Olefin Rotation Research Articles

Extensions of the applicability of the MMX molecular modelling system to determination of barriers to rotation of π-bonded ligands

The commercially available molecular mechanics package PCMODEL, which has been shown to be very useful for determining conformational energy profiles for rotation of σ-bonded ligands, cannot be utilized to determine barriers to rotation of π-bonded ligands because of limitations in the way the π-bonding interactions are defined. This paper describes a partially successful modification of the dihedral driver of PCMODEL that makes possible reasonable calculations of the conformational energy profiles for rotation of the π-bonded arene ligands in the compounds (η6-arene)Cr(CO)2(PPh3) (arene = C6H6, C6Me6) and of the π-bonded ethylene ligands in [PtCl3(η2-CH2=CH2)]− and [(η5-C5H5)Re(NO)(PPh3)(η2-CH2=CH2)]+. The steric barriers to ethylene rotation in the latter two complexes were found to be small relative to the electronic barriers to ligand rotation. Keywords: molecular mechanics, olefin rotation, conformational energy profile.

Olefin Rotation in the Solid State: A 13C, 1H, and 2H NMR Study of Rh(acac)(C2H4)2

Olefin Rotation in the Solid State: A 13C, 1H, and 2H NMR Study of Rh(acac)(C2H4)2

Rigid Bidentate Nitrogen Ligands in Organometallic Chemistry and Homogeneous Catalysis. 6. Zerovalent Palladium and Platinum Complexes Containing Rigid Bidentate Nitrogen Ligands and Alkenes: Synthesis, Characterization, Alkene Rotation and Substitution Reactions. X-ray Crystal Structure of [Bis((2,6-diisopropylphenyl)imino)acenaphthene](maleic anhydride)palladium(0)

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTRigid Bidentate Nitrogen Ligands in Organometallic Chemistry and Homogeneous Catalysis. 6. Zerovalent Palladium and Platinum Complexes Containing Rigid Bidentate Nitrogen Ligands and Alkenes: Synthesis, Characterization, Alkene Rotation and Substitution Reactions. X-ray Crystal Structure of [Bis((2,6-diisopropylphenyl)imino)acenaphthene](maleic anhydride)palladium(0)Rob van Asselt, Cornelis J. Elsevier, Wilberth J. J. Smeets, and Anthony L. SpekCite this: Inorg. Chem. 1994, 33, 7, 1521–1531Publication Date (Print):March 1, 1994Publication History Published online1 May 2002Published inissue 1 March 1994https://doi.org/10.1021/ic00085a050RIGHTS & PERMISSIONSArticle Views956Altmetric-Citations134LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (1 MB) Get e-AlertsSupporting Info (1)»Supporting Information Supporting Information Get e-Alerts

Mechanism of alkene rotation in octahedral d6 trans-bis(alkene) complexes of molybdenum

Mechanism of alkene rotation in octahedral d6 trans-bis(alkene) complexes of molybdenum

Five-Coordination in Platinum(II) and Palladium(II) Chemistry

Abstract In recent years a number of chelating ligands, when combined with a π-acid such as an alkene, have been found to stabilize five-coordination in palladium(II) and platinum(II) chemistry. The complexes have invariably a trigonal bipyramidal geometry with the bidentate ligand and the alkene in the equatorial plane. The π-acceptor capacity of the alkene and a small bite of the chelate ligand (between 70 and 85°) are both required for accommodating them in the equatorial plane where the d 2 z electron concentrations (which in the square planar d 8 complexes are located above and below the coordination plane) are also confined. Two mono-dentate ligands, in place of one chelate, do not stabilize the five-coordination since for entropic reasons the dissociation of one of them is strongly favored. A great bulk of the chelate ligand is also found to stabilize five-coordination since interligand steric interactions are smaller in the trigonal plane of a five-coordinate complex than in the square-plane of a four-coordinate species. Moreover the same steric factors that destabilize four- versus five-coordination can also destabilize four-versus three-coordination, and T-shaped ML3 species appear to provide a low energy path for the interconversion between four- and five-coordinate complexes. The alkene is strongly bound to the metal, the metal-carbon bond distances are shorter, and the bending back of the alkene substituents is greater than those found in four-coordinate compounds. The activation energy for alkene rotation is also greater in five-coordinate species, and in a number of cases atropisomers are generated. The chelate dinitrogen ligand also lying in the equatorial plane is, on the other hand, loosely bound to the metal, and dissociation of one end of this ligand appears to be the preliminary step in all substitution processes involving the alkene, the axial ligands, and the bidentate ligand itself. These processes should therefore occur through a mechanism analogous to that operating in four-coordinate complexes. The dissociation of one end of the bidentate ligand also provides a low energy path to proton exchange and inversion of configuration at coordinated nitrogens. Other aspects related to the conformation of the alkene and of the chelate ligand as well as to the preparation and reactivity of this class of compounds are discussed.

