Studien zur CH-aktivierung: II. Der einfluss der liganden auf die bildung von aryl(hydrido)ruthenium(II)- und -osmium(II)-komplexen aus aromaten

The complexes trans-MCl₂(PMe₃)₄ (M = Ru, Os) react with CO and P(OMe)₃ to give the mono- and disubstituted derivatives trans,mer-MCl₂(PMe₃)₃L (L = CO, P(OMe)₃) and all-trans-MCl₂(PMe₃)₂[P(OMe)₃]₂, respectively. On reaction of trans-RuCl₂[P(OMe)₃]₄ with CO and PMe₃, the compounds trans,mer-RuCl₂[P(OMe)₃]₃(CO) and trans,cis,cis-RuCl₂(PMe₃)₂[P(OMe)₃]₂ are synthesized. The reduction of MCl₂(PMe₃)₂[P(OMe)₃]₂ with Na/Hg in benzene or toluene via {M(PMe₃)₂[P(OMe)₃]₂} as an intermediate leads to subsequent intermolecular addition of the arene and to the aryl(hydrido)metal complexes cis,trans,cis-MH(C₆H₅)(PMe₃)₂[P(OMe)₃]₂ (M = Ru, Os) and MH(C₆H₄CH₃)(PMe₃)₂[P(OMe)₃)₂ (M = Os). For M = Ru, in the presence of P(OMe)₃, the ruthenium(0) compound Ru(PMe₃)₂(P(OMe)₃]₃ is formed. The hydrido(phenyl) complexes react with equimolar amounts of Br₂ or I₂ by elimination of benzene to produce the dihalogenometal compounds cis,trans,cis-MX₂(PMe₃)₂[P(OMe)₃]₂. The reaction of trans-RuCl₂(PMe₃)₄ with Na/Hg in the presence of PPh₃ leads to the ortho-metallated complex fac-RuH(η²-C₆H₄PPh₂)(PMe₃)₃, which reacts with CH₃I, CS₂, COS and HCl to give the compounds mer-RuI(η²-C₆H₄PPh₂)(PMe₃)₃, fac-Ru(SCHS)(η²-C₆H₄PPh₂)(PMe₃)₃, fac-Ru(S₂CO)(CO)(PMe₃)₃ and RuCl₂(PMe₃)₃, respectively. The paramagnetic 17-electron complexes [MCl₂(PMe₃)_nL_4-n]PF₆ are obtained on oxidation of MCl₂(PMe₃)_nL_4-n with AgPF₆. Their UV spectra exhibit a characteristic CT band. [RuCl₂(PMe₃)₄]PF₆ and [OsCl₂(PMe₃)₄]PF₆ react with CO and P(OMe)₃ by reduction to form the corresponding ruthenium(II) and osmium(II) compounds MCl₂(PMe₃)_nL_4-n.     

Basische metalle: LII. Synthese von carbenoid- und ylidcobalt(III)-komplexen aus substitutionslabilen cobalt(I)-vorstufen

The cyclopentadienylcobalt(I) compounds C₅H₅Co(PMe₃)P(OR)₃ (R = Me, Et, Prⁱ) and C₅H₅Co(C₂H₄)L (L = PMe₃, P(OMe)₃, CO) are prepared by ligand substitution starting from C₅H₅Co(PMe₃)₂ and C₅H₅Co(C₂H₄)₂. Whereas the reaction of C₅H₅Co(PMe₃)P(OMe)₃ with CH₂Br₂ mainly gives [C₅H₅CoBr(PMe₃)P(OMe)₃]Br, the dihalogenocobalt(III) complexes C₅H₅CoX₂(PMe₃) (X = Br, I) are obtained from C₅H₅Co(CO)PMe₃ and CH₂X₂. Treatment of C₅H₅Co(CO)PMe₃ or C₅H₅Co(C₂H₄)PMe₃ with CH₂ClI at low temperatures produces a mixture of C₅H₅CoCH₂Cl(PMe₃)I and C₅H₅CoCl(PMe₃)I, which can be separated due to their different solubilities. The same reaction in the presence of ligand L gives the carbenoidcobalt(III) compounds [C₅H₅CoCH₂Cl(PMe₃)L]PF₆ in nearly quantitative yields. If NEt₃ is used as the Lewis base, the ylide complexes [C₅H₅Co(CH₂PMe₃)(PMe₃)X]PF₆ (X = Br, I) are obtained. The PF₆ salts of the dications [C₅H₅Co(CH₂PMe₃)(PMe₃)L]²⁺ (L = PMe₃, P(OMe)₃, CNMe) and [C₅H₅Co(CH₂PMe₃)(P(OMe)₃)₂]²⁺ are prepared either from [C₅H₅Co(CH₂PMe₃)(PMe₃)X]⁺ and L, or more directly from C₅H₅Co(CO)PMe₃, CH₂X₂ and PMe₃ or P(OMe)₃, respectively. The synthesis of C₅H₅CoCH₂OMe(PMe₃)I is also described.     

