σ-Bonded Metal Carbonyl Cations and Their Derivatives: Syntheses and Structural, Spectroscopic, and Bonding Principles

[GeRu6(CO)18HI]: A Germanium-Centered Ruthenium Carbonyl Cluster with Aromatic Ring Current

The carbonyl cluster compound [GeRu6(CO)18HI] is unique in regard to its structure and bonding with a GeRu6 cluster core, a planar GeRu4HI unit, extensive multi-center bonding, and an aromatic ring current similar to benzene (9-10 nA T-1). The open-shell cluster core is a Ge-centered five-membered Ru4(Ru2) ring with CO ligands and an additional H and I atom, each bridging two Ru atoms on opposite sides of the cluster core. The compound is prepared at 130 °C in a weakly-coordinating ionic liquid.

Terminal end-on coordination of dinitrogen versus isoelectronic CO: A comparison using the charge displacement analysis

The metal dinitrogen bonding in a wide series of terminal end-on dinitrogen complexes is investigated with the charge displacement analysis based on natural orbitals of chemical valence (CD-NOCV). The effect of the σ donation and π backdonation on the NN bond are discussed and compared with the observations for a series of carbonyl complexes, published in 2016 by Tarantelli et al. The σ donation is relative invariant over the series of dinitrogen complexes and has no significant effect on the NN bond strength, whereas the π backdonation causes a considerable elongation of the NN bond. Some uncommon examples of weakly bound dinitrogen with blue-shifted stretching frequency compared to free N2 (ν = 2330 cm-1 ) are known. The dinitrogen bonding in these complexes is simulated with a point charge. Apparently, electrostatics account for the shortened N─N bond in these systems.

Metal-CO Bonding in Mononuclear Transition Metal Carbonyl Complexes

DFT calculations have been carried out for coordinatively saturated neutral and charged carbonyl complexes [M(CO) _n ] ^q where M is a metal atom of groups 2-10. The model compounds M(CO)₂ (M = Ca, Sr, Ba) and the experimentally observed [Ba(CO)]⁺ were also studied. The bonding situation has been analyzed with a variety of charge and energy partitioning approaches. It is shown that the Dewar-Chatt-Duncanson model in terms of M ← CO σ-donation and M → CO π-backdonation is a valid approach to explain the M-CO bonds and the trend of the CO stretching frequencies. The carbonyl ligands of the neutral complexes carry a negative charge, and the polarity of the M-CO bonds increases for the less electronegative metals, which is particularly strong for the group 4 and group 2 atoms. The NBO method delivers an unrealistic charge distribution in the carbonyl complexes, while the AIM approach gives physically reasonable partial charges that are consistent with the EDA-NOCV calculations and with the trend of the C-O stretching frequencies. The AdNDP method provides delocalized MOs which are very useful models for the carbonyl complexes. Deep insight into the nature of the metal-CO bonds and quantitative information about the strength of the [M] ← (CO)₈ σ-donation and [M(d)] → (CO)₈ π-backdonation visualized by the deformation densities are provided by the EDA-NOCV method. The large polarity of the M-CO π orbitals toward the CO end in the alkaline earth octacarbonyls M(CO)₈ (M = Ca, Sr, Ba) leads to small values for the delocalization indices δ(M-C) and δ(M···O) and significant overlap between adjacent CO groups, but the origin of the charge migration and the associated red-shift of the C-O stretching frequencies is the [M(d)] → (CO)₈ π-backdonation. The heavier alkaline earth metals calcium, strontium and barium use their s/d valence orbitals for covalent bonding. They are therefore to be assigned to the transition metals.

Chasing the Mond Cation: Synthesis and Characterization of the Homoleptic Nickel Tetracarbonyl Cation and its Tricarbonyl-Nitrosyl Analogue

130 years after Mond discovered the first homoleptic carbonyl complex Ni(CO)₄, we report on a [Ni(CO)₄]^.+ salt as the first synthesis of any homoleptic nickel carbonyl cation in the condensed phase. It was prepared by oxidation of nickel metal with the synergistic oxidant Ag[F{Al(OR^F)₃}₂]/0.5 I₂ (R^F=C(CF₃)₃) in CO atmosphere. This D_2d‐symmetric metalloradical represents the last missing entry among the structurally characterized homoleptic carbonyl cations of Groups 6 to 11. Additionally, the nickel tricarbonyl‐nitrosyl cation [Ni(CO)₃(NO)]⁺ was obtained by usage of NO[F{Al(OR^F)₃}₂] and all products were fully characterized by means of IR, Raman, NMR/EPR, single crystal and powder XRD.