Ligand control of agostic M ⋯ H ⋯ C three-centre, two-electron bonding in bicyclo[2.2.1]hept-2-yl complexes of platinum and palladium. X-Ray crystal structures of [Pt(η2-C7H10){But2P(CH2)2PBut2}] and [Pt(C7H11){But2P(CH2)2PBut2}][BPh4

The reaction of non-co-ordinating acids with the Pt0 and Pd0 alkene complexes [M(η2-C7H10)(L–L)][M = Pt or Pd; L–L =(C6H11)2P(CH2)2P(C6H11)2, 1a or 1f: But2P(CH2)2PBut2, 1b or 1g; and o-C6H4(CH2PBut2)2, 1e or 1h; M = Pt, L–L =(C6H11)2P(CH2)3P(C6H11)21c or But2P(CH2)3PBut21d] affords a series of cationic bicyclo[2.2.1]hept-2-yl complexes 2a–2h in which the otherwise electron-deficient metal centre is stablized by a three-centre, two-electron (agostic) interaction with the exo-3-CH bond. The complexes were characterized by 1H, 13C and 31P NMR spectroscopy and for 1b and 2b by single-crystal X-ray crystallography. In complex 1b the norborn-2-ene is bound to the platinum in a normal in-plane η2-mode with equal Pt–C distances [2.110(7), 2.108(8)A] and Pt–P distances [2.273(2), 2.274(2)A]. For 2b the crystallographic results reveal a long Pt–Cβ contact of 2.309(5)A which is bridged by a hydrogen atom forming the agostic bond, whereas the Pt–Cα bond is shortened to 2.096(4)A. The Pt–P bond trans to the weak agostic bond is significantly shorter than the cis Pt–P bond [2.256(1) and 2.311(1)A respectively], and this asymmetry in the co-ordination of the diphosphine is reflected in the 31P NMR spectrum of 2b for which 1J(PtPtrans)1J(PtPcis). The extent of agostic interaction, as indicated by NMR parameters [1J(PtH), 1J(PtPtrans), etc.], depends on the bite angle of the diphosphine and the bulk of the substituents on phosphorus such that the smallest diphosphines induce the strongest M ⋯ H ⋯ C interaction. All the agostic complexes undergo a rapid intramolecular rearrangement on the NMR time-scale at room temperature involving β-elimination and alkene rotation. However, the 31P nuclei remain non-equivalent up to 300 K.

Four-versus five-co-ordination in palladium(II) and platinum(II) complexes containing 2,9-dimethyl-1,10-phenanthroline (dmphen). Crystal structures of [PtCl2(dmphen)] and [Pt(η2-C2H4)Cl2(dmphen)