Substituted cyclopentadienyl ligands—10. Intramolecular steric interactions in [(η-C5H3(SiMe3)2)Mo(CO)2(L)I]

Reaction of C₅H₄(SiMe₃)₂ with Mo(CO)₆ yielded [(η⁵-C₅H₃(SiMe₃)₂)Mo(CO)₃]₂, which on addition of iodine gave [(η⁵-C₅H₃(SiMe₃)₂Mo(CO)₃I]. Carbonyl displacement by a range of ligands: [L  P(OMe)₃, P(OPrⁱ)₃,P(O-o-tol)₃, PMe₃, PMe₂Ph, PMePh₂, PPh₃, P(m-tol)₃] gave the new complexes [(η⁵-C₅H₃(SiMe₃)₂ MO(CO)₂(L)I]. For all the trans isomer was the dominant, if not exclusive, isomer formed in the reaction. An NOE spectral analysis of [(η⁵-C₅H₃(SiMe₃)₂)Mo(CO)₂(L)I] L  PMe₂Ph, P(OMe)₃] revealed that the L group resided on the sterically uncongested side of the cyclopentadienyl ligand and that the ligand did not access the congested side of the molecule. Quantification of this phenomenon [L  P(OMe)₃] was achieved by means of the vertex angle of overlap methodology. This methodology revealed a steric preference with the trans isomer (less congestion of CO than I with an SiMe₃ group) being the more stable isomer for L  P(OMe)₃.     

Preparative and NMR studies of 1,3-dienecobalt(I) complexes

The synthesis and the characterization of cobalt(I) complexes of the type [Co(diene)(phosphine)₃]Y (diene = 1,3-butadiene and isoprene; phosphine = PMe₃, PMe₂Ph, and HPPh₂; Y = ClO₄, BF₄, or BPh₄) are reported. The fluxional nature of the five-coordinate cations [Co(diene)(phosphine)₃]⁺ is shown by variable-temperature NMR spectroscopy (¹H and ³¹P). Low-temperature spectra are consistent with a square-pyramidal structure in which the diene occupies two basal coordination sites.     

Hydrogen transfer processes in the formation of cationic η5 -dienyl complexes of ruthenium(II): X-ray structure of [Ru(1-5-η-cyclooctadienyl)(PMe2Ph)3][PF6]

The cations [Ru(1—3:5—6-η-C₈H₁₁)(η⁶ -1,3,5-cyclooctatriene)]⁺ (2) and [RuH(COD)L₃]⁺ (5) (COD = cycloocta-1,5-diene, L = PMe₂Ph, AsMePh₂) are convenient precursors to a range of η⁵ -dienyl complexes of ruthenium(II); evidence for hydrogen transfer processes is presented.     

Phosphine substitution and phosphine induced reductive elimination reactions: synthesis of zerovalent ruthenium fluorophosphine complexes from hydrido(acetato)tris(triphenylphosphine)ruthenium(II)

Substitution of PPh₃ from the bidentate acetatohydridoruthenium(II) complex RuH(CH₃OCO)(PPh₃)₃ with various ligands, L, leads to unidentate acetatohydrido compounds with replacement of one, two or all PPh₃ ligands depending on L (L = t-BuNC, PF₂NMe₂, P(OCH₂)₃CMe, P(OMe)₃, dppe). On the other hand acetic acid elimination from RuH(CH₃OCO)(PPh₃)₃ occurs with fluorophosphines to yield zerovalent ruthenium complexes RuL′₅ (L′ = PF₂NMe₂, PF₂NC₄H₈), RuL₄(PPh₃) (L′ = PF₃) and RuL₃(PPh₃)₂ (L′ = PF₃).     

