Synthesis and properties of the tetrakis(trifluoromethyl)borate anion, [b(CF3)4]-: structure determination of Cs[B(CF3)4] by single-crystal X-ray diffraction

Lewis Acid Decorated Hexacyanodiborane(6) Dianion

First examples of diborane(6) dianions decorated with weakly coordination B(C6F5)3 (BCF) groups and SiEt3 + moieties have been synthesized demonstrating the synthetic potential of the [B2(CN)6]2- dianion. [B2{CNB(C6F5)3}6]2- (1) was isolated as potassium and tetrabutylammonium salt. 1 is a rare example for a weakly coordinating dianion and it was used for the stabilization of the carbocation [Ph3C]+ and the oxonium acid [H(OEt2)2]+. Reaction of [Ph3C]21 with HSiEt3 resulted in the silylated neutral diborane(6) [B2{CNB(C6F5)3}4(CNSiEt3)2] (2) in which two BCF groups have been selectively replaced by SiEt3 + substituents, underscoring the stability and chemical versatility of the [B2(CN)6]2- dianion. The chemical properties and physicochemical data of 1 and 2 provide insight into electronic, coordinating, and steric properties of theses novel diborane(6) compounds.

Nickel(II) Cyanoborates and Cyanoborate-Ligated Nickel(II) Complexes

Nickel(II) cyanoborates Ni[BH2(CN)2]2·H2O (1b·H2O), Ni[BH(CN)3]2·0.5H2O (1c·0.5H2O), and Ni[B(CN)4]2·0.5H2O (1d·0.5H2O) were synthesized, and their reactivity with respect to dppeO2 (=1,2-bis-(diphenylphosphinoethane dioxide)), pyNO (=pyridine-N-oxide), dppe (=1,2-bis-(diphenylphosphinoethane), and DMSO (=dimethyl sulfoxide) was examined. Using these ligands, either cyanoborate (CB) complex salts of [Ni(dppe)2]2+ (2b-d) and [Ni(pyNO)6]2+ (3c-d) were isolated or complexes [Ni(DMSO)4{NC-B(CN)3}2] (1dDMSO) and [Ni(dppeO2)2{NC-B(CN)3}2] (1ddppeO2) were formed. Salt metathesis of [Ni(dppe)Cl2] with alkali metal cyanoborates resulted in mono- and disubstituted coordination compounds [Ni(dppe){NC-BH(CN)2}Cl] (5c) and [Ni(dppe){NC-BH2CN)2}] (4b), which decomposed to salts 2b-d. The synthetical pathways explored offer convenient routes to nickel(II) cyanoborates, nickel(II) complexes ligated with cyanoborates, and nickel(II) complex salts of cyanoborates. Further, our studies demonstrate the diverse character of cyanoborates in coordination chemistry as noncoordinating counteranions and also as medium coordinating anions forming novel transition-metal complexes and salts.

Engineering considerations for practical lithium-air electrolytes

Lithium-air batteries promise exceptional energy density while avoiding the use of transition metals in their cathodes, however, their practical adoption is currently held back by their short lifetimes. These short lifetimes are largely caused by electrolyte breakdown, but despite extensive searching, an electrolyte resistant to breakdown has yet to be found. This paper considers the requirements placed on an electrolyte for it to be considered usable in a practical cell. We go on to examine ways, through judicious cell design, of relaxing these requirements to allow for a broader range of compounds to be considered. We conclude by suggesting types of molecules that could be explored for future cells. With this work, we aim to broaden the scope of future searches for electrolytes and inform new cell design.

Homoleptic Transition Metal Carbonyl Cations: Synthetic Approaches, Characterization and Follow-Up Chemistry