The four-co-ordinate complexes with 2,9-dimethyl-1,10-phenanthroline [MX2(dmphen)](M = Pt, X2= Cl21, ClBr 2, Br23, or I24; M = Pd, X2= Cl25, ClBr 6, Br27 or I28) have been prepared for the first time. The crystal structure of [PtCl2(dmphen)]1, has been determined by X-ray diffraction methods. The complex has a square-planar geometry around the metal and the steric interaction between the methyl groups of dmphen and the cis chlorine ligands causes a narrowing of the Cl–Pt–Cl angle [85.8(1)°], a displacement of the two chlorine atoms from the N–Pt–N plane [0.286(4) and 0.372(4)A respectively], a bending of the phenanthroline (ca. 17°), and a rotation of the overall ligand plane with respect to the platinum co-ordination plane (ca. 28°). All interligand steric constraints are released in the five-co-ordinate complexes obtained from the four-co-ordinate species by direct uptake of olefins, [M(η2-olefin)X2(dmphen)](olefin = ethylene, a; propene, b; but-1-ene, c; cis-but-2-ene, d; trans-but-2-ene, e; or styrene, f). The structure of [Pt(η2-C2H4)Cl2(dmphen)]1a, has been determined by X-ray diffraction methods. The complex has a trigonal-bipyramidal geometry around the Pt atom; the phenanthroline ligand and the olefinic carbons occupy the equatorial plane while the two chlorine atoms are in axial positions. Bond distances and angles are similar to those found in other five-co-ordinate complexes of platinum(II). The rate of uptake of olefin by the four-co-ordinate complexes, the equilibrium constant (Kf) of the formation reaction [MX2(dmphen)]+ olefin ⇌[M(η2-olefin)X2(dmphen)], and the activation energy for olefin rotation in the five-co-ordinate complexes have been measured for different olefins and halogen ions. The uptake, in chloroform, takes place in a one-step process and the reaction rate increases by a factor of 104 going from the chloro 1 to the iodo 4 complex. The equilibrium constant for the formation reaction decreases by a factor of 103 going from ethylene to trans-but-2-ene and increases by a factor of 102 going from the chloro 1 to the iodo 4 species. The activation energy for olefin rotation (ethylene and propene)(ca. 20.5 kcal mol–1) is higher than any reported value for platinum complexes and does not appear to depend upon the nature of the halogen atoms.

Synthesis, structure, and dynamic behaviour of (p-quinone)copper(I) complexes. Crystal structure determination of a copper p-benzoquinone complex stabilized by a tridentate oxygen ligand

The quinonecopper(I) complexes [Cu(olefin)L][olefin =p-benzoquinone (bq), methyl-p-benzoquinone, or 1,4-naphthoquinone] of the tridentate oxygen ligand L–=[(C5H5)Co{P(O)(OCH3)2}3]– have been prepared by the reaction of NaL and the appropriate quinone with [Cu(CH3CN)4]PF6 and by carbon monoxide substitution from [Cu(CO)L]. The complex [Cu(bq)L] crystallizes in space group P(Z= 2) with a= 11.358(6), b= 12.138(7), c= 12.465(6)A, α= 92.18(2), β= 134.24(2), γ= 99.49(4)°, and its X-ray crystal structure was refined to R= 0.056 for 4 140 reflections [|Fo| > 2.5σ(Fo)]. The co-ordination about the copper atom is distorted tetrahedral with one long [2.184(2)A] and two short [1.966(2), 2.001 (2)A] copper–oxygen contacts. p-Benzoquinone co-ordinates through one olefinic double bond to the copper atom. The complex dismutates in solution according to the equilibrium 2 [Cu(bq)L]⇌[LCu(µ-bq)CuL]+ bq. In addition, it exhibits several other dynamic processes, the lowest-energy of which is olefin rotation. The intramolecular exchange of the free and the co-ordinated double bond gives rise to a coalescence temperature of about 305 K in the 1H n.m.r. spectrum, corresponding to an activation barrier of ca. 60 kJ mol–1. Intermolecular exchange of co-ordinated and non-co-ordinated bq becomes rapid on the n.m.r. time-scale above ca. 370 K.

ChemInform Abstract: Widely Varying Olefin Rotation Geometry in Platinum(II)‐Olefin Complexes Containing the η3‐Allyl Ligand. Molecular Structure, Solution Stability, and Molecular Orbital Studies.

AbstractThe molecular structures of the cationic (η3‐allyl)olefin‐Pt complexes (Ia) (space group P21/n, Z=4), (Ib)·0.5 CH2Cl2 (Pccn, Z = 8), (Ic)·C6H6 and (Id) (for all P21/c, Z=4) are determined by X‐ray analysis and compared.

