DOCSLIB.ORG

Reactivity of Transition Metal Organometallics L. J. Farrugia Msc

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Reactivity of Transition Metal Organometallics L. J. Farrugia Msc L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics Reactivity of Transition Metal Organometallics L. J. Farrugia MSc Core Course C5 Text books : Inorganic Chemistry - Housecroft & Sharpe Ch 23 - the very basics Inorganic Chemistry - Shriver & Atkins Ch 16 - the very basics This course assumes familiarity with the Level-2 and Level-3 courses on Organometallic Chemistry, and this is covered in the above texts Organometallics - Elschenbroich & Salzer (library) - much more useful Topic 1 - Introduction to Cyclopentadienyl Compounds First part of course covers the role of the ligand cyclopentadienyl (Cp) and its derivatives. Cp is one of the most important ligands in organometallics after CO. A considerable percentage of organometallic compounds contain this ligand - it is also a good ligand for main group metals and the f-block metals (lanthanides & actinides). H H H Fe CO H OC CO Cyclopentadiene Cyclopentadiene complex (4 e donor ligand) very rare as a ligand Na / -H2 H - Na+ H acidic H atom planar aromatic (6-pi electron) dienyl anion - The anion C 5H5 is a very useful synthetic reagent. It is usually treated as equivalent to occupying THREE coordination sites, so that C 5H5 ≡ 3(CO). In electron counting terms, it can be treated as either as a 6-e donor ANION or a 5- e donor NEUTRAL molecule. The latter is the recommended approach because it is simpler (do not need to worry about oxidation levels). 1.1 Bonding in cyclopentadienyl compounds Cp has 5 π electrons in the 5 out-of-plane p-orbitals on the C atoms. These 5 orbitals combine as a 1 + e 1 + e 2 under five-fold symmetry. With a single Cp ring, 1 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics inspection shows the possible combinations with the metal d-orbitals are shown below Cp orbitals Metal orbitals Symmetry of bond a1 pz dz2 , s σ e1 dxz dyz px py π e2 dx2-y2 dxy δ For two Cp rings in a metallocene M(C 5H5)2 the rings may be either staggered or eclipsed - Fe(C 5H5)2 is eclipsed but Co(C 5H5)2 and Ni(C 5H5)2 are staggered. The bonds can also be divided into sigma, pi and delta symmetry as shown in OHP #1 OHP # 2 shows the formal MO interaction diagram for ferrocene - Fe(C 5H5)2 The main points to remember (i) the 9 filled orbitals have 18 electrons - hence the rule ! (ii) the LUMO is a doubly degenerate π* orbital, so Ni(C 5H5)2 is paramagnetic 2 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics This shows how the metal and ring π-orbitals match by symmetry. The resultant MO scheme for ferrocene shows the way that the basis orbitals having the same symmetry can combine to give new orbitals (NOT necessary to remember the MO scheme) The filling of the nine bonding orbitals in ferrocene explains the high stability of this compound. The mixing of metal and Cp orbitals indicates a strong covalent character to the transition metal - cyclopentadienyl bond. In Co(C 5H5)2 ONE electron fills the e* 1g while in Ni(C 5H5)2 there are TWO unpaired electrons - hence both compounds are paramagnetic 3 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics + Co(C 5H5)2 readily loses an electron to give the cobalticinium cation [Co(C 5H5)2] an 18e cation. Similarly Ni(C 5H5)2 loses one electron to give a 19e cation, but further oxidation results in decomposition. 1.2 Structural types of Cp compounds (i) Metallocenes MCp 2 These are known for the metals Ti, V, Cr , Mn, Co and Ni. Staggered or eclipsed rings leads to D5h or D5d symmetry with virtually NO barrier to rotation of the Cp ring about the metal-Cp axis. So all Cp protons appear as equivalent in the 1H NMR spectrum V, Cr, Fe, Co and Ni give the "classic" sandwich compounds illustrated above. The exceptions are 4 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics (a) Titanocene "TiCp 2" This is really a fulvalene complex made by reducing Cp 2TiCl 2. It contains bridging * hydrides and a Ti-Ti bond - gives 16 e Ti atoms. Real "titanocene" TiCp 2 has recently been made but is extremely reactive. (b) Manganocene MnCp 2 This is ionic at low temperature, with a polymeric chain like structure Above 159 oC it becomes isomorphous with ferrocene, so must adopt a sandwich