ConspectusCarbon monoxide, CO, is one of the most important ligands in organometallic chemistry. It is an excellent π-acceptor and a moderate σ-donor. Therefore, most of the known transition metal carbonyls (TMCs) exhibit a zerovalent or even negative metal oxidation state (OS) of up to -4. However, given the right conditions, the carbonyl ligand also forms homoleptic cationic complexes with one or more transition metal atoms, the transition metal carbonyl cations (TMCCs), known with an OS of up to +3. Despite their long-standing history upon discovery of the first [M(CO)6]+ examples (M = Mn, Tc, Re) by E. O. Fischer in 1962 as well as their very fundamental nature, it took until the 1990s for the scope to be widened by Aubke, Strauss and Willner. Yet, many potential TMCC entries known from gas-phase mass spectrometry work remained unknown on preparative grounds. This is due to their high reactivity, which puts scientists to new challenges and encourages the development of suitable solvents, anions and oxidants, to cope with the demands of these fundamental salts─later referred to as pseudo-gas-phase conditions and innocent deelectronators and solvents.Hence, the utilization of extremely weakly coordinating perfluorinated alkoxyaluminates [Al(ORF)4]- and [F{Al(ORF)3}2]- (ORF = -OC(CF3)3) in combination with the polar but non- or weakly coordinating innocent solvents 1,2-difluorobenzene (oDFB) and 1,2,3,4-tetrafluorobenzene (TFB) yielded the first TMCC salts containing heptacoordinate [M(CO)7]+ (M = Nb, Ta) as well as paramagnetic [M(CO)6]+· (M = Cr, Mo, W) or [Ni(CO)4]+·. However, the use of typical inorganic oxidants Ag+, [NO]+ and Ag+/0.5 I2 regularly led to unwanted side reactions. For example, the Lewis acidic silver(I) cations form Lewis pairs with various Lewis basic TMCs yielding partly clustered [Agx{TMC}y]x+ complex salts, while nitrosonium cations may substitute for carbonyl ligands, forming [M(CO)x-1(NO)]+ complexes. The synergistic oxidizing reagent Ag+/0.5 X2 can add halonium ions X+ to the TMCs (X = Cl, Br, I). This prevented the synthesis of univalent group 8 TMCC salts. Yet, the application of radical cation salts of perfluorinated arenes as innocent deelectronators finally yielded salts of [Fe(CO)5]+· and [M3(CO)14]2+ (M = Ru, Os).TMCC salts are excellent starting materials, and the reaction of [Co(CO)5]+ and [Ni(CO)4]+· with benzene led to the previously unknown bis(benzene) sandwich complexes [Co(benzene)2]+ and [Ni(benzene)2]+·. Under the right conditions, even the very weakly bound oDFB-complex salts with [M(oDFB)2]+ (M = Co, Ni) cations form, useful as naked metal(I) synthons and for small-molecule activation.

Cyano(fluoro)borate and cyano(hydrido)borate ionic liquids: low-viscosity ionic media for electrochemical applications

The synthesis and detailed characterization of low-viscosity room-temperature ionic liquids (RTILs) and [BnPh₃P]⁺ salts with the cyano(fluoro)borate anions [BF(CN)₃]^- (MFB), [BF₂(CN)₂]^- (DFB), and [BF₃(CN)]^- as well as the new mixed-substituted anion [BFH(CN)₂]^- (FHB) is described. The RTILs with [EMIm]⁺ or [BMPL]⁺ as countercations were obtained in yields of up to 98% from readily available alkali metal salts and in high purities that allow application in electrochemical devices. Trends in thermal stability, melting and freezing behavior, density, electrochemical stability, dynamic viscosity, specific conductivity and ion diffusivity have been assessed and compared to those of the related tetracyanoborate- and cyano(hydrido)borate-RTILs. The crystal structure analysis of the [BnPh₃P]⁺ salts of [BF_n(CN)_4-n]^- (n = 0-4), [BH_n(CN)_4-n]^- (n = 1-3) and [BFH(CN)₂]^- provided experimental access to anion volumina that together with ion molecular mass, electrostatic potential, shape and chemical stability have been correlated to physicochemical properties. In addition, the cytotoxicity of the [EMIm]⁺-ILs and potassium or sodium salts was studied.

Main group cyanides: from hydrogen cyanide to cyanido-complexes

Homoleptic cyanide compounds exist of almost all main group elements. While the alkali metals and alkaline earth metals form cyanide salts, the cyanides of the lighter main group elements occur mainly as covalent compounds. This review gives an overview of the status quo of main group element cyanides and cyanido complexes. Information about syntheses are included as well as applications, special substance properties, bond lengths, spectroscopic characteristics and computations. Cyanide chemistry is presented mainly from the field of inorganic chemistry, but aspects of chemical biology and astrophysics are also discussed in relation to cyano compounds.