Widely varying olefin rotation geometry in platinum(II)-olefin complexes containing the .eta.3-allyl ligand. Molecular structure, solution stability, and molecular orbital studies

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTWidely varying olefin rotation geometry in platinum(II)-olefin complexes containing the .eta.3-allyl ligand. Molecular structure, solution stability, and molecular orbital studiesKunio. Miki, Kazuhiko. Yamatoya, Nobutami. Kasai, Hideo. Kurosawa, Akira. Urabe, Mitsuhiro. Emoto, Kazuyuki. Tatsumi, and Akira. NakamuraCite this: J. Am. Chem. Soc. 1988, 110, 10, 3191–3198Publication Date (Print):May 1, 1988Publication History Published online1 May 2002Published inissue 1 May 1988https://doi.org/10.1021/ja00218a031RIGHTS & PERMISSIONSArticle Views105Altmetric-Citations17LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (3 MB) Get e-AlertsSupporting Info (1)»Supporting Information Supporting Information Get e-Alerts

Organometallic ‘helicopters.’ Characterisation of the non-rigid triosmium arene–alkene complexes [Os3(CO)8(η2-CH2CHR)(µ3-η2:η2:η2-C6H6)], (RH, Ph) and the X-ray crystal structure of [Os3(CO)8(η2-CH2CH2)(µ3-η2:η2:η2-C6H6)

The cluster-alkene complexes [Os3(CO)8(η2-CH2CHR)(µ3-η2:η2:η2-C6H6)](RH, Ph) contain benzene ligands in a face-capping co-ordination mode, and the geometry of the ethylene derivative has been confirmed by a single-crystal X-ray analysis; the intramolecular dynamics of these complexes have been investigated in solution by n.m.r. spectroscopy and are discussed in terms of arene ring-whizzing and alkene rotation.

Preparation and spectroscopic study of functionally substituted cyclopentadienides of thallium(I), potassium, rhodium(I), and iridium(I)

A 13C and 1H n.m.r. study has been carried out on a series of monosubstituted cyclopentadienide compounds of type MI(C5H4X)[M = K or Tl; X = Cl, COMe, CO2Me, CHO, Ph, COCO2Et, or C(CN)C(CN)2]. The different patterns observed for the ring nuclei reflect both the Lewis-acid character of the metal and the electronic effect of the substituent. Fluxional behaviour is observed for K(C5H4COMe). Pseudo-first-order rate constants have been evaluated for hydrogen–deuterium exchange in several of the potassium compounds. The thallium(I) compounds generally exhibit greater synthetic utility than their potassium analogues and several have been used in the synthesis of new [M(η5-C5H4X)(η2-C2H4)2](M = Rh or Ir) complexes. Activation barriers for alkene rotation and mass spectral fragmentation patterns are discussed for these compounds. The 1H n.m.r. spectra found for the cyclopentadienyl ring indicate that the M–ring bonding is not fully delocalised. Analysis of the products resulting from reaction between methyl chloroformate and cyclopentadienide ion confirms the strongly electrophilic character of the former reagent.

Electronic effect of η 5-cyclopentadienyl and η 3-allyl ligands on the nature of the metal—olefin bond: Reversal of the relative olefin affinity of palladium(II) and platinum(II) ions

The barrier to olefin rotation in [Pt(η 3-CH 2CMeCH 2)(olefin)(PPh 3)]PF 6 ( 3) (olefin = CH 2CH 2, E-MeCHCHMe) has been found to be extremely low compared to those in the other known, 4-coordinate olefin complexes of Pt II. This can be ascribed to the smaller steric congestion around the olefin in 3. The corresponding barrier in [Pt(η 5-C 5H 5)(CH 2CH 2)(PPh 3]ClO 4 ( 2), possessing likewise small steric congestion, was substantially higher than that in 3 (olefin = CH 2CH 2). The 13C and 31P NMR measurements have revealed much larger J(Pt-C(olefin)) in 2 than that in 3 (olefin = CH 2CH 2), while J(Pt-P) are comparable in these two. Stability constant data suggested that Pd II ion in the Pd(η 5-C 5H 5)(PPh 3) + moiety is a better π-donor to olefins than Pt II ion in the Pt(η 3-CH 2CMeCH 2)(PPh 3) + moiety, a reversal of the normal trend in the relative olefin affinity of these metal ions. The above spectral and stability features have been related to the electronic effect of the Cp ligand in enhancing the π back-bond interaction in one particular orientation of the CC bond.

