Functionalization of η4-Dienetricarbonyiron Complexes: A Facile Route to η4-Dienetri-Carbonyliron Complexes Bearing Carbonyl Groups VIA Darzens-Nenitzescu Reaction

The asymmetric Darzens-Nenitzescu reaction of acyl chloride adjacent to planar chiral (η⁴-diene) Fe(CO)₃ fragment with alkenes in the presence of Lewis acid afforded (η⁴-diene) Fe(CO)₃ complexes bearing β-chloroketones and unsaturated ketones in good yields.     

Synthesis,structure determination and chemoselective catalytic studies of amino acids complexes of osmium(II)

The two new half sandwich amino acids complexes of osmium, i.e. [Os(η6‐p‐cymene)(κ1‐N‐(rac)‐phenylglycine methylester)Cl₂] ( A ) and [Os(η6‐p‐cymene)(κ1‐N,N′‐(S)‐phenylalanineamido)Cl] ( B ) have been synthesized and employed for chemoselective reduction of ketones (nine α,β‐unsaturated ketones and three saturated ketones). The complexes were characterized by spectroscopic as well as analytical methods; their solid structures were confirmed by single‐crystal X‐ray analysis. Both of the osmium complexes catalyze the reduction of α,β‐unsaturated ketones to saturated ketones via isomerization of the initially produced allylic alcohols. The reducible substrates were studied to obtain information on the steric and electronic factors which may affect the interaction of the substrate with the metal center and, thus, control the selectivity of the hydrogen‐transfer reductions. Copyright © 2008 John Wiley & Sons, Ltd.     

Benzene and Its Derivatives as Bridging Ligands in Transition-Metal Complexes

Transition-metal complexes in which two or more metal atoms are bridged by one or more arene ligands led a shadowy existence in comparison to the extensive class of mononuclear arene complexes. Arene bridges can occur in a variety of coordination modes and with almost all of the transition–metal elements of the periodic table. Nowhere else are found so many forms of distorted and bent arene rings. The binuclear compounds can be divided into two classes: adducts which show relatively weak metal–arene bonding and complexes which show strong arene–metal interaction. Most of the adducts are in equilibrium with mononuclear complexes in solution or are only stable in the solid state (often as polymers). In both classes syn and anti coordination occurs; their geometries show a wide variation between the extreme cases of η¹ : η¹-bridge and η⁶ : η⁶-triple-decker structure. Metal surfaces with chemisorbed arenes can be seen as a form of multinuclear arene–metal complexes. On transition-metal surfaces, benzene can be bonded to one, two, or four surface atoms. Molecular clusters with face-capping arene ligands that are bonded to three metal atoms have until now mainly been limited to two classes. The arenes bound to {(CO)₃M}₃ (M = Ru, Os) or (CpCo)₃ clusters as μ₃-η² : η² : η² ligands show only a weak trigonal distortion towards a Kekulé structure. Detailed investigations of the molecular structure and ligand dynamics of [(CpCo)₃(μ₃-arene)] complexes considerably help the understanding of the bonding of arenes to metal clusters and to metal surfaces.     

Synthesis and 1H-, 13C-, and 57Fe-NMR spectra of mono- and bis[tricarbonyl(η4-diene)iron], and (η3-allyl)tetracarbonyliron trifluoroborate complexes

