Synthese und spektroskopische charakterisierung von cyclopentadienyleisen-komplexen mit PN-liganden des typs (C6H5)3−nP(NR2)n (n = 0−3; R = CH3, C2H5

The complex cations [C₅H₅Fe(CO)₂L]BF₄ (L = (C₆H₅)_3−nP(NR₂)_n; n = 0−3 R = CH₃, C₂H₅) have been obtained from the reaction of [C₅H₅Fe(CO)₂THF]BF₄ (I) with L. The reaction of I with E(NR₂)₃ (E = As, Sb; R= CH₃) is also described. Spectroscopic investigations (IR, ¹H, ¹³C and ³¹P NMR) indicate an increase in electron density on the iron center through increase of the number of P-bound NR₂ groups.     

Zur verwendung von ferricenium-kationen als selektivem oxidationsmittel in der metallorganischen chemie

The use of ferricenium cations [(C₅H₅)₂FE]X (X = BF₄, PF₆, SbF₆) as one-electron oxidizing agents for organometallic complexes is demonstrated. Sandwich compounds M(C₅H₅)₂ (M = Cr, Co, Ni) and Cr(C₆H₆)₂ are oxidized in nearly quantitative yield to the corresponding cations [M(C₅H₅)₂]BF₄ and [(C₆H₆)₂Cr]BF₄. The metalmetal bond in the dinuclear organometallic complexes [DienylM(CO)_n]₂ (M = Mo (n = 3), Fe (n = 2), Ni (n = 1)) and Co₂(CO)₈ is fissioned by (C₅H₅)₂Fe⁺ in the presence of neutral ligands L to form the corresponding cationic compounds [DienylM(CO)_nL_m]X (M = Mo (n = 2), Fe (n = 2), Ni (n = 0)) and [Co(CO)₃L₂BF₄ (L = VB and VIB donor ligands) in high yields.The practical applications of ferricenium cations are discussed in comparison with other methods for the preparation of cationic organometallic complexes.     

Alkylidenverbrückte diphosphane und dichlalkogenodiphosphorane als liganden in kationischen cyclopentadienyleisencarbonyl-komplexen

Oxidative cleavage of the FeFe bond in [C₅H₅Fe(CO)₂]₂ in the presence of alkylide-bridged diphosphanes LL (LL = (C₆H₅)₂P(CH₂)_n(P(C₆H₅)₂; n = 1–3), (C₆H₅)₂PCH₂As(C₆H₅)₂ and dichalcogenodiphosphoranes (X)LL(X) ((X)LL(X) = (C₆H₅)₂P(X)(CH₂)_n(X)P(C₆H₅)₂; X  O, S, Se; n = 1–3) yields the complexes [C₅H₅Fe(CO)₂L′]BF₄ (L′ = LL, (X)LL(X); X  S, Se) in high yield. the complexes react with Ni(CO)₄ under photochemical conditions to form [C₅H₅Fe(CO)₂(μ-L′)Ni(CO)₃]BF₄ in quantitative yield, and lose a CO group under irradiation (λ_max > 300 nm) to form the chelate compounds [C₅H₅Fe(CO)L′]BF₄, which are isolable for L′  LL (P,As ligand) and (X)LL(X) (X = S, Se). Some substitution reactions with phosphanes are described.     

Synthesis and Spectra of Cationic Bis(tertiary phosphine) derivatives of cycloheptatrienyliummolybdenum and cyclopentadienyliron complexes

Reaction of [(η-C₇H₇)Mo(CO)₃][PF₆] and [(η-C₅H₅)Fe(CO)₂CH₃CN][PF₆] with ditertiary phosphine ligands afforded products of three types; the monosubstituted complexes [(Ring)M(CO)₂Ph₂P(CH₂)_nPPh₂][PF₆] (Ring = η-C₇H₇, M = Mo, N = 1; Ring = η-C₅H₅, M = Fe, N = 1 and 2), the chelated complexes [(Ring)M(CO)Ph₂P(CH₂)_nPPh₂][PF₆] (Ring = η-C₇H₇, M = Mo, N = 1 and 2; Ring = η-C₅H₅, M = Fe, N = 1 and 2), and the dinuclear complex [{(η-C₇H₇)Mo(CO)₂}₂ -μ- Ph₂PCH₂CH₂PPh₂][(PF₆)₂]. Spectroscopic properties, including ³¹P NMR, are reported.     