Olefin oligomerization on nickel. A theoretical study of the barrier to olefin rotation and insertion

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTOlefin oligomerization on nickel. A theoretical study of the barrier to olefin rotation and insertionBarbara M. Trzcinska, John P. Fackler Jr., and Alfred B. AndersonCite this: Organometallics 1984, 3, 2, 319–323Publication Date (Print):February 1, 1984Publication History Published online1 May 2002Published inissue 1 February 1984https://doi.org/10.1021/om00080a027RIGHTS & PERMISSIONSArticle Views123Altmetric-Citations19LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (561 KB) Get e-Alerts Get e-Alerts

The analogy between alkenes and the dinuclear species [MRh(µ-CO)2(η-C5Me5)2](M = Co or Rh); dynamic nuclear magnetic resonance, X-ray crystallographic, and extended-Hückel molecular-orbital studies of the bonding between unsaturated CoRh and Rh2fragments and the pentacarbonylmolybdenum group

Treatment of [Mo(CO)5(thf)](thf = tetrahydrofuran) with [CoRh(µ-CO)2(η-C5Me5)2] affords the heterotrimetallic complex [MoCoRh(µ-CO)2(CO)5(η-C5Me5)2]. Variable-temperature 13C-{1H} n.m.r. studies on this species revealed that in solution rotation of the CoRh(µ-CO)2(η-C5Me5)2 fragment occurs about an axis through the Mo and the midpoint of the Co–Rh vector, and that the preferred orientation of the Co–Rh vector with respect to the four equatorial carbonyl ligands of the square-pyramidal Mo(CO)5 moiety is staggered. The estimated barrier to rotation (ΔG‡258= 52.3 ± 1.3 kJ mol –1) is in the range observed and calculated for the hindered rotation of alkenes in related d6 ML5 alkene complexes where, however, the orientational preference of the alkene is to lie so as to eclipse an LML vector. The structure of the analogous trinuclear compound [MoRh2(µ-CO)2(CO)5(η-C5Me5)2] has been determined by a single-crystal X-ray diffraction study. The core of the molecule consists of an MoRh2 isosceles triangle [Mo–Rh (mean) 3.009(1), Rh–Rh 2.623(1)A]. The molybdenum atom carries five carbonyl groups, the rhodium atoms are each ligated by an η-C5Me5 group, and the Rh–Rh vector, staggered with respect to the four equatorial carbonyl groups of the Mo(CO)5 fragment, is symmetrically bridged by two CO ligands. Relatively short non-bonded contacts between the oxygen atoms of the equatorial CO ligands and the hydrogen atoms of the η-C5Me5 groups suggest a significant steric barrier to the ‘alkene-like’ rotation of the Rh2(µ-CO)2(η-C5Me5)2 fragment. Extended-Huckel molecular-orbital calculations on the model species [MoRh2(µ-CO)2(CO)5(η-C5H5)2] indicate that the dirhodium fragment and an alkene bond to an Mo(CO)5 moiety in a similar fashion. However, whereas the barrier to alkene rotation in d6 ML5 alkene complexes derives from electronic effects, no significant electronic contribution to the barrier of rotation of the trinuclear metal complex is indicated.

Dynamic 1H NMR spectroscopy of η2‐ethylene transition metal complexes

AbstractThe low‐temperature 1H NMR spectra of bis(ethylene)trimethylphosphinenickel (1), η5‐cyclopentadienyl(ethylene)methylnickel (2a), bis(ethylene)‐η5‐cyclopentadienylcobalt (2b), [bis(1‐ethyl‐2,4,6‐triphenylphosphorinyl)nickel]P,P′‐nickel(ethylene) (3) and η5‐cyclopentadienyl(ethylene)hydrido(triphenylphosphine)ruthenium (4) exhibit {AA′B′B} (1) {ABB′A} (2a and 2b), {ABA′B′} (3) or {ABCD} (4) spin patterns, respectively, for the complexed ethylene. A full line shape analysis including all the proton couplings was performed for the ethylene rotation in 1, 2a and 2b. In addition to olefin rotation, a reversible intramolecular β‐H‐elimination was confirmed for 4 by magnetization transfer experiments.