A variety of mono- and bis[Fe(CO)₃(η⁴-diene)] complex with alky, CH₂OH, CHO, COCH₃, COOR, and CN substituents on the 1,3-diene system have been synthesized. Dienes with a (Z)-configuration terminal Me group show steric inhibition of metal complexation resulting in lower yields and formation of tetracarbonyl(η²-diene) and tricarbonyl(η⁴-heterodiene) complexes as additional products. Regioselective attack by C-nucleophiles at the carbonyl C-atoms of the functional group with or without concomitant 1,3 mogration of the Fe(CO)₃ group was used to synthesize polyenes and isoprenoid building blocks as mono- or dinucliar Fe(CO)₃ complexes. Wittig-Horner-type reactions of Fe(co)₃-complexed synthons result in sterospecific formation of (E)-configurated olefins. The ¹H-, ¹³C- and ⁵⁷Fe-NMR spectra of olefinic and allylic organoiron complexes are reported, H,H,C,H, and C,C coupling constants have been evaluated and are analyzed in terms of the geometry of the coordinated diene. The results are corroborated by the crystal structure of tricarbonyl[3–6-η-((E)-6-methyl-3,5-heptadiene-2-one)]iron( 34 ) which shows an unusual distortion of the (CH₃)₂C = group, The ⁵⁷Fe-NMR chemical shifts extend over the ranges of 0–600 ppm for [Fe(CO)₃(η⁴-diene)] complexes, 780–1710 ppm for [Fe(CO)₄(η³-allyl)] [BF₄] and [FeX(CO)₃(η⁴-allyl)] complexes, and 1270–1690 ppm for [Fe(CO)₃(η⁴-enone)] complexes, relative to Fe(CO)₅.     

Catalytic diversity of diene complexes of niobium and tantalum on polymerizations of ethylene,norbornene, and methyl methacrylate

Half‐metallocene diene complexes of niobium and tantalum catalyzed three‐types of polymerization: (1) the living polymerization of ethylene by niobium and tantalum complexes, MCl₂(η⁴‐1,3‐diene)(η⁵‐C₅R₅) ( 1‐4 ; M = Nb, Ta; R = H, Me) combined with an excess of methylaluminoxane; (2) the stereoselective ring opening metathesis polymerization of norbornene by bis(benzyl) tantalum complexes, Ta(CH₂Ph)₂(η⁴‐1,3‐butadiene)(η⁵‐C₅R₅) ( 11 : R = Me; 12 : R = H) and Ta(CH₂Ph)₂(η⁴‐o‐xylylene)(η⁵‐C₅Me₅) ( 16 ); and (3) the polymerization of methyl methacrylate by butadiene‐diazabutadiene complexes of tantalum, Ta(η²‐RN=CHCH=NR)(η⁴‐1,3‐butadiene)(η⁵‐C₅Me₅) ( 25 : R = p‐methoxyphenyl; 26 : R = cyclohexyl) in the presence of an aluminum compound ( 24 ) as an activator of the monomer.     

Niob- und Tantalkomplexe mit P2- und P4-Liganden

Niobium and Tantalum Complexes with P₂ and P₄ Ligands The photolysis of [Cp″Ta(CO)₄] 1 (Cp″ = C₅H₃^tBu₂?1,3) and P₄ affords Cp″(CO)₂Ta(η⁴?P₄) 2 , [{Cp″(CO)Ta}₂(m??η^2:2?P₂)₂] 3 and [Cp₃″(CO)₃Ta₃(P₂)₂] 4 . In a photochemical reaction 2 and [Cp*Nb(CO)₄] 5 form [{Cp*(CO)Nb}{Cp″(CO)Ta}(m??η^2:2?P₂)₂] 6 and [{Cp*(CO)₂Nb} {Cp*Nb}{Cp″(CO)Ta}(m?₃?η^2:1:1?P₂)₂] 7 , a compound with the novel m?₃?η^2:2:1?P₂-coordination mode. The reaction of 2 and [Cp*Co(C₂H₄)₂] 8 leads to [{Cp*Co} {Cp″(CO)Ta}(m??η^2:2?P₂)₂] 9 , a heteronuclear complex with an ?early”? and a ?late”? transition metal. Complexes 2, 3, 7 and 9 have been further characterized by X-ray structure analyses.     