structure. All the sandwich metallocenes apart from ferrocene are paramagnetic No. unpaired electrons Cp 2V 3e Cp 2Cr 2e 2+ Cp 2Mn 5e high spin Mn Cp 2Co 1e Cp 2Ni 2e (ii) 'bent' metallocenes These have non-parallel rings due to the presence of other ligands. Some examples are H Cl W Ti V CO Re H H Cl 18 e 16 e 17 e 18 e 5 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics (iii) half-sandwich compounds - piano stool compounds These have one Cp ring and a variety of other ligands. Some examples are OC V CO Mn Ru Co PPh3 OC CO Cl CO OC CO PPh3 OC CO 4-legged stool 3-legged stool Chem-3 lab 2-legged stool The 17 electron species CpMo(CO) 3 and CpFe(CO) 2 are not stable as such, but dimerise to give the familiar compounds with a Mo-Mo or Fe-Fe metal-metal bond. This is typical behaviour of odd-electron organometallic species. (iv) other types of bonding mode 5 So far only the so-called eta-5 η -C5H5 bonding mode has been illustrated, where (in principle) all five C atoms are equally bonded to the transition metal. However 3 1 there are other possibilities, most common are η -C5H5 and η -C5H5 H M M 1 3 η -C5H5 η -C5H5 1 e donor - like alkyl group 3e donor - like allyl group H Mo ON 5 One good example is Mo(NO)Cp 3 - with "normal" η -C5H5 bonding modes, electron counting give a 24 electron compound (!!) - simply not possible. In fact it is an example of a compound containing all three types. 1 η -C5H5 ligands are usually fluxional. Consider the compound (C 5H5)4Ti - again 5 cannot be 4 η -C5H5 bonding modes because it would be a 24 e compound. In 5 1 this case it has two η -C5H5 and two η -C5H5 ligands. Ti 6 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics At the very lowest temperature measured, the 1H NMR spectrum shows a singlet 5 for the two (equivalent) η -C5H5 groups and an AA'BB'C multiplet for the two 1 (equivalent) η -C5H5 groups (three different H environments). This compound shows two fluxional processes 1 (i) migration of metal atom around the η -C5H5 group via 1,2 shifts (ring whizzing) 1 5 (ii) exchange of the η -C5H5 and η -C5H5 groups Process (i) is a very low energy process, which is frozen out only at the lowest temperatures. Process (ii) occurs at higher temperatures ~ room temperature. The net result of (i) and (ii) is that all 20 protons appear equivalent at the highest temperatures measured. Process (i) could in principle occur also by 1,3 shifts or random shifts. How can we tell which ? We can if it is possible to assign the protons of the AA'BB' signal M c c M a M a a a b c a b b b b b a 1, 2 shift The resulting exchanges are : a→c a→b i.e. both a type protons exchange b→a b→b i.e. only half of b type protons exchange c→a So... the rate of broadening for the a type protons is twice as fast as for the b type protons 5 1 The actual spectrum of ( η -C5H5)( η -C5H5)Fe(CO) 2 which exhibits a similar exchange is shown below. 7 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics 1.3 Oxidation states in Cp compounds Since Cp is formally charged with a -ve charge, each Cp ligand present in a compound contributes a charge of +1 towards the formal oxidation state of the metal. Hence a metal must be in a +ve oxidation level. The only exception would be with +ve charged ligands - NO + is the only common example. CpNi(NO) has a zerovalent Ni atom. In terms of its ability to stabilize oxidation states, Cp is comfortable with both low and high oxidation states of the metal (unlike many pi-acid ligands like CO which are only found for low oxidation states). In general Cp is a good sigma- and pi- donor, but a poor pi-acceptor. Substituting H for Me on the Cp ring makes it an even better sigma-donor - Cp* is the common symbol for C 5Me 5 . H2O2 CO Re Re O CO O OC O CO Re (+1) Re(+7) - oxo complex 8 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics Topic 2 - Reactivity of Cyclopentadienyl-type Ligands Very often Cp is a spectator ligand (i.e. it does not take part in the reaction and it is unchanged at the end). Under some circumstances however the ring will react. Ferrocene Fe(C 5H5)2 has been the most studied (because it is a stable 18 e metallocene) but ruthenocene Ru(C 5H5)2 and osmocene Os(C 5H5)2 will give similar reactions - they are in the same periodic group ! However, most Cp compounds will not survive the reaction conditions described below.
