Paddlewheel Complexes Research Articles

Cover Feature: Paddlewheel Complexes with Azulenes: Electronic Interaction between Metal Centers and Equatorial Ligands (ChemPlusChem 6/2019)

Cover Feature: Paddlewheel Complexes with Azulenes: Electronic Interaction between Metal Centers and Equatorial Ligands (ChemPlusChem 6/2019)

From Pincer to Paddlewheel: C-H and C-S Bond Activation at Bis(2-pyridylthio)methane by Palladium(II).

The bis(2-pyridylthio)methanidopalladium(II) pincer complex (1), containing a Pd-C bond, was obtained from the reaction of bis(2-pyridylthio)methane (H2L) with palladium(II) acetate in toluene under reflux. When palladium(II) trifluoroacetate was used, H2L reacted to generate the tetrakis(pyridine-2-thiol)palladium(II) complex (2). Complex 2 was converted to a heterobimetallic palladium(II)-iron(II) paddlewheel complex (3) upon treatment with iron(II) triflate in the presence of a base in acetonitrile at room temperature.

Metal-Bonded Redox-Active Triarylamines and Their Interactions: Synthesis, Structure, and Redox Properties of Paddle-Wheel Copper Complexes.

Invited for this month's cover picture is the group of Professor Winfried Plass at the Institute of Inorganic and Analytical Chemistry, Friedrich Schiller University, Jena (Germany). The cover picture shows a scene illustrating the need to investigate the properties of building blocks for complex systems to enable the basic design of new functional materials. The utilized triphenylamine ligands are constituting parts of the currently investigated “Jena University Magnetic Polymer” (JUMP) series. Read the full text of their Full Paper at 10.1002/open.201800243.

Paddlewheel Complexes with Azulenes: Electronic Interaction between Metal Centers and Equatorial Ligands

Rhodium dinuclear complexes (1-3) with azulene moieties as equatorial ligands were obtained by reacting Rh2 (OAc)4 with guaiazulene-2-carboxylic acid, azulene-2-carboxylic acid, and azulene-1-carboxylic acid, respectively. The molecular structures in their crystalline states were determined by X-ray diffraction to be 1 ⋅ (H2 O)2 , 1 ⋅ (MeCN)2 , 2 ⋅ (MeCN)2 , and 3 ⋅ (DMF)2 , which were coordinated with the crystallization solvent at the axial positions. Among these, the crystal packing of 1 ⋅ (H2 O)2 , 1 ⋅ (MeCN)2 , and 3 ⋅ (DMF)2 revealed the formation of one-dimensional stacked chains nearly along the axial direction and of two-dimensional stacked sheets along the equatorial direction. In addition, it was found that 1 ⋅ (H2 O)2 contained cavities that could adsorb CO2 , thereby inducing structural changes. Furthermore, redox measurements revealed the stepwise one-electron redox behaviors of these complexes, indicating the intramolecular interactions between the azulene units. In addition, transient absorption measurements suggested the presence of an ultrafast intersystem crossing caused by the heavy-atom effect of rhodium, and an extended lifetime of the triplet state due to the energy migration among the azulene ligands.

Intermolecular Nitrene Insertion by Bimetallic Catalysts

AbstractAn intermolecular nitrene insertion catalyzed by heterobimetallic Co−Pd complexes is reported herein. In a previous report, paddlewheel complex CoPd(OAc)4 was found to have similar reactivity with the well‐known catalyst, Rh2(OAc)4 in the intramolecular C−H amination. For the more challenging intermolecular reaction, biscarboxylate ligand esp acid was used for the Co−Pd core. The reactivity of CoPdesp2 was compared with its dirhodium counterparts in the aziridination of cyclic alkenes. Also, a biscarboxylate, HPH acid with the skeleton of xanthene was synthesized and tested on the dirhodium core in the aziridination reactions. Both of Rh2HPH2 and Rh2esp(OAc)2 were found to be competent catalysts in the intermolecular aziridination of cyclic alkenes.

Synthesis and Catalytic Properties of Dirhodium Paddlewheel Complexes with Tethered, Axially Coordinating Thioether Ligands

Novel mixed-ligand rhodium(II) paddlewheel complexes incorporating tethered axial thioether ligands have been synthesized and characterized. The thioether moiety is essential for high yields and the suppression of byproducts in cyclopropanation reactions using an electron-deficient diazoacetate. Crystal structures, UV-vis analysis, and cyclic voltammetry experiments shed light on the catalytic performance of the complexes.