Cyclodimerization of 1-(Dimethoxyniethyl)-5,6-dimethylidene-7-oxa-bicyclo[2.2.1]hept-2-ene Induced by Nonacarbonyldiiron. Crystal Structure of (1RS, 2SR, 3RS, 4RS, 4aRS, 9aSR)- Tricarbonyl[C, 2,3, C-η-(1,4-epoxy-1,5-bis(dimethoxymethyl)-2,3-dimethylidene-1,2,3,4,4a,9,9a,10-octahydroanthracene)]iron

The transition-metal-carbonyl-induced cyclodimerization of 5,6-dimethylidene-7-oxabicyclo[2.2.1]hept-2-ene is strongly affected by substitution at C(1) While 5,6-dimethylidene-7-oxabicyclo[2.2.1]hept–2-ene-l-methanol ( 7 ) refused to undergo [4 + 2]-cyclodimerization in the presence of [Fe₂(CO)⁹] in MeOH, 1-(dimethoxymethyl)-5,6-di-methylidene-7-oxabicyclo[2.2.1]hept-2-ene ( 8 ) led to the formation of a 1.7:1 mixture of ‘trans’ ( 19, 21, 22 ) vs. ‘cis’ ( 20, 23, 24 ) products of cyclodimerization together with tricarbonyl[C, 5,6, C-η-(l-(dimethoxymethyl)-5,6-di-methylidenecyclohexa-1,3-diene)]iron ( 25 ) and tricarbonyl[C,3,4, C-η-(methyl 5-(dimethoxymethyl)-3,4-di-methylidenecyclohexa-1,5-diene-l-carboxylate)]iron ( 26 ). The structures of products 19 and of its exo ( 21 ) and endo ( 22 ) [Fe(CO)₃(1,3-diene)]complexes) and 20 (and of its exo ( 23 ) and endo (24) (Fe(CO)₃(1,3-diene)complexes) were confirmed by X-ray diffraction studies of crystalline (1RS, 2SR, 3RS, 4RS, 4aRS, 9aSR)-tricarbonyl[C, 2,3, C-η-(1,4-epoxy-1,5-bis(dimethoxymethyl])-2,3-dimethylidene-1,2,3,4,4a,9,9a,10-octahydroanthracene)iron ( 21 ). In the latter, the Fe(CO)₃(1,3-diene) moiety deviates significantly from the usual local C_s symmetry. Complex 21 corresponds to a ‘frozen equilibrium’ of rotamers with η-alkyl, η³-allyl bonding mode due to the acetal unit at the bridgehead centre C(1).     

Tris(η4-1-oxa-1,3-diene)molybdenum and -tungsten: Preparation and properties of new homoleptic enone complexes

Homoleptic tris(η⁴-1-oxa-1,3-diene)complexes of tungsten and molybdenum have been prepared in moderate to high yields starting from the α, β-unsaturated ketones 1–3 and tricarbonyltris(propionitrile)tungsten(0) or from 1 and η⁶-benzenetricarbonylmolybdenum(0), respectively. The method is limited to enone ligands without substituents in the β-position. The new crystalline and air-stable yellow complexes have been characterized, and preliminary reactivity studies carried out. Their NMR-spectra show the compounds to have three-fold symmetry.     

Photochemische Reaktionen von Cyclopentadienylbis(ethen)rhodium mit Phenanthren,Acenaphthylen und Triphenylen,sowie ungewöhnlicher H-Austausch zwischen η2-koordinierten Phenanthren- bzw. Acenaphthylen- und η5-Cyclopentadienyl-Liganden