Fluorinated Boron-Based Anions for Higher Voltage Li Metal Battery Electrolytes

Lithium metal batteries (LMBs) require an electrolyte with high ionic conductivity as well as high thermal and electrochemical stability that can maintain a stable solid electrolyte interphase (SEI) layer on the lithium metal anode surface. The borate anions tetrakis(trifluoromethyl)borate ([B(CF₃)₄]⁻), pentafluoroethyltrifluoroborate ([(C₂F₅)BF₃]⁻), and pentafluoroethyldifluorocyanoborate ([(C₂F₅)BF₂(CN)]⁻) have shown excellent physicochemical properties and electrochemical stability windows; however, the suitability of these anions as high-voltage LMB electrolytes components that can stabilise the Li anode is yet to be determined. In this work, density functional theory calculations show high reductive stability limits and low anion–cation interaction strengths for Li[B(CF₃)₄], Li[(C₂F₅)BF₃], and Li[(C₂F₅)BF₂(CN)] that surpass popular sulfonamide salts. Specifically, Li[B(CF₃)₄] has a calculated oxidative stability limit of 7.12 V vs. Li⁺/Li⁰ which is significantly higher than the other borate and sulfonamide salts (≤6.41 V vs. Li⁺/Li⁰). Using ab initio molecular dynamics simulations, this study is the first to show that these borate anions can form an advantageous LiF-rich SEI layer on the Li anode at room (298 K) and elevated (358 K) temperatures. The interaction of the borate anions, particularly [B(CF₃)₄]⁻, with the Li⁺ and Li anode, suggests they are suitable inclusions in high-voltage LMB electrolytes that can stabilise the Li anode surface and provide enhanced ionic conductivity.

Tris(pentafluoroethyl)difluorophosphorane: A Versatile Fluoride Acceptor for Transition Metal Chemistry

Fluoride abstraction from different types of transition metal fluoride complexes [Ln MF] (M=Ti, Ni, Cu) by the Lewis acid tris(pentafluoroethyl)difluorophosphorane (C2 F5 )3 PF2 to yield cationic transition metal complexes with the tris(pentafluoroethyl)trifluorophosphate counterion (FAP anion, [(C2 F5 )3 PF3 ]- ) is reported. (C2 F5 )3 PF2 reacted with trans-[Ni(iPr2 Im)2 (ArF )F] (iPr2 Im=1,3-diisopropylimidazolin-2-ylidene; ArF =C6 F5 , 1 a; 4-CF3 -C6 F4 , 1 b; 4-C6 F5 -C6 F4 , 1 c) through fluoride transfer to form the complex salts trans-[Ni(iPr2 Im)2 (solv)(ArF )]FAP (2 a-c[solv]; solv=Et2 O, CH2 Cl2 , THF) depending on the reaction medium. In the presence of stronger Lewis bases such as carbenes or PPh3 , solvent coordination was suppressed and the complexes trans-[Ni(iPr2 Im)2 (PPh3 )(C6 F5 )]FAP (trans-2 a[PPh3 ]) and cis-[Ni(iPr2 Im)2 (Dipp2 Im)(C6 F5 )]FAP (cis-2 a[Dipp2 Im]) (Dipp2 Im=1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) were isolated. Fluoride abstraction from [(Dipp2 Im)CuF] (3) in CH2 Cl2 or 1,2-difluorobenzene led to the isolation of [{(Dipp2 Im)Cu}2 ]2+ 2 FAP- (4). Subsequent reaction of 4 with PPh3 and different carbenes resulted in the complexes [(Dipp2 Im)Cu(LB)]FAP (5 a-e, LB=Lewis base). In the presence of C6 Me6 , fluoride transfer afforded [(Dipp2 Im)Cu(C6 Me6 )]FAP (5 f), which serves as a source of [(Dipp2 Im)Cu)]+ . Fluoride abstraction of [Cp2 TiF2 ] (7) resulted in the formation of dinuclear [FCp2 Ti(μ-F)TiCp2 F]FAP (8) (Cp=η5 -C5 H5 ) with one terminal fluoride ligand at each titanium atom and an additional bridging fluoride ligand.