A NUCLEAR MAGNETIC RESONANCE AND MASS SPECTRAL STUDY OF SOME ALKENE COMPLEXES OF RHODIUM(I) AND IRIDIUM(I): COORDINATED ALKENE ROTATIONAL BARRIERS, APPEARANCE POTENTIALS AND ESTIMATED HEATS OF FORMATION

Abstract The activation energies (ΔG) for olefin rotation in ethylene complexes of Rh(I) and Ir(I) have been estimated by signal coalescence from their temperature dependent 1H nmr spectra in solution. The barriers decrease with increase in the electron accepting nature of the associated ligand (η5-cyclopentadienyl, pentane-2, 4-dionate, carboxylate and substituted derivatives thereof) in accordance with the expected decrease in the importance of the π-contribution to the metal-olefin bond. Barriers are substantially higher for Ir(I) than for corresponding Rh(I) complexes. The complex temperature dependent spectra of di-μ-chlorotetrakis(4-methylpenta-1, 3-diene dirhodium(I) are interpreted in terms of both intra- and intermolecular processes. The greater ΔG values found for Ir(I) compared to Rh(I) complexes are reflected in the different mass spectral fragmentation patterns for corresponding members of the two series. Appearance potentials and heats of formation are estimated for [cpRh(C2 H4)2] and [cpRh(...

Gehinderte Ligandbewegungen in Übergangsmetallkomplexen, XIX [1]. Oktaedrische Wolfram-Olefin-Komplexe mit Bisphospbinoethan-Liganden / Hindered Ligand Motions in Transition Metal Complexes, XIX [I]. Octahedral Tungsten Olefin Complexes with Bisphosphinoethane Ligands

(OC-6-32)-W(CO)3[(CH3)2PC2H4P(CH3)2] (olefin) complexes (9-14) were prepared photochemically from W(CO)4[(CH3)2PC2H4P(CH3)2](l) via W(CO)3[(CH3)2PC2H4P(CH3)2]- (THF) (2) and the electron poor olefins dimethyl inalonate(3), fumarate (4), inothylfumarate (5), fluorofumarato (6), chlorofumarato (7) and bromofumarate (8). The stereoche­mistry of 9-14 was elucidated by IR and NMR spectroscopy. The hindered rotations of the olefin ligands in 9-14 were studied by D-NMR spectroscopy. 11-14 form two diastereomeric isomers when the olefin rotation is freezed. The spectroscopic behaviour of the dimethyl fumarato complex 10 gives unambiguous proof for the rotation of the olefin ligand around the metal-olefin-bond. Alternative motions are ruled out.

η 5-cyclopentadienyl-η 2-olefin-alkylnickel

The η 5-cyclopentadienyl-η 2-ethylenealkylnickel complexes II (ethyl), IV (methyl) and V (trimethylsilylmethyl) have been prepared by treatment of nickelocene (I) with the appropriate alkylmagnesium halides and ethylene. Other alkylmagnesium halides (propyl, butyl and their isomers) also react with I and ethylene with formation of II. I reacts with isopropylmagnesium halide and propene to give a mixture of the isomeric propyl complexes IIIa and IIIB. The 1H NMR spectra are temperature dependent indicating the presence of conformers in IIIa and IIIb as well as olefin rotation in II- -VI

Some alkyl olefin cobalt complexes and their reactions with olefins

The complexes [Co(C 5Me 4Et)LMe 2] (I, L  C 2H 4; II, L  C 3H 6; III, L  Ph 3P) are reported, which undergo reactions with olefins. Following olefin exchange, I and II react with C 2H 3R giving [Co(C 5Me 4Et) (C 2H 3R) 2], CH 4 and CH 2 CMER (R  H or Me). The strong stereochemical specificity, together with deuteration experiments and 1H NMR spectrum of the reaction mixture all point to a simple olefin insertion reaction mechanism. Variable temperature 1H NMR shows olefin rotation in I.