Photochemical Reactions of Cyclopentadienylbis(ethene)rhodium with Phenanthrene, Acenaphthylene, and Triphenylene, and Unusual H Exchange between η²-Coordinated Phenanthrene or Acenaphthylene and η⁵-Cyclopentadienyl Ligands During UV irradiation of [CpRh(C₂H₄)₂] (Cp = η⁵-C₅H₅) in hexane/ether in the presence of phenanthrene one ethene ligand is displaced by coordination of the 9,10 double bond of phenanthrene, and (η⁵-cyclopentadienyl) (η²-ethene)(η²-9,10-phenanthrene)rhodium ( 1 ) is formed. The analogous reaction in hexane in the presence of acenaphthylene occurs with formation of the complexes (η²-1,2-acenaphthylene)(η⁵-cyclopentadienyl)(²-ethene)rhodium 2 and bis(η²-1,2-acenaphthylene)(η⁵-cyclopentadienyl)rhodium 3 in which one and two ethene molecules of [CpRh(C₂H₄)₂], respectively, are substituted by η²-1,2-acenaphthylene. The irradiation of [CpRh(C₂H₄)₂] with triphenylene in hexane yields the compounds [CpRh(η⁴-1,2,3,4-triphenylene)] ( 4 ), [(CpRh)₂(μ-η³: η³-triphenylene)] ( 5 ), and [(CpRh)₃(μ₃-η²: η²: η²-triphenylene)] ( 6 ). Despite the partially very low yields the new complexes could be unequivocally characterized spectroscopically and in the case of 1 and 3 by X-ray structural analysis. The compounds 1 and 2 in solution reveal a novel dynamic behaviour; via an intramolecular C? H activation, exchange occurs between the protons of the η²-coordinated arene and the Cp ligand. The complex 4 in solution is fluxional, too.     

One-Bond 13C, 13C-Spin Coupling in η4-, η3- and η2-Olefin Transition Metal Complexes

One-bond ¹³C, ¹³C-spin-coupling constants have been measured, with natural isotope abundance, in η⁴-diene, η³-allyl and η²-ene transition-metal carbonyl complexes. Typical values of ¹J(C,C) are given for Fe-, Ru- and Os-complexes with the three types of olefinic ligands. The effects of substituents and the structural significance of the C,C-coupling constants are discussed.     

A Density Functional Theory Investigation of the Cobalt‐Mediated η5‐Pentadienyl/Alkyne [5+2] Cycloaddition Reaction: Mechanistic Insight and Substituent Effects

Alkyl‐substituted η⁵‐pentadienyl half‐sandwich complexes of cobalt have been reported to undergo [5+2] cycloaddition reactions with alkynes to provide η²,η³‐cycloheptadienyl complexes under kinetic control. DFT studies have been used to elucidate the mechanism of the cyclization reaction as well as that of the subsequent isomerization to the final η⁵‐cycloheptadienyl product. The initial cyclization is a stepwise process of olefin decoordination/alkyne capture, C? C bond formation, olefin arm capture, and a second C? C bond formation; the initial decoordination/capture step is rate‐limiting. Once the η²,η³‐cycloheptadienyl complex has been formed, isomerization to η⁵‐cycloheptadienyl again involves several steps: olefin decoordination, β‐hydride elimination, reinsertion, and olefin coordination; also here the initial decoordination step is rate limiting. Substituents strongly affect the ease of reaction. Pentadienyl substituents in the 1‐ and 5‐positions assist pentadienyl opening and hence accelerate the reaction, while substituents at the 3‐position have a strongly retarding effect on the same step. Substituents at the alkyne (2‐butyne vs. ethyne) result in much faster isomerization due to easier olefin decoordination. Paths involving triplet states do not appear to be competitive.     

d6 and d8 Metal Carbonyl Complexes of 7,7-Dimethoxy-5,6-dimethylidenebicyclo[2.2.1]hept-2-ene. Stereoselective Hydroformylation of an [Fe(CO)4(olefin)] Complex

