The coordination of azepine to transition-metal complexes: A DFT analysis

DFT calculations with full geometry optimizations have been carried out on a series of real and hypothetical compounds of the CpM(C₆NH₇) and (CO)₃M(C₆NH₇) (M = transition-metal) type. A rationalization of the bonding in all the known compounds and in hypothetical complexes is provided. Depending on the electron count and the nature of the metal, the azepine ligand can bind to the metal through the η¹, η², η⁴, η⁶, or η⁷ coordination mode.     

Preparation of planar chiral ferrocenes containing a fluorous alcohol function

Ferrocene reacts with hexafluoroacetone trihydrate in refluxing octane to afford >80% yields of [CpFe(η⁵-C₅H₄C(CF₃)₂OH)] (X-ray), carrying out the reactions at 180 °C gives an additional 5% yield of [Fe(η⁵-C₅H₄C(CF₃)₂OH)₂] (X-ray).The mono alcohol is lithiated with Bu^tOK/BuⁿLi/TMEDA affording partial conversion to mixtures of [CpFe(1,2-η⁵-C₅H₃C(CF₃)₂OH)(X)] and [Fe(η⁵-C₅H₄X)(1,2-η⁵-C₅H₃C(CF₃)₂OH)(X)] (X = SMe, CPh₂OH) upon reaction with Me₂S₂ or OCPh₂.For X = CPh₂OH both structures are crystallographically characterised.Enantiopure [CpFe(1,2-η⁵-C₅H₃C(CF₃)₂OH)(SMe)] can be prepared from (R)-[CpFe(η⁵-C₅H₄S(O)C₆H₄Me)] via [CpFe(1,2-η⁵-C₅H₃S(O)C₆H₄Me)(C(CF₃)₂OH)] (X-ray) or [CpFe(1,2-η⁵-C₅H₃S(O)C₆H₄Me)(SMe)].Related procedures allow the preparation of [CpFe(1,2-η⁵-C₅H₃CPh₂OH)(Y)] (Y = SMe, CHO (X-ray), C(CF₃)₂OH) and[CpFe(1,2-η⁵-C₅H₃C(CF₃)₂OH)(CHO)].     

Coordination chemistry of molybdenum relevant to hydrodenitrogenation: Reactivity of Mo(PMe3)6 towards 6-membered heterocyclic aromatic nitrogen compounds involving C-H bond cleavage and η-coordination

The reactivity of Mo(PMe₃)₆ towards 6-membered heterocyclic aromatic nitrogen compounds, namely pyridine, pyrazine, pyrimidine and triazine, has been investigated as part of an effort to define the coordination chemistry of molybdenum relevant to hydrodenitrogenation. For example, Mo(PMe₃)₆ reacts with pyridine to yield initially (η²-N,C-pyridyl)Mo(PMe₃)₄H, an uncommon example of an η²-pyridyl-hydride complex. The formation of (η²-N,C-pyridyl)Mo(PMe₃)₄H is reversible and treatment with PMe₃ regenerates Mo(PMe₃)₆ and pyridine. At elevated temperatures, (η²-N,C-pyridyl)Mo(PMe₃)₄H dissociates PMe₃ and converts to the η⁶-pyridine complex (η⁶-pyridine)Mo(PMe₃)₃. Pyrazine, pyrimidine and 1,3,5-triazine likewise react with Mo(PMe₃)₆ to yield (η²-N,C-pyrazinyl)Mo(PMe₃)₄H, (η²-N,C-pyrimidinyl)Mo(PMe₃)₄H and (η²-N,C-triazinyl)Mo(PMe₃)₄H, respectively. At elevated temperatures (η²-N,C-pyrazinyl)Mo(PMe₃)₄H and (η²-N,C-pyrimidinyl)Mo(PMe₃)₄H dissociate PMe₃ and convert to (η⁶-pyrazine)Mo(PMe₃)₃ and (η⁶-pyrimidine)Mo(PMe₃)₃ in which the heterocycle coordinates to molybdenum in an unprecedented η⁶-manner.     