Weak acids with super-electron-donor dimetal complexes: Synergy in bifunctional activity

Dimetal paddlewheel complexes with bicyclic guanidine ligands are extremely strong thermodynamic electron donors. As a probe of the chemical potential and sites for chemical reactivity of these complexes, the interaction of Mo2(TEhpp)4 with weak acids was investigated (TEhpp is the anion of the bicyclic guanidine 3,3,9,9-tetraethyl-1,5,7-triazabicyclo[4.4.0.]dec-4-ene). Mo2(TEhpp)4 is readily protonated by acetic acid and trifluoroacetic acid as expected, but surprisingly by a mechanism that is more complicated than a simple acid-base proton exchange. Electrochemistry measurements of the shifts in potentials with successive additions of acid reveals that the conjugate base of the acid plays a critical role throughout the reaction. Computations indicate that initially the acid hydrogen bonds to a TEhpp nitrogen atom bound to the metal, and then a facile rearrangement of the conjugate base toward the axial site of the Mo–Mo bond concomitantly results in protonation of the nitrogen atom. Interestingly, the dimetal complex is bifunctional in this process, acting as a nucleophile at the nitrogen atoms of the TEhpp ligands, and as an electrophile at the Mo–Mo axial bond sites. The protonation requires a novel synergism of these disparate bonding interactions, in which protonation is not favored without enhancement by the coordinated base, and coordination of the base is enhanced by the protonation.

Enhanced Electrophilicity of Heterobimetallic Bi-Rh Paddlewheel Carbene Complexes: A Combined Experimental, Spectroscopic, and Computational Study.

Dirhodium paddlewheel complexes are indispensable tools in modern organometallic catalysis for the controlled decomposition of diazo-compounds. Tuning the reactivity of the thus-formed transient carbenes remains an active and dynamic field of research. Herein, we present our findings that the distal metal center plays an as yet underappreciated role in modulating this reactivity. Replacement of one rhodium atom in the bimetallic core for bismuth results in the formation of a significantly more electrophilic carbene complex. Bismuth-rhodium catalysts thereby facilitate previously unknown modes of reactivity for α-diazoester compounds, including the cyclopropanation of alkenes as electron deficient as trichloroethylene. While dirhodium paddlewheel complexes remain the catalysts of choice for many carbene-mediated transformations, their bismuth-rhodium analogues exhibit complementary reactivity and show great potential for small molecule and solvent activation chemistry. DFT calculations highlight the importance of metal-metal bonding interactions in controlling carbene electrophilicity. The paucity of these interactions between the 4d orbitals of rhodium and the 6p orbitals of bismuth results in weaker π-back-bonding interactions for bismuth-rhodium carbene complexes compared to dirhodium carbene complexes. This leads to weakening of the rhodium-carbene bond and to a more carbene-centered LUMO, accounting for the observed enhancement in bismuth-rhodium carbene electrophilicity. These findings are supported by a detailed spectroscopic study of the "donor-donor" carbene complexes Rh2(esp)2C( p-MeOPh)2 (19) and BiRh(esp)2C( p-MeOPh)2 (20), employing a combination of UV-vis and resonance Raman spectroscopy. The results reveal that carbene chemoselectivity in MRh(L)4 catalysis can be modulated to a previously unrecognized extent by the distal metalloligand.

New Oxypyridinate Paddlewheel Ligands for Alkane-Soluble, Sterically-Protected Ru2(II,III) and Ru2(II,II) Complexes.