Altering Charges on Heterobimetallic Transition-Metal Carbonyl Clusters

The homoleptic group 5 carbonylates [M(CO)6 ]- (M=Nb, Ta) serve as ligands in carbonyl-terminated heterobimetallic Agm Mn clusters containing 3 to 11 metal atoms. Based on our serendipitous [Ag6 {Nb(CO)6 }4 ]2+ (4 a2+ ) precedent, we established access to such Agm Mn clusters of the composition [Agm {M(CO)6 }n ]x (M=Nb, Ta; m=1, 2, 6; n=2, 3, 4, 5; x=1-, 1+, 2+). Salts of those molecular cluster ions were synthesized by the reaction of [NEt4 ][M(CO)6 ] and Ag[Al(ORF )4 ] (RF =C(CF3 )3 ) in the correct stoichiometry in 1,2,3,4-tetrafluorobenzene at -35 °C. The solid-state structures were determined by single-crystal X-ray diffraction methods and, owing to the thermal instability of the clusters, a limited scope of spectroscopic methods. In addition, DFT-based AIM calculations were performed to provide an understanding of the bonding within these clusters. Apparently, the clusters 3+ (m=6, n=5) and 42+ (m=6, n=4) are superatom complexes with trigonal-prismatic or octahedral Ag6 superatom cores. The [M(CO)6 ]- ions then bind through three CO units as tridentate chelate ligands to the superatom core, giving overall structures related to tetrahedral AX4 (42+ ) or trigonal bipyramidal AX5 molecules (3+ ).

Synthesis and characterization of crystalline niobium and tantalum carbonyl complexes at room temperature

A variety of homoleptic transition metal carbonyl complexes are known as bulk compounds for group 7-12 metals. These metals typically feature a maximum of 6 CO ligands to form complexes with 18 valence electrons. In contrast, group 3-5 metals, with fewer valence electrons, have been shown to form highly coordinated heptacarbonyl and octacarbonyl complexes-although they were only identified by gas-phase mass spectrometry and/or matrix isolation spectroscopy work. Now we have prepared heptacarbonyl cations of niobium and tantalum as crystalline salts that are stable at room temperature. The [M(CO)₇]⁺ (M = Nb or Ta) complexes were formed by the oxidation of [M(CO)₆]^- with 2Ag⁺[Al(OR^F)₄]^- (R^F, C(CF₃)₃) under a CO atmosphere; their experimental characterization was supported by density functional theory calculations. Other unusual carbonyl compounds were also synthesized: two isostructural salts that contained the 84-valence-electron cluster cation [Ag₆{Nb(CO)₆}₄]²⁺, the piano-stool complexes [(1,2-F₂C₆H₄)M(CO)₄]⁺ and two polymorphs of neutral Ta₂(CO)₁₂ with a long, unsupported Ta-Ta bond.

Completing the triad: synthesis and full characterization of homoleptic and heteroleptic carbonyl and nitrosyl complexes of the group VI metals

Oxidation of M(CO)6 (M = Cr, Mo, W) with the synergistic oxidative system Ag[WCA]/0.5 I2 yields the fully characterized metalloradical salts [M(CO)6]+˙[WCA]- (weakly coordinating anion WCA = [F-{Al(ORF)3}2]-, RF = C(CF3)3). The new metalloradical cations with M = Mo and W showcase a similar structural fluxionality as the previously reported [Cr(CO)6]+˙. Their reactivity increases from M = Cr < Mo < W and their syntheses allow for in-depth insights into the properties of the group 6 carbonyl triad. Furthermore, the reaction of NO+[WCA]- with neutral carbonyl complexes M(CO)6 gives access to the heteroleptic carbonyl/nitrosyl cations [M(CO)5(NO)]+ as salts of the WCA [Al(ORF)4]-, the first complete transition metal triad of their kind.

Stable salts of the hexacarbonyl chromium(I) cation and its pentacarbonyl-nitrosyl chromium(I) analogue

Homoleptic carbonyl radical cations are a textbook family of complexes hitherto unknown in the condensed phase, leaving their properties and applications fundamentally unexplored. Here we report on two stable 17-electron [Cr(CO)6]•+ salts that were synthesized by oxidation of Cr(CO)6 with [NO]+[Al(ORF)4]- (RF = C(CF3)3)) in CH2Cl2 and with removal of NO gas. Longer reaction times led to NO/CO ligand exchange and formation of the thermodynamically more stable 18-electron species [Cr(CO)5(NO)]+, which belongs to the family of heteroleptic chromium carbonyl/nitrosyl cations. All salts were fully characterized (IR, Raman, EPR, NMR, scXRD, pXRD, magnetics) and are stable at room temperature under inert conditions over months. The facile synthesis of these species enables the thorough investigation of their properties and applications to a broad scientific community.