Preparation,properties and crystal structures of nickel(II) complexes with acyclic schiff bases derived from 2,6-diformyl-4-chlorophenol and 1,5-diamino-3-thiapentane or 3,3′-diamino-N-methyl-dipropylamine

Nickel(II) complexes with the compartmental Schiff bases derived from 2,6-diformyl-4-chlorophenol and 1,5-diamino-3-thiapentane (H₂L¹) or 3,3′-diamino-N-methyl-dipropylamine (H₂L²) were synthesized, and the crystal structures of [Ni(L¹)- (py)₂] and [Ni(L²)(dmf)]·H₂0 were determined by X-ray crystallography.Ni(L¹)(py)₂ is monoclinic, space group C2/c, with a= 18.457(6), b = 11.116(7), c= 16.098(6) Å, and β = 115.79(5)°; D_c = 1.49 g cm⁻³ for Z = 4. The structure was refined to the final R of 6.9%. The molecule has C₂ symmetry. The nickel atom is six-coordinated octahedral. Selected bond lengths are: NiO 2.04(1) Å, NiN (L¹) 2.08(1) Å, NiN(py) 2.17(1) Å.[Ni(L²)(dmf)]·H₂O is monoclinic, space group P2₁/n, with a = 17.329(6), b = 13.322(7), c = 12.476(7) Å and β = 95.43(5)°; D_c = 1.45 g cm⁻³ for Z = 4. The structure was refined to the final R of 5.1%. The nickel atom is bonded in the octahedral geometry to the bianionic pentadentate ligand L² and to one molecule of dimethylformamide. Selected bond lengths are: NiO (charged) 2.063(3) Å (mean value), NiO (neutral) 2.120(3) Å, NiN (planar) 2.050(3) Å (mean value), NiN (tetrahedral) 2.177(3) Å.     

Hardness maximization or equalization? New insights and quantitative relations between hardness increase and bond dissociation energy

It has been overlooked that the change of hardness, η, upon bonding is intimately connected to thermochemical cycles, which determine whether hardness is increased according to Pearson’s “maximum hardness principle” (MHP) or equalized, as expected by Datta’s “hardness equalization principle” (HEP). So far the performances of these likely incompatible “structural principles” have not been compared. Computational validations have been inconclusive because the hardness values and even their qualitative trends change drastically and unsystematically at different levels of theory. Here I elucidate the physical basis of both rules, and shed new light on them from an elementary experimental source. The difference, Δη = η _mol – <η _at>, of the molecular hardness, η _mol, and the averaged atomic hardness, <η _at>, is determined by thermochemical cycles involving the bond dissociation energies D of the molecule, D ⁺ of its cation, and D ^? of its anion. Whether the hardness is increased, equalized or even reduced is strongly influenced by ΔD = 2D – D ⁺  ? D ^?. Quantitative expressions for Δη are obtained, and the principles are tested on 90 molecules and the association reactions forming them. The Wigner-Witmer symmetry constraints on bonding require the valence state (VS) hardness, η _VS, instead of the conventional ground state (GS) hardness, η _GS. Many intriguingly “unpredictable” failures and systematic shortcomings of said “principles” are understood and overcome for the first time, including failures involving exotic and/or challenging molecules, such as Be_2, B₂, O₃, and transition metal compounds. New linear relationships are discovered between the MHP hardness increase Δη _VS and the intrinsic bond dissociation energy D _i . For bond formations, MHP and HEP are not compatible, and HEP does not qualify as an ordering rule.     

Mercaptoethanesulfonic acid (CoM imitator) interaction studies with nickel(II) complexes of pyridyl groups containing tetradentate ligands: Synthesis, structure, spectra and redox properties

Nickel(II) complexes of N,N′-dimethyl-N,N′-bis(pyridyl-2yl-methyl)ethylene-diamine (L¹), N,N′-dimethyl-N,N′-bis(pyridyl-2-ylmethyl)-1,2-diaminopropane (L²) and N,N′-dimethyl-N,N′-bis(pyridyl-2-ylmethyl)-1,3-diaminopropane (L³) were prepared and their spectroscopic and redox properties studied. The distorted octahedral structure was determined for [NiL³ClCH₃OH](ClO₄) by using X-ray crystallography. The electronic spectral behavior of the complexes at different pHs was analyzed; it is shown that a new band grew at the expense of the other band intensity in acid media. The redox properties of ligands and their complexes show the peaks of Ni(II) → Ni(III) and Ni(II) → Ni(0) as these were detected at low concentration while Ni(II) → Ni(I) process was detectable clearly at high concentration. Furthermore, the interaction studies of 2-mercaptoethanesulfonic acid as a simulator of coenzyme M reductase (CoM) with NiN₄ chromophores are discussed.     