Synthesis and reactivity of naphthalene sandwich complexes of molybdenum. X-ray structure of [Mo(η6-naphthalene) {P(OMe)3}3] and [Mo(H)(η6-naphthalene){P(OMe)3}3][BF4]

The sandwich complexes bis(η⁶-naphthalene)molybdenum(0) ( 1 ), bis(η⁶-1-methylnaphthalene)molybdenum(0) ( 2 ), and bis(η⁶-1,4-dimethylnaphthalene)molybdenum(0) ( 3 ) are synthesized by cocondensation of Mo-atoms with the naphthalene ligands. Complexes 1–3 are also obtained by reduction of MoCl₅ or MoCl₄. 2THF with highly activated Mg in the presence of the naphthalene ligands. Mg was activated by sublimation of the metal in a simple rotating solution reactor. Complex 2 exists as a mixture of regio- and stereoisomers. Three regioisomers, 3a–c , are formed in reactions of Mo-atoms with 1,4-dimethylnaphthalene, whereas 3a , the isomer with the Mo-atom coordinated to the unsubstituted rings, is formed selectively via the reductive method. The ligands in 1–3 are highly labile. CO displaces both naphthalene rings in 2 and 3 to give [Mo(CO)₆], while PF₃, P(OMe)₃, and PMe₃ displace only one coordinated naphthalene in 1 to yield the [Mo(η⁶-naphthalene)L₃] complexes 4–6 . In toluene, arene exchange is a competitive process in reactions of 1 with PF₃. Complexes 5 (L = P(OMe)₃) and 6 (L = PMe₃) react with HBF₄ to give the cationic metal hydride complexes 8 and 9 . The X-ray crystal structures of [Mo(η⁶-naphthalene) {P(OMe)₃}₃] ( 5 ) and [Mo(H)(η⁶-naphthalene) {P(OMe)₃}₃][BF₄] ( 8 ) are reported.     

Announcement

[Ru(2–6-η-bicyclo[5.1.0]octadienyl)(PMe₂Ph)₃][PF₆], formed from the reaction of cyclooctatetraene with [RuH(COD)(PMe₂Ph)₃][PF₆] (COD = cycloocta-1,5-diene), has been characterised spectroscopically from ¹J(CH) coupling constants and an X-ray structural analysis; the bicyclic ligand contains an elongated bridging CC bond (1.63 Å).     

Oxidative addition of dialkylphosphites to complexes of iridium(I) and rhodium(I)

The PH bond of dialkylphosphites (dimethylphosphite, 5,5-dimethyl-1,3-dioxa-2-phosphorinane and 4,4,5,5-tetramethyl-1,3-dioxa-2-phospholane) oxidatively adds to irClL₂(L = PPh₃, AsPh₃) and IrCl(PMe₂Ph)₃ generated in situ to give six-coordinate hydrido(dialkylphosphonato)iridium(III) complexes, e.g. IrHClL₂[{(MeO)₂-PO}₂H] and IrHCl(PMe₂Ph)₃[PO(OMe)₂]. Addition of triphenylphosphine to a solution containing [IrCl(C₈H₁₄)₂]₂ and dimethylphosphite in a 1:2 mol ratio gives a five-coordinate hydrido (dimethylphosphonato)iridium(III) complex IrHCl(PPh₃)₂{PO(OMe)₂}, from which six-coordinate pyridine and acetylacetonato complexes IrHCl(PPh₃)₂(C₅H₅N){PO(OMe)₂} and IrH(PPh₃)₂(acac){PO(OMe)₂} can be obtained. The ligand arrangements in the various complexes are inferred from IR, ¹H and ³¹P NMR data.     