Alkoxycyanoborates: metal salts and low-viscosity ionic liquids

Alkoxycyanoborates including low-viscosity ionic liquids which are promising materials in particular for electrochemical applications and Li[CH3OB(CN)3], which is a potential conducting salt, are presented.

Pentafluoroethylaluminates: A Combined Synthetic, Spectroscopic, and Structural Study

Salts of the tetrakis(pentafluoroethyl)aluminate anion [Al(C2 F5 )4 ]- were obtained from AlCl3 and LiC2 F5 . They were isolated with different counter-cations and characterized by NMR and vibrational spectroscopy and mass spectrometry. Degradation of the [Al(C2 F5 )4 ]- ion was found to proceed via 1,2-fluorine shifts and stepwise loss of CF(CF3 ) under formation of [(C2 F5 )4-n AlFn ]- (n=1-4) as assessed by NMR spectroscopy and mass spectrometry and supported by results of DFT calculations. In addition, the [(C2 F5 )AlF3 ]- ion was structurally characterized.

Improved Access to Organo-Soluble Di- and Tetrafluoridochlorate(I)/(III) Salts

A facile one-pot gram-scale synthesis of tetraalkylammonium tetrafluoridochlorate(III) [cat][ClF4 ] ([cat]=[NEt3 Me]+ , [NEt4 ]+ ) is described. An acetonitrile solution of the corresponding alkylammonium chloride salt is fluorinated with diluted fluorine at low temperatures. The reaction proceeds via the [ClF2 ]- anion which is structurally characterized for the first time. The potential application of [ClF4 ]- salts as fluorinating agents is evaluated by the reaction with diphenyl disulfide, Ph2 S2 , to pentafluorosulfanyl benzene, PhSF5 . The CN moieties in acetonitrile and [B(CN)4 ]- are transferred in CF3 groups. Exposure of carbon monoxide, CO, leads to the formation of carbonyl fluoride, COF2 , and elemental gold is dissolved under the formation of tetrafluoridoaurate [AuF4 ]- .

Innovative Syntheses of Cyano(fluoro)borates: Catalytic Cyanation, Electrochemical and Electrophilic Fluorination

Different types of high-yield, easily scalable syntheses for cyano(fluoro)borates Kt[BFn (CN)4-n ] (n=0-2) (Kt=cation), which are versatile building blocks for materials applications and chemical synthesis, have been developed. Tetrafluoroborates react with trimethylsilyl cyanide in the presence of metal-free Brønsted or Lewis acid catalysts under unprecedentedly mild conditions to give tricyanofluoroborates or tetracyanoborates. Analogously, pentafluoroethyltrifluoroborates are converted into pentafluoroethyltricyanoborates. Boron trifluoride etherate, alkali metal salts, and trimethylsilyl cyanide selectively yield dicyanodifluoroborates or tricyanofluoroborates. Fluorination of cyanohydridoborates is the third reaction type that includes direct fluorination with, for example, elemental fluorine, stepwise halogenation/fluorination reactions, and electrochemical fluorination (ECF) according to the Simons process. In addition, fluorination of [BH(CN)2 {OC(O)Et}]- to result in [BF(CN)2 {OC(O)Et}]- is described.

Boron: Its Role in Energy-Related Processes and Applications

Boron's unique position in the Periodic Table, that is, at the apex of the line separating metals and nonmetals, makes it highly versatile in chemical reactions and applications. Contemporary demand for renewable and clean energy as well as energy-efficient products has seen boron playing key roles in energy-related research, such as 1) activating and synthesizing energy-rich small molecules, 2) storing chemical and electrical energy, and 3) converting electrical energy into light. These applications are fundamentally associated with boron's unique characteristics, such as its electron-deficiency and the availability of an unoccupied p orbital, which allow the formation of a myriad of compounds with a wide range of chemical and physical properties. For example, boron's ability to achieve a full octet of electrons with four covalent bonds and a negative charge has led to the synthesis of a wide variety of borate anions of high chemical and electrochemical stability-in particular, weakly coordinating anions. This Review summarizes recent advances in the study of boron compounds for energy-related processes and applications.