Darstellung und Charakterisierung von kationischen η2-1-Buten- und Acetonitrilkomplexen

Preparation and Characterization of Cationic η²-1-Butene and Acetonitrile Complexes The reaction of the species η⁵-C₅H₅M(CO)_n-σ-C₄H₇ (M = Fe, Mo, W; n = 2, 3) with (C₆H₅)₃CBF₄ yielded – instead of the expected cationic butadiene complexes of the type [η⁵-CpM(CO)_n?1-η⁴-C₄H₆][BF₄], which would have been formed in case of hydride cleavage – compounds of the type [η⁵-CpM(CO)_n η²-C₄H₈][BF₄], which were formed by protonation of the σ-C₄H₇ ligands. The reaction proceeded quantitatively. The BF₄^? anion can be substituted by other anions, such as ClO₄^?, B(C₆H₅)₄^?, PF₄^?, and [Cr(SCN)₄(NH₃)₂]^? in the complexes obtained. The mechanism of the reaction leading to the η²-bonded 1-butene complexes was determined by isotope experiments. In trying to recrystallize the butene complexes from acetonitrile the cationic complexes [η⁵-C₅H₅ Fe(CO)₂CH₃CN]BF₄ and [η⁵-C₅H₅ M(CO)₃CH₃CN]BF₄ were observed; the X-ray structure analysis of the former is reported.     

Synthese und reaktivität von dienylmetall-verbindungen XXVIII. Über kationen des typs [C5H5Fe(CO)L(EME2)]+ (E = S,Se, Te)

The chiral cations, [CpFe(CO)(EMe₂)L]⁺, are obtained both by reaction of [CpFe(CO)(EMe₂)₂]⁺ with the ligands (L) by heating, and by irradiation of the cations [C₅H₅Fe(CO)₂EMe₂]⁺ in the presence of L (E = S, Se, Te; L = PR₃, AsR₃, SbR₃). The inversion about the chalcogen atom is investigated by DNMR spectrocopy. Compounds of the type [C₅H₅Fe(TeMe₂)L₂]⁺] are formed by irradiation of [C₅H₅Fe(CO)₂(TeMe₂)]⁺ and the ligands (L₂ = 2 PR₃, R = CH₃, OCH₃, OC₆H₅; L₂ = R₂P(CH₂)_nPR₂, R = C₆H₅, n = 1,2,3). ⁷⁷Se and ¹²⁵Te NMR data vary according to the donor properties of the ligand L in the complexes.     

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.     

Reactions of coordinated propargyl and allene ligands in cyclopentadienyliron dicarbonyl complexes

Reactions of η⁵-C₅H₅Fe(CO)₂CH₂CCR (R  CH₃, C₆H₅, and CH₂Fe(CO)₂(η⁵-C₅H₅)) with HBF₄ in acetic anhydride yield [η⁵-C₅H₅Fe(CO)₂(η²CH₂CCHR)]⁺BF^?₄. The resultant cationic iron-η²-allene complexes react with a wide range of nucleophiles (Nu) to give the following types of behavior: (a) addition of Nu to carbon-1 of the η²-allene fragment (with NaBH₄, (C₂H₅)₂NH, and P(C₆H₅)₃, inter alia), (b) addition of Nu to carbon-2 of the η²-allene fragment (with NaOCH₃), (c) addition of Nu to the carbonyl carbon (with NaOC₂H₅), (d) deprotonation of the iron-η²-allene cation to the parent propargylic complex (with N(C₂H₅)₃), and (e) nonselective reactions to yield a mixture of products (with CH₃Li). Of these, the most common is behavior (a); together with the protonation of η⁵-C₅H₅Fe(CO)₂CH₂CCR it stimulates the two-step (3 + 2) cycloaddition reactions between electrophilic molecules and these iron-propargyl complexes.     