Reaction of 7,7-dimethoxy-5,6-dimethylidenebicyclo[2.2.1]hept-2-ene ( 2 ) with various metal carbonyls and their derivatives gave the η²-M(CO)₄ (M = Fe ( 17 ), Ru ( 18 )), η⁴-M(CO)₃ (M = Fe ( 19x, 19n ), Ru ( 20n )), and η²-M(CO)₅ and η⁶-M(CO)₃ (M = Cr, Mo, W) complexes. The trigonal bipyramidal η²-M(CO)₄ complexes present an exceptional C_3v symmetry at the metal with the C,C-double bond in an axial position. In all the η²-complexes, this double bond is stereospecifically coordinated by its exo-vs. endo-η⁴-Fe(CO)₃ configuration was established by chemical correlation (hydrolysis, hydrogenation) with the corresponding complexes ( 24x, 24n ) of 7,7-dimethoxy-2,3-dimethylidenebicyclo[2.2.1]heptane ( 5 ). The relative rates of hydrolysis (AcOH/H₂O 2:1, 50°C) of ligands 2 and 5 and of complexes 19x, 19n, 24x , and 24n to the corresponding ketones showed an acceleration effect only when the metal is coordinated to the exo-face. This was attributed to an F-strain effect on the leaving group of the substrate. Compound 17 was further metallated by [Fe₂(CO)₉] giving the bimetallic isomers 21xn and 21xx . The endocyclic C,C-double bond of the latter can be stereospecifically hydroformylated (1 atm CO, AcOH/H₂O, 25°C) giving 29x (49%). Hydroformylation of 17 gave the corresponding uncoordinated aldehydes 30x/30n in better yields (76%) but with lower selectivity (3:1). These are the first examples of hydroformylation of an isolated [Fe(CO)₄(olefin)] complex.     

Oxidation of Tricarbonyl (η1 η2-but-3-en-1-yl)iron(0)and Tricarbonyl (η3-allyl)iron(0) Anion Complexes with Dioxygen

The addition of reactive carbanions to tricarbonyl(η⁴-1,3-diene)iron(0) complexes proceeded at ?78 °C to give putative tricarbonyl(η¹,η²-but-3-en-1-y1)iron(0) anion complexes and at 25 °C to produce postulated tricarbonyl(η³-allyl)iron(O) anion complexes; trapping of reactive intermediates with dioxygen produced γ,δ-unsaturated acids and allylic alcohols, respectively.     

Base Stabilized σ-Borane Complex of Group 5 Metals: Synthesis and Structure of [(η5-C5Me5)Ta(H3BNC5H4) (C5H4NS)(η2-S2)]

Azametallacyclopropane-containing base stabilized borane complexes of group 5 transition metals have been synthesized and their structural aspects have been described. Treatment of Cp* based Ta and Nb chlorides, Cp*TaCl₄ and Cp*NbCl₄ with [LiBH₄ ⋅ THF] followed by addition of ligands, such as 2-mercaptobenzothiazole, MBT, (C₇H₅NS₂) and 2-mercaptobenzoxazole, MBO (C₇H₅NSO) led to the formation of complexes [Cp*M-[BHS(CH₂ENC₆H₄)(C₇H₄NSE)] ( 1 : M=Ta, E=S; 2 ; M=Nb, E=S; 3 : M=Ta, E=O; 4 ; M=Nb, E=O, Cp*=pentamethyl-η⁵-cyclopentadienyl). By means of UV-vis absorption spectra, the electronic properties of these complexes associated with central metal atoms and heteroatoms (S or O) have been evaluated. In contrast, treatment of Cp*TaCl₄ with 2-mercaptopyridine, MP, (C₅H₅NS) under the same reaction conditions yielded the agostic σ-borane Ta complex, [Cp*Ta(H₃BNC₅H₄) (C5H₄NS)(η²-S₂)], 5 . Unlike 1 – 4 , where the metals interact with boron through bridging sulphur, 5 shows a notable σ-B−H bond interaction with Ta. All spectroscopic data of 1 – 5 along with the X-ray diffraction studies suggest complexes 2 , 4 , and 5 are base (amine) stabilized borane species. Computational studies based on Density Functional Theory (DFT) also supported this conclusion.     