Hydroxytricyanoborate Anion: Synthetic Aspects and Structural, Chemical, and Spectroscopic Properties

In recent years, salts of the hydridotricyanoborate anion [BH(CN)₃]^- (MHB) have become readily available. In spite of the unusually high stability of the MHB anion, it can be used as a valuable starting material for the preparation of selected tricyanoborates, for example, the boron-centered nucleophile B(CN)₃^2-. A further unprecedented example is the hydroxytricyanoborate anion [B(OH)(CN)₃]^- that is accessible by oxidation of (H₃O)MHB with elemental bromine in water. The Brønsted acid (H₃O)[B(OH)(CN)₃] was isolated as a crystalline solid. It serves as a versatile starting material for the synthesis of coordination compounds, metal salts, and ionic liquids. The [B(OH)(CN)₃]^- anion shows a rich coordination chemistry and a high tendency to form hydrogen-bonded motifs as demonstrated by a series of salts with different types of cations. Furthermore, the [B(OH)(CN)₃]^- anion itself serves as starting material for new tricyanoborates such as the unusual trianion [B{OB(CN)₃}₃]^3- and the silylated anions [B(OSiR₃)(CN)₃]^- (R = Me, Et, Ph). Some of these follow-up products have been characterized by single-crystal X-ray diffraction, e.g., [nBu₄N]₃[B{OB(CN)₃}₃] and [nBu₄N][B(OSiPh₃)(CN)₃].

Oxygen-Bridged Ga2 (Et)3 (OTeF5 )3 and the Weakly Coordinating Anions [Ga(Et)(OTeF5 )3 ]- and [Ga(OTeF5 )4 ]. Chemistry 2019;25:10441-10449. [PMID: 31090983 DOI: 10.1002/chem.201901651]

Salts of the weakly coordinating anions [Ga(OTeF₅ )₄ ]^- as well as [Ga(Et)(OTeF₅ )₃ ]^- and the neutral Ga₂ (Et)₃ (OTeF₅ )₃ were synthesized and characterized by spectroscopic methods and single-crystal X-ray diffraction. Ga₂ (Et)₃ (OTeF₅ )₃ was formed by treating GaEt₃ with pentafluoroorthotelluric acid (HOTeF₅ ) and reacted with PPh₄ Cl and CPh₃ Cl to [PPh₄ ][Ga(Et)(OTeF₅ )₃ ] and [CPh₃ ][Ga(Et)(OTeF₅ )₃ ]. In contrast, Ag[Ga(OTeF₅ )₄ ] was prepared from AgOTeF₅ and GaCl₃ and was used as a versatile starting material for further reactions. Starting with Ag[Ga(OTeF₅ )₄ ] the substrates [PPh₄ ][Ga(OTeF₅ )₄ ] and [CPh₃ ][Ga(OTeF₅ )₄ ] were formed from PPh₄ Cl and CPh₃ Cl.

The pentafluoroethyltrihydridoborate anion: from shock sensitive salts to stable room temperature ionic liquids

Salts of the first monoperfluoroalkylhydridoborate anion [C₂F₅BH₃]⁻, which is weakly hydridic, range from explosive metal salts to stable ionic liquids.

Taming the Cationic Beast: Novel Developments in the Synthesis and Application of Weakly Coordinating Anions

This Review gives a comprehensive overview of the most topical weakly coordinating anions (WCAs) and contains information on WCA design, stability, and applications. As an update to the 2004 review, developments in common classes of WCA are included. Methods for the incorporation of WCAs into a given system are discussed and advice given on how to best choose a method for the introduction of a particular WCA. A series of starting materials for a large number of WCA precursors and references are tabulated as a useful resource when looking for procedures to prepare WCAs. Furthermore, a collection of scales that allow the performance of a WCA, or its underlying Lewis acid, to be judged is collated with some advice on how to use them. The examples chosen to illustrate WCA developments are taken from a broad selection of topics where WCAs play a role. In addition a section focusing on transition metal and catalysis applications as well as supporting electrolytes is also included.