Reactions of metal carbonyl derivatives : XI. Tertiary phosphine,phosphite and stibine and ditertiaryphosphine and arsine derivatives of bis(μ-phenyl- sulphidotricarbonyliron)

The ditertiary phosphines (C₆H₅)₂P(CH₂)_nP(C₆H₅)₂ (n = 1 and 2), cis(C₆H₅)₂PC₂H₂P(C₆H₅)₂ and (C₆H₅)₂PN(C₂H₅)P(C₆H₅₂ and the ditertiary arsines (C₆H₅)₂As(CH₂)_nAs(C₆H₅)2 (_n = 1 and 2) react with [Fe(CO)₃SC₆H₅]₂ to give a wide range of products, the nature of which depends on the reaction conditions and the ligand involved. Examples of the different types of comp isolated include, (i) Fe₂(CO)₅[(C₆H₅)₂PCH₂P(C₆H₅)₂](SC₆H₅)₂, in which the ligand acts as a monodentate, (ii) {[Fe(CO)₂SC₆H₅]₂[(C₆H₅)₂PC₂H₄P(C₆H₅)₂]}₂, in which two [Fe(CO)₂SC₆H₅]₂ moieties are bridged by two diphosphine ligands, (iii) [Fe(CO)₂SC₆H₅]₂[(C₆H₅)₂PN(C₂H₅)P(C₆H₅)₂], in which the ligand bridges the two iron atoms, and (iv) Fe(CO)₃(SC₆H₅)₂Fe(CO)[(C₆H₅)₂PC₂H₂P(C₆H₅)₂], which contains the ligand chelated to a single iron atom. The tertiary phosphines PR₃ (R=C₂H₅ and C₆H₅), phosphites P(OR′)₃(R′ = CH₃, C₂H₅, i-C₃H₇ and C₆H₅) and the stibine Sb(C₆H₅)₃ bring about mono-, bis- or tris-substitution in [Fe(CO)₃SC₆H₅]₂ depending on the reaction conditions and the ligand involved. Whereas in solution [Fe(CO)₂L(SC₆H₅)]₂ [L = PR₃ (R = C₂H₅ and C₆H₅), P(OC₆H₅)₃ and Sb(C₆H₅)₃] exist as a single isomer, [Fe(CO)₂L′(SC₆H₅)]₂ [L′=P(OR′)₃ (R'=CH₃, C₂H₅ and i-C₃H₇)] occur as a mixture of isomers.     

Optisch aktive übergangsmetall-komplexe LXIV. Neue optisch aktive Cp(CO)Fe-acyl-komplexe mit

The novel complexes CpFe(CO)(COR)P(C₆H₅)₂NR'R^* with Cp = C₅H₅,C₉H₇ (indenyl); R = CH₃, C₂H₅, CH(CH₃)₂, CH₂C₆H₅;R` = H, CH<sub>3</sub>, C<sub>2</sub>H<sub>5</sub>, CH<sub>2</sub>C<sub>6</sub>H<sub>5</sub> and R<sup>*</sup> = (S)-CH(CH<sub>3</sub>)(C<sub>6</sub>H<sub>5</sub>), have been synthesized by reaction of CpFe(CO)<sub>2</sub>R wiht P(C<sub>6</sub>H<sub>5</sub>)<sub>2</sub>NR`R^* and characterized analytically as well as spectroscopically. The pairs of diastereoisomers RS/SS have been separated by preparative liquid chromatography and fractional crystallization, respectively. The optically pure complexes (+)₄₃₆- und ()₄₃₆-CpFe(CO)(COR)P(C₆H₅)₂NR`R^* are configurationally stable at room temperature. At higher temperatures they equilibrate with CpFe(CO)₂R and epimerize with respect to the Fe configuration.     