The paddlewheel complex Ru2(chp)4Cl (1-Cl, chp = 6-chloro-2-oxypyridinate), upon reduction with Zn, has been previously shown to dimerize to [Ru2(chp)4]2 (2), blocking further chemistry at the Ru2(II,II) axial site [ Inorg. Chem. 2015 , 54 , 8571 - 8589 ]. Functionalization of the chp ligand at the 3 and 5 positions with either bromine (dbchpH = 3,5-dibromo-6-chloro-2-pyridone) or trimethylsilyl (TMS) groups (dsichpH = 6-chloro-3,5-bis(trimethylsilyl)-2-pyridone) allows for the preparation of the Ru2(II,II) paddlewheel complexes Ru2(dbchp)4 (3) and Ru2(dsichp)4 (6), respectively, neither of which shows evidence of dimerization. Though the utilization of 3 is limited due to insolubility, complex 6 is soluble even in typically non-coordinating solvents, forming a stable κ1-axial adduct in CH2Cl2 (6-CH2Cl2) and showing evidence of an axial interaction with n-decane. The first example of an axially free Ru2(II,II) complex with a 3A ground state is observed upon crystallization of 6 from benzene (6-C6D6). Complex 6 is accessed via Zn reduction of Ru2(dsichp)4Cl (4-Cl), which along with Ru2(dsichp)4N3 (4-N3), show similar structural and electronic properties to their non-TMS-substituted analogues, 1-Cl and 1-N3. Photolysis of 4-N3 in frozen solution generates Ru2(dsichp)4N (5); no N atom transfer to PPh3 is observed upon room temperature photolysis in fluid solution.

Enantiomeric resolution of helicochiral paddlewheel complexes and their infrared, Raman, UV–vis and X-ray optical activity

Linear polynuclear paddlewheel complexes-extended atom chains or metal strings-have provided attractive models for the study of metal-metal bonding, magnetism and conductivity since their discovery in the 1990s [1]. Their helicoidal chirality, arising from mutual steric hindrance of the 3-pyridyl protons, resulting in the twisting of the equatorial ligand around the axis (see figure), has been less studied. Nonetheless, in one of the few examples of chiral resolution, the obtained enantiomers of a trinickel complex showed a remarkably high specific rotation of 5000 deg•mL•g −1 •dm −1 [2], motivating us to seek a general technique for the chiral resolution of such racemates. We have developed a procedure based on anion exchange for the chiral resolution of [M3(dpa)4] 2+ salts (M = Co(II) or Ni(II), Hdpa = 2,2'-dipyridylamine). Homochiral arsenyl tartrate (AsT) salts promoted the selective crystallization of [-M3(dpa)4(MeCN)2](NBu4)2[-AsT]2, or [-M3(dpa)4(MeCN)2](NBu4)2[-AsT]2 in the P4212 space group. The enantiopure compounds demonstrated surprisingly large optical activities using UV-vis, Raman and infrared spectroscopy in solution and, for the cobalt derivatives, in the X-ray range at the Co K-edge in single crystals. An intense X-ray linear dichroism was observed in the orthoaxial crystal orientation, whereas it vanished in the axial confirmation, while the angular dependence of the circular dichroism spectra followed the expected (3cos 2 − 1) function, thus spectroscopically confirming the D4 crystal symmetry. X-ray magnetic circular dichroism and X-ray magnetochiral dichroism signals at the Co K-edge were not detected, likely due to a strongly delocalized spin density on the metal-metal bonded tricobalt core. Nevertheless, these results establish that chiral polynuclear paddlewheel complexes can be cleanly resolved using selective crystallization and demonstrate considerable optical activity in the infrared, UV-vis and X-ray energy ranges, thus potentially offering future perspectives in non-linear optics and asymmetric synthesis [3].

Synthesis and characterization of a heteroleptic nickel paddlewheel complex

A heteroleptic paddlewheel Ni complex of formula [Ni2(μ2(N,S)-2-pyS)3(μ2-dppm)](BF4) (2-pyS: 2-pyridinethiolate; dppm: 1,2-bis(diphenylphosphino)ethane) was synthesized and characterized by X-ray diffraction, elemental analysis, and electrospray ionization mass spectroscopy. A single-crystal X-ray diffraction study revealed that the complex has μ2(N,S)-2-pyS bridging ligands. Furthermore, spectroscopic and electrochemical studies were performed, and the corresponding results were confirmed by computational chemistry.

Heterobimetallic Lantern Complexes and Their Novel Structural and Magnetic Properties.