Reaction of platinum complexes with (+)-α-pinene and (+)-limonene. Synthesis, molecular structure, and catalytic activity of dichloro(η4-[p-mentha-1,8{9}-diene])platinum(II)

The transformations of platinum(II) and platinum(IV) complexes with inner-and outer-sphere ligands by the action of (+)-α-pinene and (+)-limonene were studied. Reduction of the metal complex is the main process whose rate increases in the following outer-sphere ligand series: (Me₂SO)₂H⁺ < Et₃NH⁺ < K^? < H⁺. The reaction of K₂PtCl₄ with α-pinene gave cis-terpine monohydrate and dichloro-η⁴-[p-mentha-1,8(9)-diene]platinum(II), and their structure was proved by X-ray analysis. The complex belongs to monoclinic crystal system, the Pt-Cl and Pt-C bonds therein have different lengths, the ClPtCl angle is 85.88°, and the C=C bond plane is orthogonal to the square coordination core. Dichloro-η⁴-[p-mentha-1,8(9)-diene]-platinum(II) was tested as catalyst in the hydrosilylation of acetophenone with diphenylsilane.     

Fine Tuning of Metallaborane Geometries: Chemistry of Metallaboranes of Early Transition Metals Derived from Metal Halides and Monoborane Reagents

Reaction of [Cp_nMCl_4?x] (M=V: n=x=2; M=Nb: n=1, x=0) or [Cp*TaCl₄] (Cp=η⁵‐C₅H₅, Cp*=η⁵‐C₅Me₅), with [LiBH₄?thf] at ?70 °C followed by thermolysis at 85 °C in the presence of [BH₃?thf] yielded the hydrogen‐rich metallaboranes [(CpM)₂(B₂H₆)₂] ( 1 : M=V; 2 : M = Nb) and [(Cp*Ta)₂(B₂H₆)₂] ( 3 ) in modest to high yields. Complexes 1 and 3 are the first structurally characterized compounds with a metal–metal bond bridged by two hexahydroborate (B₂H₆) groups forming a symmetrical complex. Addition of [BH₃?thf] to 3 results in formation of a metallaborane [(Cp*Ta)₂B₄H₈(μ‐BH₄)] ( 4 ) containing a tetrahydroborate ligand, [BH₄]^?, bound exo to the bicapped tetrahedral cage [(Cp*Ta)₂B₄H₈] by two Ta‐H‐B bridge bonds. The interesting structural feature of 4 is the coordination of the bridging tetrahydroborate group, which has two B? H bonds coordinated to the tantalum atoms. All these new metallaboranes have been characterized by mass, ¹H, ¹¹B, and ¹³C NMR spectroscopy and elemental analysis and the structural types were established unequivocally by crystallographic analysis of 1 – 4 .     

Reaction of Tricarbonyl(η3-allyl)iron(0) Anions with Bromine

The addition of reactive carbanions to tricarbonyl(η⁴-1,3-diene)iron(0) complexes proceeded at 23°C to give putative tricarbonyl(η³-allyl)iron(0) anion complexes. Trapping of the reactive intermediates with bromine produced nucleophilic-substituted tricarbonyl(η⁴-1,3-diene)iron(0) complexes.     

Synthesis and Structure of a New Type of Chiral Organoiron and Organoruthenium Complexes