Alkylsulfito,bisulfito and sulfito complexes of η5-cyclopentadienyldicarbonyliron(II)

Alkylsulfito complexes η⁵-C₅H₅Fe(CO)₂S(O)₂OR (R = CH₃, C₂H₅, n-C₃H₇, i-C₃H₇) have been prepared by reaction of [η⁵-C₅H₅Fe(CO)₂H₂O]BF₄ with Na[S(O)₂OR] (R = CH₃, C₂H₅) and the respective alcohol as solvent. These products may be interconverted by the use of the appropriate alcohol at reflux; such exchange occurs also at 25°C in the presence of HBF₄. Reaction of η⁵-C₅H₅Fe(CO)₂S(O)₂OCH₃ with (+)₅₈₉-2-C₈H₁₇OH and HBF₄ followed by treatment of the optically active product (+)₅₈₉-η⁵-C₅H₅Fe(CO)₂S(O)₂OC₈H₁₇ with CH₃OH and HBF₄ regenerates (+)₅₈₉-2-C₈H₁₇OH with unchanged specific rotation. Hydrolysis of η⁵-C₅H₅Fe(CO)₂S(O)₂OR affords η⁵-C₅H₅Fe(CO)₂S(O)₂OH, which is a strong acid.     

Synthetic applications of the photolysis of the cyclopentadienyliron-(ρ-xylene) cation

Photolysis of CpFe(ρ-xylene)⁺ (Cp = η⁵-cyclopentadienyl) in the presence of suitable 6- or 2-electron donor ligands results in replacement of the aromatic ring with one 6-electron or three 2-electron donor ligands. The compounds [CPFe(η⁶-C₇H₈)]BF₄ (C₇H₈ = cycloheptatriene), [CpFe(η⁶-C₈H₈)]PF₆, (C₈H₈ = cyclooctatetraene), [CpFe(η⁶-PCP)]PF₆ (PCP = 2,2-paracyclophane), [CpFe-(P(OCH₃)₃)₃]PF₆ and [CpFe(P(OCH₂CH₃)₃)₃]PF₆ were prepared in this manner. The compound [CpFe(TM4)₃FeCp](PF₆)₂ · CH₃COCH₃ (TM4 = 2,5-dimethyl-2,5 diisocyanohexane) was prepared in two steps. First, [CpFe(ρ-xylene)]PF₆ was irradiated with an excess of the free TM4 ligand producing a mixture of [CpFe(TM4)₃]⁺ and [CpFe(TM4)₃FeCp]²⁺. An additional equivalent of [CpFe(p-xylene)]PF₆ was added to this mixture and photolysis yielded [CpFe(TM4)₃FeCp](PF₆)₂ · CH₃COCH₃.     

Stereo- and regio-specific CC bond formation via the coupling of electrophilic and nucleophilic organotransition-metal complexes: The x-ray crystal structure of [Ru(CO)2(PPh3)(η2,η3-C8H8R)][PF6]·0.5CH2Cl2 (R = CH2C(Me)CH2)

The coupling of [Ru(CO)₂L(η⁴-cot)] (L = CO or PPh₃, cot = cyclooctatetraene) with [Fe(CO)₃(η⁵-cyclohexadienyl)]⁺ or [Fe{P(OMe)₃}(NO)₂(η³-allyl)]⁺ yields respectively the dimetallic species [Ru(CO)₂L(η²,η³-C₈H₈{Fe(CO)₃(η⁴-C₆H₇)}] (3) and the allyl-substituted derivative [Ru(CO)₂L(η⁵-C₈H₈CH₂C(Me)CH₂)][PF₆] (5) whose X-ray structure is reported; paramagnetic [Co(η-C₅H₅)₂] and [Ru(CO)₃(η⁵-cyclohexadienyl)]⁺ give diamagnetic [Ru(CO)₃(η⁴-C₆H₇C₅H₆(o-C₅H₅)] (8) via CC bond formation and one-electron reduction.     

Preparation and properties of new ferrocenyl heterobimetallic complexes with counterion dependent NLO responses

New ferrocenyl-based bimetallic cationic compounds of the type of (E)-[CpFe(η⁵-C₅H₄)(CHCH)(C₆H₄)CNRuCp(PPh₃)₂]X (X=PF₆, BF₄) and of (E)-[CpFe(η⁵-C₅H₄)(CHCH)(C₆H₄)CNFeCp(CO)₂]PF₆ have been obtained and characterized. The crystal structure of (E)-[CpFe(η⁵-C₅H₄)(CHCH)(C₆H₄)CNRuCp(PPh₃)₂]BF₄ has been established by means of X-ray diffractometry. The NLO responses of the compounds have been studied by the hyper-Rayleigh scattering technique and the hyperpolarizability is found to be dependent on the nature of the counterion.     