How does the Environment Influence a Given Cation? A Systematic Investigation of [Co(CO)5 ]+ in Gas Phase, Solution, and Solid State

IR spectroscopic studies of the gaseous metal carbonyl cations [Co(CO)₅ ]⁺ ⋅mCO (m=1-4) indicated that the weakly bound CO molecules in a second coordination sphere perturb the structure of [Co(CO)₅ ]⁺ causing the CO stretching frequencies ν(CO) to become noticeably redshifted. In this work, we aimed to establish the relationship between such gas phase IR spectra and those recorded in condensed phases, either as a solid salt or as a solution in the weakly basic solvent o-difluorobenzene. For this purpose, a series of [Co(CO)₅ ]⁺ [WCA]^- salts (WCA=weakly coordinating anion), with the counterions varying between more coordinating (WCA=F-Al(OR^F )₃ , (R^F O)₃ Al-F-Al(F)(OR^F )₂ ; R^F =C(CF₃ )₃ ) and almost non-coordinating (WCA=Al(OR^F )₄ , F{Al(OR^F )₃ }₂ ), were synthesized and characterized by vibrational spectroscopy as well as X-ray structure analysis. The experimental spectra differ considerably from that of the undisturbed gaseous [Co(CO)₅ ]⁺ ion, as the structural deformation of [Co(CO)₅ ]⁺ requires very little energy. Together with previously reported data, the perturbed condensed phase [Co(CO)₅ ]⁺ ions were analyzed and compared with the gaseous [Co(CO)₅ ]⁺ ⋅mCO ions. DFT calculations were performed on simply adapted [Co(CO)₅ ]⁺ structures to allow the assignment of all the ν(CO) modes and a qualitative interpretation of structural deformations by external influences as a function of the environment (ligands, solvation, crystal packing). The analysis showed that especially the degenerate E' mode ν_e and the averaged asymmetric equatorial CO stretch ν ‾ _e , which originates from a split of the E' mode, are a function of the interaction with the environment. Whereas for the more coordinating counterions ν ‾ _e values of 2112-2120 cm^-1 were obtained, for the less coordinating counterions ν ‾ _e values of up to 2133 cm^-1 were found, which is very close to that of gaseous [Co(CO)₅ ]⁺ ⋅4CO, with a ν ‾ _e value of 2135 cm^-1 . This indicates the possibility of approximating the gas phase conditions in the condensed phases with the [Co(CO)₅ ]⁺ ion probably being the prototypical probe molecule for investigating the strengths of interactions in different environments.

From an Easily Accessible Pentacarbonylcobalt(I) Salt to Piano-Stool Cations [(arene)Co(CO)2 ]. Chemistry 2017;23:14658-14664. [PMID: 28796933 DOI: 10.1002/chem.201703589]

The facile synthesis of a pentacarbonyl cobalt(I) salt without the need for a superacid as solvent is presented. This salt, [Co(CO)₅ ]⁺ [Al(OR^F )₄ ]^- {R^F =C(CF₃ )₃ }, readily accessible on a multigram scale, undergoes substitution reactions with arenes yielding the hitherto unknown class of two-legged cobalt piano-stool complexes [(arene)Co(CO)₂ ]⁺ with four different arene ligands. Such a substitution chemistry would have been impossible in superacid solution, as the arenes used would have been oxidized and/or protonated. Thus, the general approach described herein may have a wide synthetic use. Additionally, the thermochemistry of the piano-stool complexes is shown to be not easy to describe computationally and most of the established DFT methods overestimate the reaction energies. Only CCSD(T) calculations close to the basis set limit gave energies fully agreeing with the experiment.

An element through the looking glass: exploring the Au-C, Au-H and Au-O energy landscape

Gold, the archetypal "noble metal", used to be considered of little interest in catalysis. It is now clear that this was a misconception, and a multitude of gold-catalysed transformations has been reported. However, one consequence of the long-held view of gold as inert metal is that its organometallic chemistry contains many "unknowns", and catalytic cycles devised to explain gold's reactivity draw largely on analogies with other transition metals. How realistic are such mechanistic assumptions? In the last few years a number of key compound classes have been discovered that can provide some answers. This Perspective attempts to summarise these developments, with particular emphasis on recently discovered gold(iii) complexes with bonds to hydrogen, oxygen, alkenes and CO ligands.