Diastereoselectivity at chiral metal center of half-sandwich-type ruthenium complexes with planar-chiral cyclopentadienyl ligands in multiple ligand transfer reaction

The triple ligand transfer reaction between planar-chiral cyclopentadienyl-ruthenium complexes [Cp′Ru(NCMe)₃][PF₆] (1) (Cp′ = 1-(COOR²)-2-Me-4-R¹C₅H₂; R¹ = Me, Ph, t-Bu) and iron complexes CpFe(CO)(L)X (2) (L = PMe₃, PMe₂Ph, PMePh₂, PPh₃; X = I, Br) resulted in the formation of metal-centered chiral ruthenium complexes Cp′Ru(CO)(L)X (3) in moderate yields with diastereoselectivities of up to 68% de. The configurations of some major diastereomers were determined to be by X-ray crystallography. The diastereoselectivity of 3 was under kinetic control and not affected by the steric effect of the substituents on the Cp′ ring of 1 and the phosphine of 2. Although the double ligand transfer reaction between [Cp′Ru{P(OMe)₃}(NCMe)₂][PF₆] (7) and CpFe(CO)₂X (8) produced Cp′Ru{P(OMe)₃}(CO)X (9), the selectivity at the ruthenium center was low.     

Studies on the reactions of bis(acetylacetonato)platinum(II) with lewis bases : IV. Preparations and characterization of novel acetylacetonato complexes [Pt(C5H6O2)L2]

Novel complexes [Pt(C₅H₆O₂)L₂] (IVa, L = PPh₃; IVb, L = PMePh₂, IVc, L = PMe₂Ph) were prepared by the reactions of [Pt(acac)₂] with tertiary phosphines either at elevated temperature (when L = PPh₃) or at room temperature (L = PMePh₂ and PMe₂Ph), whereas AsPh₃ yielded [Pt(acac)(γ-acac)AsPh₃] (Id) by the reaction with [Pt(acac)₂] even under rigorous conditions. Complexes IV were characterized on the basis of their IR and NMR spectra, elemental analyses and chemical reactions, and a structure which possesses a chelate type “acetylacetonato” ligand involving π-oxoallyl bonding is proposed.     

Preparation and properties of some cationic binuclear platinum(I) complexes

Reaction of [Pt₂Cl₂(μ-dppm)₂] with ligands, L, in the presence of [PF₆^- gave stable cationic diplatinum(I) complexes [Pt₂L₂(μ-dppm)₂][PF₆]₂ where L = PMe₂Ph, PMePh₂, PPh₃, NH₃, C₅H₅N. Reaction of [Pt₂(NH₃)₂(μ-dppm)₂][PF₆]₂ with CO gave [Pt₂(CO)₂(μ-dppm)₂][PF₆]₂ and an unsymmetrical complex [Pt₂(CO)(C₅H₅N)(μ-dppm)₂][PF₆]₂ was also prepared. The compounds were characterized by vibrational and ¹H and ³¹P NMR spectroscopy and the presence of direct platinumplatinum bonds is indicated.     

Ethylen(hydrido)-metallkomplexe aus M(CH2)CH3-vorstufen: modellreaktionen für einen elementarschritt der fischer-tropsch-synthese

The compounds C₆Me₆Ru(Ch₃)₂, C₆H₆Os(CH₃)₂PMe₃ and C₅H₅Ir(CH₃)₂-Pprⁱ₃ react with [CPh₃]PF₆ in Ch₂Cl₂ to give the ethylene(hydrido)metal complexes [C₆Me₆RuH(C₂H₄)PR₃]PF₆, [C₆H₆OsH(C₂H₄)PMe₃]PF₆ and [C₅H₅IrH(C₂H₄)PPrⁱ₃]PF₆, respectively. Treatment of C₆Me₆RuCH₃(C1)PMe₃ with [CPh₃]PF₆ leads to cleavage of the RuCH₆ bond instead of hydride elimination; in the presence of PMe₂Ph the compound [C₆Me₆RuCl(PMe₂Ph)PMe₃]PF₆ is obtained. The reaction of C₅H₅RhCH₂OMe(PMe₃)CH₃ with HBF₄ gives [C₅H₅RhH(C₂H₄)PMe₃]BF₄ and methanol. It is assumed that the formation of the ethylene(hydrido)metal complexes always occur via a M(CH₂)CH₃ intermediate, radical intermediates not being observed. The crystal structure of [C₆Me₆RuH(C₂H₄)PPh₃]PF₆ has been determined. The cation of 1.50 Å. The CC distance in the ethylene ligand is 1.41(1) Å and thus is significantly longer than in the free olefin.     