Cationic arenechromium-nitrosyls and -hydrides

Complexes Cr(CO)₂L(C₆Me_6-nH_n), n = 0-3, L = CO and PPh₃, react with NOPF₆ in methanol/toluene to give [Cr(CO)L(NO)(C₆Me_6-nH_n)] PF₆, n = 0-3, L = CO; n = 0, L = PPh₃, and these react with nucleophiles (X^-) to give cyclohexadienyl derivatives Cr(CO)₂(NO)(C₆Me_6-nH_nX); the compounds Cr(CO)₂(PhCCPh)(C₆Me_6-nH_n) react with NOPF₆ to yield [Cr(H)(CO)₂(PhCCPh)(C₆Me_6-nH_n)] PF₆, n = 0 and 1.     

Direktsynthese von orthometallierten Ketonen des Typs RCO (o-C6H4)Mn(CO)4−nLn (R = Alkyl- und Arylrest,n = 0, 1, 2, L = Ligand)

Direct Synthesis of Orthometallated Ketones of the Type RCO(o-C₆H₄)Mn(CO)_4?nL_n (R = Alkyl and Aryl Groups, n = 0, 1, 2, L = Ligand) The starting materials of the type RMn(CO)_5?nL_n und (C₆H₅)₂ Hg react to the products of the type RCO(o-C₆H₄)Mn(CO)_4?nL_n[n = 0, R = Ch₃, C₂H₅, C₃H₇, C₆H₅,CH₂; R = C₆H₅, n = 1, L = E(C₆H₅)₃, E = P, As, Sb; R = C₆H₅, n = 2, L = P(OR′)₃, R′ = C₆H₅, CH₃, C₂H₅, C₃H₇]. Steps of their complex reaction pathway are proposed. These orthometallated substances have been characterized by means of ¹H-n.m.r., i.r. and u.v. spectroscopic measurements. The determination of the molecular structure of the two compounds RCO(o-C₆H₄)Mn(CO)₃L [R = C₂H₅, L = CO; R = C₆H₅, L = As(C₆H₅)₃] show that both contain a planar heterocyclic five-membered ring of the type .     

Stereoselektive Hydrid-Addition an von N-Methylbenzimidchlorid abgeleitete Carbenchelatkomplexe

Upon reaction with NaBH₄ the carbene chelates [C₅H₅(CO)_xMC(C₆H₅N(CH₃)C(C₆H₅)N(CH₃)]PF₆ (I,M = Mo, x = 2; II,M = Fe, x = 1) are reduced at the carbene carbon with formation of the neutral compounds C₅H₅(CO)_xMC(H)(C₆H₅)N(CH₃)C(C₆H₅)N(CH₃) (III and IV). Depending on the orientation of the incoming H substituent with respect to the C₅H₅ ligand two different isomers A and B are obtained which can be separated by column chromatography. Whereas the H^? addition to the Fe compound II is almost stereospecific (formation of 95% IVB), the stereoselectivity of the H^? addition to the Mo compound I is influenced by a competitive metal centered rearrangement of III in opposite direction. The approach to the equilibrium IIIA/IIIB 85/15 can be measured by ¹H NMR spectroscopy (ΔG³₃₂₈ 26.6 kcal/mol).     

Synthese und reaktivität von dienylmetall-verbindungen: XXI. Cyclopentadienylnickel-komplexemit sterisch anspruchsvollen phosphan-liganden

Bulky phosphanes PR₃ (R = C₆H₁₁, iC₃H₇, t-C₄H₉, C₆H₄CH₃-o) stabilize complexes of type [C₅H₅Ni(PR₃)L]BF₄ (L=S(CH₃)₂, (CH₃)₃PS), from which [C₅H₅Ni(PR₃)₂]⁺ cations are obtained. Iodide replaces the sulfur ligands to yield neutral C₅H₅Ni(PR₃)I compounds. No stable [C₅H₅Ni(PR₃)]⁺ cations could be obtained by iodide abstraction, but [C₅H₅Ni(PR₃)CO]⁺ cations were formed in the presence of carbon monoxide.     