A new tetranuclear copper(II) complex with oximate bridges: Structure, magnetic properties and DFT study

A new tetranuclear complex, [Cu₄L₄](ClO₄)₄·2H₂O (1), has been synthesized from the self-assembly of copper(II) perchlorate and the tridentate Schiff base ligand (2E,3E)-3-(2-aminopropylimino) butan-2-one oxime (HL). Single-crystal X-ray diffraction studies reveal that complex 1 consists of a Cu₄(NO)₄ core where the four copper(II) centers having square pyramidal environment are arranged in a distorted tetrahedral geometry. They are linked together by a rare bridging mode (μ₃-η¹,η²,η¹) of oximato ligands. Analysis of magnetic susceptibility data indicates moderate antiferromagnetic (J₁ = −48 cm⁻¹, J₂ = −40 cm⁻¹ and J₃ = −52 cm⁻¹) exchange interaction through σ-superexchange pathways (in-plane bridging) of the oxime group. Theoretical calculations based on DFT technique have been used to obtain the energy states of different spin configurations and estimate the coupling constants and to understand the exact magnetic exchange pathways.     

Stabilization of dimethyliron(II) and methylcobalt(I) complexes bearing a hemilabile 2-benzoylpyridine ligand in octahedral and square-pyramidal coordination

Octahedral cis-Fe(CH₃)₂{2-(benzoyl)pyridyl-N,O}(PMe₃)₂ (1), square-pyramidal Co(CH₃){2-(benzoyl)pyridyl-N,O}(PMe₃)₂ (2), and triangular-planar Ni{2-(benzoyl)pyridyl-η²-C,O}(PMe₃)₂ (3) have been synthesized by reaction of 2-benzoylpyridine with thermally labile Fe(CH₃)₂(PMe₃)₄ and Co(CH₃)(PMe₃)₄ complexes. With Ni(CH₃)₂(PMe₃)₃, reductive elimination of ethane is observed when a η²-C,O-coordination is constituted. The complexes were investigated by NMR spectroscopic methods and the molecular structures of 1 and 2 were determined by X-ray crystallography.     

Coordination chemistry of half-open trozircenes: Adduct formation of [(η-C7H7)Zr(η-2,4-C7H11)] with isocyanides, phosphines and N-heterocyclic carbenes

The reactions of the half-open trozircene [(η⁷-C₇H₇)Zr(η⁵-2,4-C₇H₁₁)] (1) with the two-electron donor ligands tert-butyl isocyanide (CN-tBu), 1,2-bis(dimethylphosphino)ethane (dmpe), trimethylphosphine (PMe₃) and 1,3,4,5-tetramethylimidazolin-2-ylidene (IMe, :C[N(Me)C(Me)]₂) have led to the 1:1 adducts 3, 4, 5 and 6, respectively. The latter three were structurally characterized by X-ray diffraction analysis. Additionally, the stability of the adducts was probed by DFT calculations employing the B3LYP and M05-2X functionals showing that the strongly σ-basic N-heterocyclic carbene forms a thermodynamically much more stable adduct than the other three.     

Magnetic and structural studies of novel tetranickel hydroxamates

The dinickel(II) compound [Ni₂(μ-OAc)₂(OAc)₂(μ-H₂O)(asy·dmen)₂]·2.5H₂O, 1; undergoes facile reaction in a 1:2 molar ratio with benzohydroxamic acid (BHA) in ethanol to give the novel nickel(II) tetranuclear hydroxamate complex [Ni₄(μ-OAc)₃(μ-BA)₃(asy·dmen)₃][OTf]₂·H₂O, 2, in which the bridging acetates, bridging two nickel atoms in 1, undergo a carboxylate shift from the μ₂-η¹:η¹ bridging mode of binding to the μ₃-η¹:η² bridging three nickel atoms in the tetramer. The structure of complex 2 was determined by single-crystal X-ray crystallography. The two monodentate acetates, water and two bidentate bridging acetates of two moles of complex 1 are replaced by three monodentate bridging acetates and three benzohydroxamates. Three nickel atoms in the tetramer, Ni(2), Ni(3) and Ni(4) are in a N₂O₄ octahedral environment, while the fourth nickel atom Ni(1) is in an O(6) octahedral environment. The Ni-Ni separations are Ni(1)-Ni(2) = 3.108 Å, Ni(1)-Ni(3) = 3.104 Å and Ni(1)-Ni(4) = 3.110 Å, which are longer than previously studied in dinuclear urease inhibited models but shorter than in the nickel(II) tetrameric glutarohydroxamate complex [Ni₄(μ-OAc)₂(μ-gluA₂)₂(tmen)₄][OTf]₂, isolated and characterized previously in this laboratory. Magnetic studies of the tetrameric complex show that the four Ni(II) ions are ferromagnetically coupled, leading to a total ground spin state S_T = 4. Three analogous tetranuclear nickel hydroxamates were prepared from AHA and BHA and the appropriate dinuclear complex with either sy·dmen or asy·dmen as capping ligands.     