The synthesis of 36 [Fe(CO)₂L¹(η⁴-diene)], three [Fe(CO)₂L¹(η⁴-encne)], and five [Ru(CO)₂L¹(η⁴-diene)] complexes (L¹ = Ph₃P, Et₃P, (EtO)₃P, (MeO)₃p, C₆H₁₁NC) by thermal, selective CO ligand displacement in the corresponding tricarbonyl precursor complexes is described. In a second step, photochemical CO displacement by another phosphorus ligand L² leads to a new type of η⁴-diene complexes with a centre of chirality at the metal atom (Fe, Ru). 23 Fe and three Ru complexes of this type have been prepared and characterized. In the case of complexes with unsymmetrical dienes, racemic diastereoisomers are formed which can be separated by chromatographic methods. The molecular structures of [Fe(CO)(Ph₃P)((MeO)₃P)(buta-1,3-diene)] ( 52 ), [Fe(CO)(Ph₃P)((MeO)₃P)(isoprene)] ( 58 ) and [Fe(CO)(Et₃P)(EtO)₃P(hexa-2,4-dienal)] ( 62a ) were determined by X-ray diffraction. All complexes were investigated by ¹³C-, ³¹P- and, in part, ¹H-NMR spectroscopy. At low temperatures, conformational isomers (rotamers) can be differentiated which probably arise from ψ rotation at the coordinated metal centre.     

Dianionic Mononuclear Cyclo‐P4 Complexes of Zero‐Valent Molybdenum: Coordination of the Cyclo‐P4 Dianion in the Absence of Intramolecular Charge Transfer

Relative to other cyclic poly‐phosphorus species (that is, cyclo‐P_n), the planar cyclo‐P₄ group is unique in its requirement of two additional electrons to achieve aromaticity. These electrons are supplied from one or more metal centers. However, the degree of charge transfer is dependent on the nature of the metal fragment. Unique examples of dianionic mononuclear η⁴‐P₄ complexes are presented that can be viewed as the simple coordination of the [cyclo‐P₄]^2? dianion to a neutral metal fragment. Treatment of the neutral, molybdenum cyclo‐P₄ complexes Mo(η⁴‐P₄)I₂(CO)(CNAr^Dipp2)₂ and Mo(η⁴‐P₄)(CO)₂(CNAr^Dipp2)₂ with KC₈ produces the dianionic, three‐legged piano stool complexes, [Mo(η⁴‐P₄)(CO)(CNAr^Dipp2)₂]^2? and [Mo(η⁴‐P₄)(CO)₂(CNAr^Dipp2)]^2?, respectively. Structural, spectroscopic, and computational studies reveal a similarity to the classic η⁶‐benzene complex (η⁶‐C₆H₆)Mo(CO)₃ regarding the metal‐center valence state and electronic population of the planar‐cyclic ligand π system.     

Variants of Solid-State and Solution Structures of (η3-Allyl)- {2-[2′-(diphenylphosphino)phenyl]-4,5-dihydrooxazole-P,N}palladium(II) hexafluorophosphates and tetraphenylborates

Crystal and solution structures of the enantiomerically pure and the racemic pairs of (η³-allyl) {2-[2′-(diphenylphosphino)phenyl]-4,5-dihydro-4-phenyloxazole}palladium(II) hexafluorophosphates ( 1 , and rac- 1 , resp.) and tetraphenylborates ( 2 , and rac- 2 , resp.) as well as (η³-allyl){2-[2′-(diphenylphosphino)phenyl]-4,5-dihydro-4-isopropyloxazole}palladium(II) tetraphenylborate ( 3 ) were characterized by X-ray crystallography and ¹H-NMR spectroscopy. In the solid state, rac- 1 and rac- 2 proved to be disordered with both diastereoisomeric complexes in the crystal. The complexes 2 and 3 exist only in the ‘exo’ form. The X-ray structures show that the [Pd^II(η³-allyl)] moiety may adopt different configurations between a nearly symmetrical three-electron Pd^II(π³-allyl) system and an asymmetrical allyl group with a η¹- and a η²-bonding to the metal center. The [Pd^II(η³-allyl)] system of rac- 1 and of ‘endo’ rac- 2 is closer to the former, and that of 2 , ‘exo’-rac- 2 , and 3 closer to the later geometry. The ¹H-NMR spectra of the hexafluorophosphates 1 and rac- 1 show two sets of signals of the allylic protons in an ‘exo’/‘endo’ ratio of 2:3. The tetraphenylborates 2, rac- 2 , and 3 give only one set of broad signals of the allylic protons.