As the scale of microelectronic circuit devices approaches the atomic limit, the study of molecular-based wires and magnets has become more prevalent. Compounds with quasi-1D geometries have been investigated for their electronic conductivity and magnetic properties with potential use as nanoscale circuit components and information storage devices. To increase the number of compositionally tailored molecular systems available to study, we have taken a building-block, bottom-up approach to the development of improved electronic structure and magnetic properties of quasi-1D arrays. Over the past decade, a large family of asymmetric complexes that can assemble into extended arrays has resulted. Lantern (or paddle-wheel) complexes with conventional {O, O} donor carboxylates are legion, but by the use of monothiocarboxylate ligands and hard-soft Lewis acid-base principles, dozens of new lantern complexes of the form [PtM(SOCR)4(L)] (M = Mg, Ca, Cr, Mn, Fe, Co, Ni, Zn; R = Ph (tba = thiobenzoate), CH3 (SAc = thioacetate); L = neutral or anionic ligand) have been prepared. Depending on M and L, new intermolecular arrangements have resulted, and the magnetic properties have proven particularly interesting. In the solid state, the [PtM(SOCR)4(L)] building blocks are sometimes isolated, sometimes form dimers, and can be induced to form infinite chains. The versatility of the lantern motif was demonstrated with a range of axial ligands to form both terminal and bridged complexes with various 3d metals and two different substituted thiocarboxylate backbone ligands. Within the dozens of crystallographically characterized compounds that make up this family of lanterns, several different structural motifs of solid-state dimerization were observed and divided into four distinct categories on the basis of their Pt···Pt and Pt···S distances and relative monomer orientations. Among all of these compounds, three novel magnetic phenomena were observed. Initially, long-range antiferromagnetic coupling between two metals more than 8 Å apart was observed in solid-state dimers formed via metallophilic Pt···Pt interactions and could induced by choice of the terminal L group. An infinite chain was prepared in [PtCr(tba)4(NCS)]∞ that displays ferromagnetic coupling between Cr centers with J/ kB = 1.7(4) K. Homobimetallic quasi-1D chains of the form [Ni2(SOCR)4(L)]∞ (R = Ph, CH3; L = DABCO, pyz) were also prepared with S = 1 {Ni2} building blocks in which the Ni centers have two different spin states with weak antiferromagnetic coupling along the chain, such that -0.18 > J/ kB > -0.24 K. In the [Ni2(tba)4(quin)] derivative, a solid-state dimer forms with a bridging square conformation by interlantern Ni2S2 interactions and displays unusual S = 1 configurations on both Ni centers and weak antiferromagnetic coupling between them.

Effect of Fluorinated Ligand on Structural, Electronic and DNA-binding Properties of Copper Paddlewheel Complex: Synthesis, Structure and Properties

This paper presents synthesis, structural description and properties of a binuclear paddlewheel copper(II) carboxylate complex with formula [(py)Cu(μ-L)4Cu(py)], where py = pyridine and L = 4-fluorophenylacetate. Structural characteristics, electronic absorption and DNA-binding properties of the synthesized complex have been compared to those of the non-fluorinated analogues (where F is replaced by H, -CH3 and -OCH3) already reported and the modifications successfully ascribed to fluorine. The complex exhibits typical paddlewheel structure and there are two crystallographically independent molecules in the unit cell. The electronic absorption spectrum of complex is also different and the mode and extent of DNA-binding ability of complex are significantly altered owing to the presence of suitably substituted fluorine. The effect of fluorine is clearly manifested in the modified properties of synthesized complex.

Temperature dependence of the spin state and geometry in tricobalt paddlewheel complexes with halide axial ligands.

Trinuclear cobalt paddlewheel complexes, [Co3(dpa)4X2] (dpa = the anion of 2,2'-dipyridylamine, X = Cl-, Br-, -NCS-, -CN-, (NC)2N-), are known to demonstrate a thermally-induced spin-crossover (SCO). Despite a wealth of structural and magnetic information about such complexes, the role of the axial ligand on the characteristic SCO temperature (T1/2) remains ambiguous. The situation is complicated by the observation that the solid state geometry of the complexes, symmetric or unsymmetric, with respect to the central cobalt ion, also appears to influence the SCO behavior. In order to seek trends in the relationship between the nature of the axial ligand, geometry and magnetic properties, we have prepared the first examples of tricobalt paddlewheel complexes with axial fluorido and iodido ligands, as well as two new chlorido and bromido solvates. Their SCO properties are discussed in the context of an examination of previously reported chlorido and bromido adducts. The main conclusions are: (1) T1/2 values follow the trend I- < Br- ≈ Cl- < F-; (2) while the molecular geometry is predominantly guided by crystal packing for the Cl-, Br- and I- derivatives, the presence of an axial fluoride may favor a more symmetric core; (3) the magnetic characterization of a second example of an unsymmetric complex supports the observation that they display dramatically lower T1/2 values than their symmetric analogues; and (4) SCO in crystallographically symmetric compounds apparently occurs without loss of molecular or crystallographic symmetry, while a gradual geometric transformation linking the temperature dependence of quasi-symmetric to unsymmetric in crystallographically unconstrained compounds was found.