Electronic property of Group IV phthalocyanine dimers: SiPcMPc

A series of Group IV phthalocyanine (Pc) dimers, (n-C₆H₁₃)₃SiOSiPcOSiPcOSi(n-C₆H₁₃)₃ (SiPcSiPc), (n-C₆H₁₃)₃SiOSiPcOGePcOSi(n-C₆H₁₃)₃ (SiPcGePc), and (n-C₆H₁₃)₃SiOSiPcOSnPcOH (SiPcSnPc), was characterized by cyclic voltammetry and DFT calculation. Two oxidations and two reductions were observed for (n-C₆H₁₃)₃SiOSiPcOSiPcOSi(n-C₆H₁₃)₃ and (n-C₆H₁₃)₃SiOSiPcOGePcOSi(n-C₆H₁₃)₃, while there were two oxidations and three reductions for (n-C₆H₁₃)₃SiOSiPcOSnPcOH. The Pc with a bigger size of the central metal in one part of the dimeric compound is more difficult to be oxidized but it is easier to be reduced at the same time: i.e., both oxidation and reduction potentials showed a positive shift with the increase of the size of the central metal atom. Density functional theory was used to optimize the structures of the Pc dimers and to understand the electrochemical properties. The optimized structures of HOSiPcOSiPcOH, HOSiPcOGePcOH and HOSiPcOSnPcOH as model compounds for SiPcSnPc, SiPcGePc, SiPcSiPc, respectively, show that all the Pc dimers are staggered, the plane-to-plane distances are 3.394, 3.538 and 3.722 Å, respectively. Tin generates a saddle-type structure of phthalocyanine, but silicon or germanium does not greatly distort the ring structure, and yields a planar ring structure. A large plane-to-plane distance and a high degree of plane distortion yield a red-shift of Q-band, a low ring current, high oxidation and low reduction potentials and high ionization energies.     

Stabilization of a hydrated ketone by metal complexation. The crystal and molecular structures of bis-2,2′,N,N′-bipyridyl ketone-hydrate nickel(II) sulfate,copper(II) chloride and copper(II) nitrate

The crystal structure of the complexes (I)Ni[C₁₁N₈N₂(OH)₂]₂SO₄, (II) Cu[C₁₁H₈N₂(OH)₂]₂Cl₂· 4H₂O and (III) Cu[C₁₁H₈N₂(OH)₂]₂(NO₃)₂·2H₂O have been determined by three-dimensional X-ray analysis methods. Crystal data are: (I), monoclinic, space group C2/c, Z = 4, a = 19.666(4), b = 7.994(2), c = 16.045(6) /rA, /gb = 111.231(9)°, (II), monoclinic, space group C2/c, Z = 4, a = 14.504(4), b = 12.333(8), c = 14.630(3) Å, /gb = 90.92°; and (IIl), monoclinic, space group P2₁/n, Z = 2, a = 7.601(5), b = 11.977(4), c = 14.463(6) Å, β = 93.10(8)°. These structural investigations clearly demonstrate that in each case hydration occurs across the ketone double bond in the ligand and that the resulting hydroxyl group coordinates to the metal. Two di-2-pyridyl ketone ligands are thus bonded to the metal atom in a tridentate fashion. In the nickel complex (I), all six coordination interactions appear to have approximately the same strength. However, in the copper complexes (II) and (III), the pyridyl nitrogens are strongly coordinating to the metal in the equatorial plane, while the hydroxyl groups are more weakly coordinating in the axial direction. The metal to ligand bond distances are: (I) d_Ni−O = 2.098(4), d_NiN = 2.062(4), 2.087(4) Å, (II) d_CuO = 2.465(5), d_CuN = 1.994(5), 2.006(5) Å, (III) d_CuO = 2.464(5), d_CuN = 1.990(5), 2.036(5) Å. The neutral diol that results from hydrolysis of di-2-pyridyl ketone is stabilized by coordination to the metal and such coordination is little affected by changes in the metal, the anion or the extent of hydration.     

