blocks, whereas in 3 the accumulation of weak noncovalent interactions controls the metal geometry. Other N co-ligands have also been reacted with 1 to give the compounds [Mn(tda)(phen)](2)·6H(2)O (7) (phen=1,10-phenanthroline), [Mn(tda)(terpy)](n) (8) (terpy=2,2':6,2''-terpyridine), [Mn(tda)(pyterpy)](n) (9) (pyterpy=4'-(4-pyridyl)-2,2':6,2''-terpyridine), [Mn(tda)(tpt)(H(2)O)]·2H(2)O (10) and [Mn(tda)(tpt)(H(2)O)](2)·2H(2)O (11) (tpt=2,4,6-tris(2-pyridyl)-1,3,5-triazine). Their identified mono-, bi- or polynuclear structures clearly indicate that hydrogen bonding is variously competitive with π-π stacking.

A bonding quandary--or--a demonstration of the fact that scientists are not born with logic.

We document here a spirited debate among three colleagues and friends who have strong opinions on a specific bonding problem, the presence or absence of a cross-ring sulfur-sulfur bond in a trinuclear Cu(3)S(2) cluster. The example may seem esoteric, but through their struggles with this specific bond (and with each other) the authors approach the more general problematic of chemistry, the chemical bond. The discussion focuses on bond lengths and the population of bonding and antibonding orbitals, and on oxidation states, electron counting, and associated geometries. It expands to encompass other bonding criteria, and introduces examples ranging far across organic and inorganic chemistry. The authors suggest molecules that might test their ideas. An Appendix to the paper discusses a matter rarely broached in the chemical literature--should one review for publication a paper which criticizes one of your own contributions.

Is 2.07 A a record for the shortest Pt-S distance? Revision of two reported X-ray structures.

The comparison of the very similar compounds (Ph(3)P)(2)Pt(mu-S)(2)Pt(PPh(3))(2) (1) and (Ph(2)PyP)(2)Pt(mu-S)(2)Pt(PPh(2)Py)(2) (2) raises intriguing questions about the reliability of the reported Pt(2)S(2) core in 1, where the Pt-S bonds are the shortest ever reported. Also, the trans-annular S...S separation of 2.69 A is surprisingly shorter in 1 than in 2 (3.01 A), but no incipient coupling between two S(2-) bridges seems reasonable in this case. Various considerations lead to reformulate 1 as [(Ph(3)P)(2)Pt(mu-OH)(2)Pt(PPh(3))(2)](BF(4))(2), 3. The sets of cell parameters for 1 and 3 are not equal but two axes match, and the volume of 1 is exactly double. Simple matrices may be constructed to interconvert the direct and reciprocal crystalline cells, thus corroborating their identity of the compounds. It is concluded that, in the structure solution of 1, some atoms were either neglected (BF(4)(-) counterions) or ill identified (sulfido in place of hydroxo bridges), while the structure of 3 was solved by collecting only one-half of the possible reflections (hence, also the different space groups). A new preparation, crystallization and X-ray structure of 3 confirms the above points and dismisses any other theoretical conjecture about two electronically different Pt(2)S(2) cores in 1 and 2.

Cover picture: "half-bonds" in an unusual coordinated S4(2-) rectangle (Chem. Asian J. 2/2009).

This stone lantern is known as the "Koto-ji", or "harp-tuner", as it resembles the tuning bridge of the Japanese koto. The six windows of the lantern are said to symbolize the six essential attributes of a perfect garden, such as the one it overlooks: spaciousness, seclusion, artifice, antiquity, abundant water, and broad views. R. Hoffmann et al. report on two synthesized compounds containing the unusual S4(2-) rectangles bound to either Ir or Rh fragments, illuminated by the two lamp windows. Like the Koto-ji lantern, this paper, which suggests the presence of S-S half-bonds, sheds some light in the garden of beautiful compounds. For more information, see their Full Paper on page 302 ff.

"Half-bonds" in an unusual coordinated S(4) (2-) rectangle.

Don't be square! A rare S(4) (2-) rectangle bridging two M(2)Cp(2)(mu(2)-CH(2))(2) (M=Rh, Ir) fragments is found to contain two "half-bonds" with S-S distances of 2.70 or 2.90 A. Computational studies explore the connection between these "half-bonds" and a Jahn-Teller distortion, as well as possible intermediates that form M(4)S(4) (2+) clusters having the S(4) (2-) rectangle rotated by 90 degrees. The bonding of a rare S(4) (2-) rectangle coordinated to four transition metals (synthesized by Isobe, Nishioka, and co-workers), [{M(2)(eta(5)-C(5)Me(5))(2)(mu-CH(2))(2)}(2)(mu-S(4))](2+) (M=Rh, Ir) is analyzed. DFT calculations indicate that, while experiment gives the rectangle coordinated with its long edge parallel to Rh-Rh bonds and perpendicular to the Ir-Ir bonds, either orientation is feasible for both metals. Although rotation of the S(4) rectangle is likely a multi-step process, a calculated barrier of 46 kcal mol(-1) for a simple interconversion pathway goes through a trapezoidal, not a square, transition state. An argument is presented, based on molecular orbital (MO) calculations, that the long S-S contacts (2.70 and 2.90 A) in the rectangle are in fact two-center, three-electron bonds (or "half-bonds"). Moreover, the 2- charge on the S(4) rectangle is related to a Jahn-Teller distortion from a square to a rectangle. Finally, DFT is used to explore possible stable intermediates in the oxidative process giving these M(4)S(4) (2+) compounds: for Ir, the coupling of two Ir(2)S(2) (+) molecules appears feasible, as opposed to a possible two-electron oxidation of a neutral Rh(4)S(4) molecule.