Some trimethyl phosphine and trimethyl phosphite complexes of tungsten(IV)

Direct reduction of WCl₆ with PMe₃ in toluene at 120°C in a sealed tube affords the complexes [WCl₄(PMe₃)_x] (x = 2, 3). [WCl₄(PMe₃)₃] abstracts oxygen from equimolar amounts of water in wet acetone or tetrahydrofuran to give [WOCl₂(PMe₃)₃] in very high yields. This procedure has been successfully applied to the high yield synthesis of other known oxotungsten(IV) complexes, [WOCl₂(PR₃)₃] (PR₃ = PMe₂Ph and PMePh₂). Metathesis reactions of [WOCl₂(PMe₃)₃] with NaX give [WOX₂(PMe₃)₃] (X = NCO, NCS) and [WOX₂(PMe₃)] (X = Me₂NCS₂). The synthesis of the trimethylphosphite analogue, [WOCl₂(P(OMe)₃)₃], is also described and the structures of the new complexes assigned on the basis of IR and ¹H and ³¹P NMR spectroscopy.     

The Synthesis and Structure of Hexakis(dimethylphenylphosphonite)ruthenium(II) bis[tetraphenylborate(–1)]

Abstract . Treatment of the hydrazine salt [Ru(COD)(H₂NNH₂)₄][BPh₄]₂ with excess of P(OMe)₂Ph in acetone gave a homoleptic complex trans‐[Ru{P(OMe)₂Ph}₆][BPh₄]₂, which was characterized by IR, ³¹P{¹H}, ¹³C{¹H}, and ¹H NMR spectroscopy, elemental analysis, and X‐ray crystallography. The ruthenium in the complex has distorted octahedral coordination arrangement and bonded to all the six P(OMe)₂Ph molecules through the phosphorus atoms.     

Pentamethylcyclopentadienyl ruthenium(II) complexes of para-substituted N-(pyrid-2-ylmethylene)-phenylamine ligands: syntheses,spectral and structural studies

The reaction of [(η⁵-C₅Me₅)Ru(PPh₃)₂Cl] (1) with acetonitrile in the presence of excess NH₄PF₆ leads to the formation of the cationic ruthenium(II) complex [(η⁵-C₅Me₅)Ru(PPh₃)₂(CH₃CN)]PF₆ (2). The complex (2) reacts with a series of N,N′ donor Schiff base ligands viz. para-substituted N-(pyrid-2-ylmethylene)-phenylamines (ppa) in methanol to yield pentamethylcylopentadienyl ruthenium(II) Schiff base complexes of the formulation [(η⁵-C₅Me₅)Ru(PPh₃)(C₅H₄N-2-CHN-C₆H₄-p-X)]PF₆ [3a]PF₆–[3f]PF₆, where C₅Me₅ = pentamethylcylopentadienyl, X = H, [3a]PF₆, Me, [3b]PF₆, OMe, [3c]PF₆, NO₂, [3d]PF₆, Cl, [3e]PF₆, COOH, [3f]PF₆. The complexes were isolated as their hexafluorophosphate salts. The complexes were fully characterized on the basis of elemental analyses and NMR spectroscopy. The molecular structure of a representative complex, [(η⁵-C₅Me₅)Ru(PPh₃)(C₅H₄N-2-CHN-C₆H₄-p-Cl)]PF₆ [3e]PF₆, has been established by X-ray crystallography.     

Methyl- and phenyl-bis(tertiary phosphine) N-bonded carboxamido complexes of platinum(II)

Methyl- or phenylN-carboxamido-complexes of platinum(II) Pt(NHCOR')RL₂ (L = PEt₃, R = Me, R′ = Me, CH = CH₂; L = PEt₃, R = Ph, R′ = Me; L = PMe₂Ph, R = Ph, R′ = Me, Ph; L = PMePh₂, R = Ph, R′ =₃, R = Ph, R′ = Me) have been prepared by the reaction of KOH with cationic nitrile complexes [PtR(NCR′)L₂]BF₄. Thermally unstable hydrido-N-carboxamido-complexes could be detected spectroscopically. IR and NMR (¹H, ³¹P) spectra of some of the complexes indicate the existence of a solvent- and temperature-dependent equilibrium between syn-and anti-isomers arising from restricted rotation about the NC bond of the carboxamido-group. The anti-isomer is favoured by nonpolar solvents and by increasing bulk of L. In the complex [PtH(NCCH CH₂)(PEt₃)₂]BF₄, IR and NMR spectra show acrlonitrile to be bound through nitrogen, not through the olefinic CC bond.     