Actinide-transition metal bonding in heterobimetallic uranium– and thorium–molybdenum paddlewheel complexes

We report the preparation of four heterobimetallic uranium- and thorium-molybdenum paddlewheel complexes. The characterisation data suggest the presence of Mo → An σ-interactions in all cases. These complexes represent unprecedented actinide-group 6 metal-metal bonds, where before heterobimetallic uranium-metal bonds were restricted to group 7-11 metals.

Expanding the Chemistry of Rhenium Metal-Metal Bonded Fluoro Complexes: Facile Preparation and Characterization of Paddlewheel Complexes.

Quadruply bonded rhenium(III) dimers with the stoichiometry Re2L4F2 (1, L = hexahydro-2H-pyrimido[1,2a]pyrimidinate (hpp-); 2, L = diphenyl formamidinate (dpf-)) were prepared from the solid-state melt reactions (SSMRs) between (NH4)2[Re2F8]·2H2O and HL. Those compounds were characterized in the solid state by single-crystal X-ray diffraction and in solution by UV-visible spectroscopy and cyclic voltammetry. The compound [Re2(hpp)4F2]PF6 (3) was prepared from the one-electron oxidation of Re2(hpp)4F2 with [Cp2Fe]PF6. Compounds 1-3 are isostructural with the corresponding chloro derivatives. In solution, compound 1 undergoes two one-electron oxidations. Comparison with its higher halogen homologues reveals that Re2(hpp)4F2 (1) is more easily oxidized than its chloro and bromo analogues.

Aromaticity-Driven Molecular Structural Variation and Electronic Configuration Alternation: An Example of Cyclic π Conjugation Involving a Mo-Mo δ Bond.

We have synthesized and characterized the mixed-ligand dimolybdenum paddlewheel complex Na[(DAniF)3Mo2(C3S5)] (Na[1]; DAniF = N,N'-di-p-anisylformamidinate, dmit = 1,3-dithiole-2-thione-4,5-dithiolate), which has a six-membered chelating [Mo2S2C2] ring created by equatorial coordination of the dmit (C3S5) ligand to the Mo2 unit. One-electron oxidation of Na[1] using Cp2FePF6 yields the neutral complex [(DAniF)3Mo2(C3S5)] ([1]), and removal of two electrons from Na[1] using AgBPh4 gives [(DAniF)3Mo2(C3S5)]BPh4 ([1]BPh4). In the crystal structures, [1]- and [1] present dihedral angles of 118.9 and 142.3° between the plane defined by the Mo-Mo bond vector and the dmit ligand, respectively, while DFT calculations show that in [1]+ the Mo-Mo bond and the dmit ligand are coplanar. Complex [1] is paramagnetic with a g value of 1.961 in the EPR spectrum and has a Mo-Mo bond distance of 2.133(1) Å, increased from 2.0963(9) Å for [1]-. Consistently, a broad absorption band is observed for [1] in the near-IR region, which arises from charge transfer from the dmit ligand to the cationic Mo25+ centers. Interestingly, complex [1]+ has an aromatic [Mo2S2C2] core, as evidenced by a large diamagnetic anisotropy, in addition to the coplanarity of the core structure, which shifts downfield the 1H NMR signal of the horizontal methine proton (ArN-(CH)-NAr) but upfield those of the vertical protons, relative to the methine proton resonances for the precursor ([1]-). The magnetic anisotropy (Δχ = χ⊥ - χ∥) for the [Mo2S2C2] ring in [1]+ is -105.5 ppm cgs, calculated from the McConnell equation, which is about 2-fold larger than that for benzene. The aromaticity of the [Mo2S2C2] ring is supported by theoretical studies, including single-point calculations and gauge-including atomic orbital (GIAO) NMR spectroscopic calculations at the density functional theory (DFT) level. DFT calculations also show that the [Mo2S2C2] core in [1]+ possesses a set of three highest occupied and three lowest unoccupied molecular orbitals in π character, corresponding to those of benzene in symmetry, and six π electrons that conform to the Hückel 4n + 2 rule for aromaticity. Therefore, this study shows that an aromatic [Mo2S2C2] core is formed by coupling the δ orbital of the Mo≣Mo bond with the π orbital of the C═C bond through the bridging atoms (S), thus validating the equivalency in bonding functionality between δ and π orbitals.