Perfluoroalkyltricyanoborate and Perfluoroalkylcyanofluoroborate Anions: Building Blocks for Low-Viscosity Ionic Liquids

The potassium perfluoroalkyltricyanoborates K[C_n F_2 n+1 B(CN)₃ ] [n=1 (1 d), 2 (2 d)] and the potassium mono(perfluoroalkyl)cyanofluoroborates K[C_n F_2 n+1 BF(CN)₂ ] [n=1 (1 c), 2 (2 c)] and [C_n F_2 n+1 BF₂ (CN)]^- [n=1 (1 b), 2 (2 b), 3 (3 b), 4 (4 b)] are accessible with perfect selectivities on multi-gram scales starting from K[C_n F_2 n+1 BF₃ ] and Me₃ SiCN. The K⁺ salts are starting materials for the preparation of salts with organic cations, for example, [EMIm]⁺ (EMIm=1-ethyl-3-methylimidazolium). These [EMIm]⁺ salts are hydrophobic room-temperature ionic liquids (RTILs) that are thermally, chemically and electrochemically very robust, offering electrochemical windows up to 5.8 V. The RTILs described herein, exhibit very low viscosities with a minimum of 14.0 mPa s at 20 °C for [EMIm]1 c, low melting points down to -57 °C for [EMIm]2 b and extraordinary high conductivities up to 17.6 mS cm^-1 at 20 °C for [EMIm]1 c. The combination of these properties makes these ILs promising materials for electrochemical devices as exemplified by the application of selected RTILs as component of electrolytes in dye-sensitised solar cells (DSSCs, Grätzel cells). The efficiency of the DSSCs was found to increase with a decreasing viscosity of the neat ionic liquid. In addition to the spectroscopic characterisation, single crystals of the potassium salts of the anions 1 b-d, 2 d, 3 b and 4 c as well as of [nBu₄ N]2 c have been studied by X-ray diffraction.

Syntheses, characterisation and solid-state study of alkali and ammonium BArF salts

Syntheses, characterisation in solution and solid-state studies of alkali and ammonium BArF salts are reported.

Syntheses of tricyanofluoroborates M[BF(CN)3] (M = Na, K): (CH3)3SiCl catalysis, countercation effect, and reaction intermediates

Potassium tricyanofluoroborate, K[BF(CN)3], which is the starting material for tricyanofluoroborate room-temperature ionic liquids [N. Ignat'ev et al. J. Fluorine Chem., submitted] was obtained on a molar scale (140 g) from Na[BF4] and (CH3)3SiCN with a purity of up to 99.9%. The initial product of the reaction that was catalyzed by (CH3)3SiCl was Na[BF(CN)3]·(CH3)3SiCN that was characterized by multinuclear NMR and vibrational spectroscopy, elemental analysis, differential scanning calorimetry, and single-crystal X-ray diffraction. Na[BF(CN)3]·(CH3)3SiCN was converted to K[BF(CN)3] via a simple extraction protocol. The catalytic effect of (CH3)3SiCl was evaluated and some intermediates of the reaction, including the isocyanoborate anion [BF(NC)(CN)2](-), were identified using multinuclear NMR and vibrational spectroscopy. K[BF2(CN)2] also reacted with (CH3)3SiCN in the presence of (CH3)3SiCl, to result in K[BF(CN)3]. The interpretation of the experimental observations was supported by data derived from density functional theory (DFT) calculations. In addition, the influence of selected countercations of the tetrafluoroborate anion on the progress of the (CH3)3SiCl-catalyzed reaction was studied. The fastest reaction was observed for Na[BF4], while the conversion of [BF4](-) to [BF(CN)3](-) was slower with the countercation K(+). Li[BF4] and [Et4N][BF4] were converted under the reaction conditions applied to Li[BF2(CN)2] and [Et4N][BF2(CN)2] only.

Structural diversity and spectral and thermal properties of the first alkaline earth metal tetracyanidoborates: [Mg(H2O)6][B(CN)4]2, [Mg(H2O)2][B(CN)4]2, [Mg(DMF)6][B(CN)4]2, [Ca(H2O)3][B(CN)4]2, and [Ca(H2O)2(CH3CN)][B(CN)4]2

The syntheses, structures, and spectral and thermal properties of the first tetracyanidoborate salts coordinating weakly to solvated alkaline earth cations are discussed.

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.