Synthesis of cyclopentadienyl ruthenium(II) complexes containing tethered functionalities

The reaction of [C₅H₄(CH₂)_nX]Tl (1: n = 2, X = NMe₂, OMe, CN; n = 3, X = NMe₂) with [(η⁶-C₆H₆)RuCl(μ-Cl)]₂, 2, afforded the sandwich compounds [{η⁵-C₅H₄(CH₂)_nX}Ru(η⁶-C₆H₆)]PF₆, 3, and [η⁵-C₅H₄(CH₂)_nX]₂Ru, 4. Photolytic cleavage of 3 in acetonitrile afforded the tethered products [{η⁵:κN-C₅H₄(CH₂)_nX}Ru(CH₃CN)₂]PF₆, 5.     

The reaction between orthodiphenylphosphinobenzaldehyde and [(η-C5Me5)MCl(μ-Cl)]2 (M=Rh and Ir)

The benzaldehyde functionalized phosphine Ph₂PC₆H₄CHO-2 underwent reaction with [(η⁵-C₅Me₅)MCl(μ-Cl)]₂ (M=Rh, Ir) to form (η⁵-C₅Me₅)MCl₂(κP-Ph₂PC₆H₄CHO-2), which underwent activation of the aldehyde C-H bond to form (η⁵-C₅Me₅)MCl(κP,κC-Ph₂PC₆H₄CO-2). Formally the reaction involves oxidative addition of C-H across the metal and reductive elimination of HCl. The structure of (η⁵-C₅Me₅)RhCl(κP,κC-Ph₂PC₆H₄CO-2) has been determined by single-crystal X-ray diffraction.     

Anionic ruthenium(II) alkyls with ancillary diene and dienyl ligands: Synthesis and structures of [(η,η-C7H8)RuMe4] and [(η,η-C8H11)RuMe3]

Treatment of the ruthenium(II) diene complexes [(η²,η²-nbd)RuCl₂]_n or [(η²,η²-cod)RuCl₂]_n with 4 equiv. of methyllithium in the presence of N,N,N′,N′-tetramethylethylenediamine (tmed) yields the methyl complexes [Li(tmed)]₂[(η²,η²-nbd)RuMe₄] (1) and [Li(tmed)]₂[(η³,η²-C₈H₁₁)RuMe₃] (2), respectively, where nbd = norbornadiene and cod = 1,5-cyclooctadiene. In the latter compound, the cyclooctadiene ligand has been deprotonated to afford a η³,η²-1,2,3:5,6-cyclooctadienyl group. Both complexes were studied by ¹H and ¹³C{¹H} NMR spectroscopy, and the crystal structure of 2 was determined. One lithium atom in 2 is four-coordinate and bridges between one ruthenium-bound methyl group and one of the wingtip allylic carbon atoms in the η³,η²-C₈H₁₁ ligand. The other lithium atom is five-coordinate, and forms contacts with the other two Ru-Me groups and with the other wingtip carbon atom of the allyl unit.     

Sheet and chain polymeric transition metal complexes formed by N,N-o-phenylenedimethylenebis(pyridin-4-one)

The preparations are reported of the ‘extended reach’ ligand N,N^′-o-phenylene-dimethylenebis(pyridin-4-one) (o-XBP4) and of a range of its metal complexes with Mn(II), Co(II), Ni(II), Cu(II) and Zn(II), two of which have been shown by X-ray studies to have polymeric structures. In the compound [Mn(o-XBP4)(H₂O)₂(NO₃)](NO₃) the o-XBP4 ligands link ‘Mn(H₂O)₂(NO₃)’ units into chains which are then cross-linked into sheets by the bridging action of the coordinated nitrate. In [Cu(o-XBP4)(NO₃)₂] chains are also formed by the bridging action of the o-XBP4 ligands but here they simply pack trough-in-trough with no nitrate cross-linking. X-band EPR spectra are reported for these and the other Mn and Cu compounds as are relevant spectroscopic results for the other complexes.     