New Rh2(II,II) Complexes for Solar Energy Applications: Panchromatic Absorption and Excited-State Reactivity.

The new heteroleptic paddlewheel complexes cis-[Rh2(μ-form)2(μ-np)2][BF4]2, where form = p-ditolylformamidinate (DTolF) or p-difluorobenzylformamidinate (F-form) and np = 1,8-napthyridyine, and cis-Rh2(μ-form)2(μ-npCOO)2 (npCOO- = 1,8-naphthyridine-2-carboxylate), were synthesized and characterized. The complexes absorb strongly throughout the ultraviolet (λmax = 300 nm, ε = 20 300 M-1 cm-1) and visible regions (λmax = 640 nm ε = 3500 M-1 cm-1), making them potentially useful new dyes with panchromatic light absorption for solar energy conversion applications. Ultrafast and nanosecond transient absorption and time-resolved infrared spectroscopies were used to characterize the identity and dynamics of the excited states, where singlet and triplet Rh2/form-to-naphthyridine, metal/ligand-to-ligand charge-transfer (ML-LCT) excited states were observed in all four complexes. The npCOO- complexes exhibit red-shifted absorption profiles extending into the near-IR and undergo photoinitiated electron transfer to generate reduced methyl viologen, a species that persists in the presence of a sacrificial donor. The energy of the triplet excited state of each complex was estimated from energy-transfer quenching experiments using a series of organic triplet donors (E(3ππ*) from 1.83 to 0.78 eV). The singlet reduction (+0.6 V vs Ag/AgCl) potentials, and singlet and triplet oxidation potentials (-1.1 and -0.5 V vs Ag/AgCl, respectively) were determined. Based on the excited-state lifetimes and redox properties, these complexes represent a new class of light absorbers with potential application as dyes for charge injection into semiconductor solar cells and in sensitizer-catalyst assemblies for photocatalysis that operate with irradiation from the ultraviolet to ∼800 nm.

A Binaphthyl-Based Scaffold for a Chiral Dirhodium(II) Biscarboxylate Ligand with α-Quaternary Carbon Centers

A chiral dirhodium(II) paddlewheel complex has been synthesized from biscarboxylate ligands derived from BINOL, and the resulting complex has been used in enantioselective carbene/alkyne cascade reactions. The ligand design was guided by requirements of α,α-dimethyl substituted carboxylates and bidentate ligands to ensure high levels of catalytic activity. Previously disclosed chiral complexes lack these features, resulting in low product yields. The design successfully replicated or exceeded the yields of the unusually effective achiral catalyst for the cascade reaction, Rh2(esp)2, which often shows unique reactivity. Promising enantioselectivity was observed for aldehyde-derived hydrazone substrates (29–96% ee), showing that the new scaffold has significant potential.

Exploring energetics of dirhodium paddlewheel complexes with π-ligands of different topologies

The first comprehensive theoretical exploration of the energetics of the interaction between π-ligands with isolated multiple carbon-carbon bonds (acetylene and ethylene), aromatic (benzene), planar polyaromatic (naphthalene, acenaphthalene, pyrene), curved polyaromatic (corannulene) and closed-cage C60-fullerene systems and Lewis acidic [Rh2(O2CCF3)4] complex has been carried out. Importantly, two energy decomposition schemes (EDA-NOCV and NBO-NEDA) that are based on very different approaches and algorithms were utilized in order to shed light on the energy and nature of bonding. The total bonding energy was sequentially represented as a combination of attractive covalent (ΔEorb and ECT in EDA and NBO approaches, respectively) and ionic (ΔEelstat and EEL), as well as repulsive (ΔEPauli and ECO) terms. This approach provides a balanced and complete picture of bonding between two (or more) interacting molecular fragments. All computational methods applied unambiguously indicate that the actual stability of adducts formed by unsaturated hydrocarbons with the selected Lewis acidic dirhodium complex is indeed a multifaceted phenomenon. Important contributions come from a different interplay between attractive (ΔEorbvs. ΔEelstat or ECTvs. EEL) and repulsive (ΔEPauli or ECO) components of the bonding and from variations in aromatic behavior of the selected π-systems.