A hexanuclear gold carbonyl cluster

The hexanuclear gold carbonyl cluster [PPh4]2[Au6(CF3)6Br2(CO)2] (4) has been obtained by spontaneous self-assembly of the following independent units: CF3AuCO (1) and [PPh4][Br(AuCF3)2] (3). The cyclo-Au6 aggregate 4, in which the components are held together by unassisted, fairly strong aurophilic interactions (Au···Au ∼310 pm), exhibits a cyclohexane-like arrangement with chair conformation. These aurophilic interactions also result in significant ν(CO) lowering: from 2194 cm-1 in the separate component 1 to 2171 cm-1 in the mixed aggregate 4. Procedures to prepare the single-bridged dinuclear component 3 as well as the mononuclear derivative [PPh4][CF3AuBr] (2) are also reported.

A systematic investigation of coinage metal carbonyl complexes stabilized by fluorinated alkoxy aluminates

The reaction of Cu(I), Ag(I), and Au(I) salts with carbon monoxide in the presence of weakly coordinating anions led to known and structurally unknown non-classical coinage metal carbonyl complexes [M(CO)n][A] (A = fluorinated alkoxy aluminates). The coinage metal carbonyl complexes [Cu(CO)n(CH2Cl2)m](+)[A](-) (n = 1, 3; m = 4-n), [Au2(CO)2Cl](+)[A](-), [(OC)nM(A)] (M = Cu: n = 2; Ag: n = 1, 2) as well as [(OC)3Cu⋅⋅⋅ClAl(OR(F))3] and [(OC)Au⋅⋅⋅ClAl(OR(F))3] were analyzed with X-ray diffraction and partially IR and Raman spectroscopy. In addition to these structures, crystallographic and spectroscopic evidence for the existence of the tetracarbonyl complex [Cu(CO)4](+)[Al(OR(F))4](-) (R(F) = C(CF3)3) is presented; its formation was analyzed with the help of theoretical investigations and Born-Fajans-Haber cycles. We discuss the limits of structure determinations by routine X-ray diffraction methods with respect to the C-O bond lengths and apply the experimental CO stretching frequencies for the prediction of bond lengths within the carbonyl ligand based on a correlation with calculated data. Moreover, we provide a simple explanation for the reported, partly confusing and scattered CO stretching frequencies of [Cu(I)(CO)n] units.

Isolable, copper(I) dicarbonyl complexes supported by N-heterocyclic carbenes

Cationic copper(I) dicarbonyl complexes supported by N-heterocyclic carbene ligands, SIPr and IPr*, have been synthesized. [(SIPr)Cu(CO)(2)][SbF(6)] and [(IPr*)Cu(CO)(2)][SbF(6)] have a trigonal planar, three-coordinate copper atom with an average Cu-CO distance of 1.915 Å and display C-O stretching frequencies higher than that of the free CO (2143 cm(-1)). The high CO stretching frequencies suggest that the Cu(I)-CO interaction in these cationic adducts is dominated by electrostatic and OC → Cu σ-donor components. [(SIPr)Cu(CO)(2)][SbF(6)] and [(IPr*)Cu(CO)(2)][SbF(6)] readily form the corresponding [(SIPr)Cu(CO)(H(2)O)][SbF(6)] and [(IPr*)Cu(CO)(H(2)O)][SbF(6)] with loss of a CO even with traces of water, but they can be converted back to the dicarbonyl adducts using excess CO. The synthesis and structure of [(IPr*)Cu(H(2)O)][SbF(6)] are also reported. It is a two-coordinate copper adduct with a Cu-O distance of 1.874(2) Å. It reacts with excess CO to form [(IPr*)Cu(CO)(2)][SbF(6)].

Octahedral perfluoroalkyl complexes of Ir(III) formed by oxidative addition of perfluoroalkyl iodides to Ir(acac)(CO)2

Oxidative addition of primary, secondary, or benzylic perfluoroalkyl iodides (RF–I) to the phosphine free Ir(I) precursor Ir(acac)(CO)2 1 (acac = 2,4-pentanedionato) proceeds smoothly to afford octahedral Ir(III) products Ir(acac)(I)(RF)(CO)2, A combination of X-ray crystallographic studies and solution spectroscopy shows that these products are the result of overall trans-addition of the C–I bond to iridium, probably a result of thermodynamic control; evidence for a kinetic product resulting from net cis-addition is obtained in one case. Treatment of the Ir(III) compounds with AgOTf (Tf = CF3SO3) illustrates that the iodo ligand is replaced by triflate with retention of stereochemistry at Ir. The resulting triflate complexes are inert to displacement by H2O or H2. The Ir(III) products exhibit very high CO stretching frequencies in the IR, indicating that the CO ligands may be non-classical. A quantitative estimation of the degree of backbonding to the CO ligands in these compounds, and a comparison of the π-acceptor properties of CO and fluoroalkyl ligands, is made using an approach based on Density Functional Theory (DFT) and Natural Bond Orbital analyses.Key words: iridium, fluoroalkyl, oxidation, carbonyl, DFT.