Preparation of cobalt sandwich diphosphine ligand [(η-C5H4Pr)Co(η-C4(PPh2)2Ph2)] and its chelated palladium complex: Application of diphosphine ligand in the preparation of mono-substituted ferrocenylarenes

The reaction of (η⁵-C₅H₄ⁱPr)Co(PPh₃)₂ with PhCCPPh₂ furnished two isomeric cyclobutadiene-substituted Cp′CoCb diphosphines, [(η⁵-C₅H₄ⁱPr)Co(η⁴-1,2-(PPh₂)₂C₄Ph₂)] (5-cis) and [(η⁵-C₅H₄ⁱPr)Co(η⁴-1,3-(PPh₂)₂C₄Ph₂)] (5-trans). Further reaction of 5-cis with one molar equivalent of Pd(COD)Cl₂ gave palladium complex [(η⁵-C₅H₄ⁱPr)Co(η⁴-1,2-(PPh₂)₂C₄Ph₂)-PdCl₂] (6) in good yield. Both of the molecular structures of 5-cis and 6 were determined by single-crystal X-ray diffraction methods. Unexpectedly, the palladium complex 6 was found to be more efficient than the combination of the commonly used Buchwald’s ligand, biphenyl-2-yl-di-tert-butyl-phosphane, with Pd(OAc)₂ as the catalytic precursor in the Suzuki-Miyaura reaction between ferroceneboronic acid and 4-bromoaldehyde. The X-ray structural analysis and DFT study of several palladium complexes containing sandwich-type diphosphine chelating ligands revealed that the variations in bite angles are much larger than those in bite distances. The more energetically favorable conformation in the Pd(II) complexes is the one with bite angle close to 90°.     

Experimental and computational studies of two new mono- and dinuclear iridium complexes containing a Buchwald biphenyl phosphine ligand

The reaction of [(η⁴-1,5-C₈H₁₂)₂Ir₂(μ-Cl)₂] with 2-di-t-butylphosphino-2′-methylbiphenyl (t-Bu₂Pbiph^Me) in the presence of AgBF₄ afforded the dichlorido-bridged Ir–Ag complex [(η⁴-1,5-C₈H₁₂)Ir(μ-Cl)₂Ag(t-Bu₂Pbiph^Me)] (1) which was fully characterized by a single crystal X-ray diffraction study. Sequential treatment of the diiridium precursor first with the silver salt and then with the phosphine yielded cyclometalated [(η⁴-1,5-C₈H₁₂)Ir(t-Bu₂Pbiph^Me–H⁺)] (2). Detailed DFT calculations gave evidence that the phosphine ligand of 2 forms a strained four-membered iridaheterocycle through orthometalation rather than a sterically congested six-membered chelate structure through C–H activation on the remote phenyl ring. The phosphonium salt [t-Bu₂P(H)biph^Me]BF₄ was isolated as a by-product of the preparations of 1 and 2; its crystal structure was determined.     

EPR study of the oxidation reaction of nickel(0) phosphine complexes with Lewis and Brønsted acids

The oxidation of Ni(PPh₃)₄ with BF₃ · OEt₂, H₃CCOOH, and F₃CCOOH, and that of (PPh₃)₂Ni(C₂H₄) with BF₃ · OEt₂ is studied by EPR spectroscopy. The reaction of the Ni(0) complexes with BF₃ · OEt₂ gives Ni(II) complexes with which they react to form Ni(I) compounds with covalent Ni-F and Ni-B bonds that transform with excess BF₃ · OEt₂ into cationic paramagnetic Ni(I) complexes. Acetic acid also adds oxidatively to Ni(PPh₃)₄ to form a Ni(II) complex that reacts further to give Ni(I) hydride and carboxylate complexes. The Ni(I) hydride is transformed by the acid into the Ni(I) carboxylate with release of hydrogen, the amount of which depends on the rate of acid addition. The following Ni(I) complexes are identified in the reaction medium: [Ni(PPh₃)₃]BF₄, [(PPh₃)₂Ni(OEt₂)]BF₄, [(PPh₃)Ni(OEt₂)_n]BF₄, (PPh₃)₂NiBF₂, (PPh₃)₃NiOOCCH₃, and [(PPh₃)₂Ni(OEt₂)P(OEt)₃]BF₄. Oxidation schemes of Ni(0) complexes by Lewis and Brønsted acids are given.