A study of β-hydride abstraction from alkanediyl homobimetallic complexes [{Cp(CO)2Fe}2{μ-(CnH2n)}] (n = 4-10, Cp = η-C5H5)

The alkyl-bridged iron(II) complexes [{Cp(CO)₂Fe}₂{μ-(C_nH_2n)}] (n = 6-10, Cp = η⁵-C₅H₅) undergo both single and double hydride abstraction when reacted with one equivalent of Ph₃CPF₆ to give both the monocationic complexes, [{Cp(CO)₂Fe}₂{μ-(C_nH_2n−1)}]PF₆, and the dicationic complexes, [{Cp(CO)₂Fe}₂{μ-(C_nH_2n−2)}](PF₆)₂. The ratios of monocationic to dicationic complexes decrease with the increase in the value of n. The complexes where n = 4 and 5 undergo only single hydride abstraction under similar conditions. When reacted with two equivalents of Ph₃CPF₆, the complexes where n = 6-10 undergo double hydride abstraction to give dicationic complexes only. In contrast, the complex where n = 5 gives equal amounts of the monocationic and the dicationic complexes, while the complex where n = 4 only gives the monocationic complex. ¹H and ¹³C NMR data show that in the monocationic complexes one metal is σ-bonded to the carbenium ion moiety while the other is bonded in a η²-fashion forming a chiral metallacylopropane type structure. In the dicationic complexes both metals are bonded in the η²-fashion. The monocationic complexes where n = 4-6, react with methanol to give η¹-alkenyl complexes[Cp(CO)₂Fe(CH₂)_nCHCH₂] (n = 2-4) as the major products and σ-bonded ether products [{Cp(CO)₂Fe}₂{μ-(CH₂)_nCH(OCH₃)CH₂}] as the minor products. The complex where n = 8 reacted with iso-propanol to give the η¹-alkenyl complex [Cp(CO)₂Fe(CH₂)₆CHCH₂]. The dicationic complexes where n = 5, 8 and 9 were reacted with NaI to give the respective α, ω-dienes and [Cp(CO)₂FeI].     

Reactions of metal carbonyl derivatives : XIII. Reaction of bis[μ-(tert-butylsulphido)tricarbonyl-iron] with tertiary and ditertiary phosphines and phosphites

The ligands L  P(C₂H₅)₃, P(C₆H₅)₃, P(OCH₃)₃ and P(OC₆H₅)₃ react with [Fe(CO)₃(S-t-C₄H₉)]₂ to give mono-substituted Fe₂(CO)₅L(S-t-C₄H₉)₂ or bis-substituted [Fe(CO)₂L(S-t-C₄H₉)]₂ depending on the reaction conditions. With the exception of [Fe(CO)₂P(C₂H₅)₃(S-t-C₄H₉)]₂, the latter derivatives occur both in solution and in the solid state as a single isomer in which the ligands L are bonded trans to the metal-metal bond. Whereas an asymmetrically bis-substituted product, Fe(CO)₃(S-t-C₄H₉)₂Fe(CO)L' is formed in the reaction of [Fe(CO)₃(S-t-C₄H₉)]₂ with L' &2.dbnd; cis-(C₆H₅)₂PC₂H₂P(C₆H₅)₂, symmetrically bis-substituted derivatives [Fe(CO)₂(S-t-C₄H₉)]₂L', in which the ligand bridges the two iron atoms are produced in the corresponding reactions involving L'  (C₆H₅)₂P(CH_2nP(C₆H₅)₂ (n  1 and 2). The NMR spectrum of [Fe(CO)₂P(OCH₃)₃(S-t-C₄H₉)]₂, as well as those of the complexes [Fe(CO)₂P(OCH₃)₃SR]₂ (R  CH₃ and i-C₃H₇) which have also been synthesised in this study, is interpreted in terms of a virtual coupling effect.