Basic metals : XXXIV. Synthesis and reactivity of RuH(η2-CH2PMe2)(PMe3)3

The complex RuH(η²-CH₂PMe₂)(PMe₃)₃ is obtained by reduction of trans-RuCl₂(PMe₃)₄ with Na/Hg in benzene. In contrast to the iron analogue, this complex is configurationally stable on the NMR time scale and does not react with CO or P(OMe)₃ under normal conditions, but it does react with the electrophiles MeI, CS₂ and NH₄PF₆ to form RuI(η²-CH₂PMe₂)(PMe₃)₃, Ru(η³-S₂CHPMe₂CH₂)(PMe₃)₃ and [RuH(PMe₃)₅]PF₆, respectively.     

The preparation and molecular structure oftrans-[MoCl2(PMe2Ph)4]

Summary The compoundtrans-[MoCl₂(PMe₂Ph)₄] has been prepared by the reduction of MoCl₅ (by Mg) or of [MoCl₃(PMe₂Ph)₃] (by LiBuⁿ) in the presence of PMe₂Ph in tetrahydrofuran (THF). It has _eff=2.84 B.M. and crystallises in space group P1 witha=11.591(3),b=12.931(3),c=12.703(3) Å, = 95.28(2), =105.97(2), =103.54(2)°. Refinement of the structure gave R=0.036. The Mo-Cl and Mo-P distances average 2.443(6) and 2.534(8) Å, respectively.Low-valent phosphine complexes of the Group VI metals continue to attract much attention because of their involvement in studies of the catalytic activation of dinitrogen⁽¹⁾, dihydrogen(^{2, 3}), alkenes and alkynes(⁴). As a by-product during our studies of dinitrogen(¹) and hydride(²) complexes of molybdenum and tungsten, we obtainedtrans-[MoCl_2- (PMe₂Ph)₄] as yellow, paramagnetic crystals (_eff= 2.84 B.M.). We first obtained the compound during the attempted synthesis ofcis-[Mo(N₂)₂(PMe₂Ph)₄] by reduction of MoCl₅ with Mg in the presence of PMe₂Ph (see Experimental). Upon identification of the compound we found that it could be readily synthesised by treatment of [MoCl₃(PMe₂Ph)₃]⁽⁵⁾ with LiBuⁿ in THF in the presence of PMe₂Ph (experimental).The complex was shown to have thetrans structure by x-ray analysis (Figure). Analogues oftrans-[MoCl₂(PMe₂Ph)₄] have been prepared, namely [CrCl₂(Me₂PCH₂CH₂PMe₂)₂]⁽⁶⁾,trans- [MoCl₂(PMe₃)₄]⁽⁷⁾, [WCl₂(PMe₂Ph)₄]⁽⁸⁾ and [WCl₂(PMe₃)₄]⁽⁴⁾, of which onlytrans-[MoCl₂(PMe₃)₄] has been examined by X-rays⁽⁷⁾. Its principal structural parametersi.e. d(Mo-Cl)= 2.420(6), d(Mo-P)_av=2.496(3) Å⁽⁶⁾ are close to those found here fortrans-[MoCl₂(PMe₂Ph)₄].     

Some diolefin complexes of iridium(I) and a trans-influence series for the complexes [IrCl(cod)L]

A high-yield synthesis of [IrCl(cod)]₂ (cod = 1,5-cyclooctadiene) is described. The ¹H and ¹³C NMR spectra of a number of complexes [IrCl(cod)L] are interpreted in terms of a trans-effect series Cl^? < sym-collidine < 2-picoline < PCy₃ < P-i-Pr₃ < Pet₃ ～ AsPh₃ < PMe₂Ph < PMePh₂ < PPh₂ <P(MeO)Ph₂ < PClPh₂ < P(OPh)₃ < PCl₂Ph. Some ligand exchange reactions of [IrCl(cod)L] are discussed. A number of complexes of the type [Ir(cod)L_n]PF₆ (L = a variety of amines (n = 2) and phosphines (n = 2 or 3)) are described. Exchange reactions of the sort: [Ir(cod)(PR₃)₂]PF₆ + [Ir(cod)(py)₂]PF₆ ? [Ir(cod)(PR₃)Py]PF₆ are reported in which, surprisingly, the isolable mixed ligand complexes are the only detectable species at equilibrium (py = pyridine).