Load more

Recommended publications
  • Indene CH Activation, Indole Π Vs. Nitrogen Lone-Pair Coordinati
    HHS Public Access Author manuscript Author ManuscriptAuthor Manuscript Author Organometallics Manuscript Author . Author Manuscript Author manuscript; available in PMC 2016 April 15. Published in final edited form as: Organometallics. 2007 ; 26(2): 281–287. doi:10.1021/om0606643. Reactions of Indene and Indoles with Platinum Methyl Cations: Indene C-H Activation, Indole π vs. Nitrogen Lone-Pair Coordination Travis J. Williams, Jay A. Labinger, and John E. Bercaw Arnold and Mabel Beckman Laboratories of Chemical Synthesis, California Institute of Technology, Pasadena, CA 91125 (U. S. A.) Abstract Reactions of indene and various substituted indoles with [(diimine)PtII(Me)(TFE)]+ cations have been studied (diimine = ArN = C(Me) − C(Me) = NAr; TFE = 2,2,2-trifluoroethanol). Indene displaces the TFE ligand from platinum to form a stable π coordination complex that, upon heating, undergoes C-H activation with first order kinetics, ΔH‡ = 29 kcal/mol, ΔS‡ = 10 eu, and a kinetic isotope effect of 1.1 at 60 °C. Indoles also initially form coordination complexes through the C2=C3 olefin, but these undergo rearrangement to the corresponding N-bound complexes. The relative rates of initial coordination and rearrangement are affected by excess acid or methyl substitution on indole. Introduction Selective C-H bond activation is a potentially valuable approach to synthetic problems in areas ranging from fuels and bulk chemicals to fine chemicals and pharmaceutical synthesis. 1 Studies of C-H activation in our laboratory have focused on models of the Shilov system,2 particularly [(diimine)PtII(Me)(solv)]+ (2, diimine = ArN = C(Me) − C(Me) = NAr; solv = 2,2,2-trifluoroethanol (TFE), H2O).3 These cations are capable of activating a variety of II carbon-hydrogen bonds.4 Cations 2 can be generated by protonolysis of (diimine)Pt Me2 species 1 in TFE with aqueous HBF43, 4ab or BX3 (X = C6F5,4cd F5), the latter producing H+ by boron coordination to TFE.4c In deuterated solvent 2 is formed as a mixture of two isotopologs.
    [Show full text]
  • Iron-Catalysed Transformation of Molecular Dinitrogen Into Silylamine Under Ambient Conditions
    ARTICLE Received 6 Sep 2012 | Accepted 6 Nov 2012 | Published 4 Dec 2012 DOI: 10.1038/ncomms2264 Iron-catalysed transformation of molecular dinitrogen into silylamine under ambient conditions Masahiro Yuki1, Hiromasa Tanaka2, Kouitsu Sasaki1, Yoshihiro Miyake1, Kazunari Yoshizawa2 & Yoshiaki Nishibayashi1 Although stoichiometric transformations using transition metal–N2 complexes have been well investigated towards the goal of nitrogen fixation under mild reaction conditions, only a few examples of the catalytic transformations of N2 using transition metal–N2 complexes as catalysts have been reported. In almost all the catalytic systems, the use of Mo is essential to realize the catalytic transformation of N2, where Mo–N2 complexes are considered to work as effective catalysts. Here we show the first successful example of the Fe-catalysed transfor- mation of N2 into N(SiMe3)3 under ambient conditions, in which iron complexes such as iron pentacarbonyl [Fe(CO)5] and ferrocenes have been found to work as effective catalysts. A plausible reaction pathway is proposed, where Fe(II)–N2 complex bearing two Me3Si groups as ancillary ligands has an important role as a key reactive intermediate, with the aid of density-functional-theory calculations. 1 Institute of Engineering Innovation, School of Engineering, University of Tokyo, Yayoi, Bunkyo-ku, Tokyo 113-8656, Japan. 2 Institute for Materials Chemistry and Engineering, International Research Center for Molecular Systems, Kyushu University, Nishi-ku, Fukuoka 819-0395, Japan. Correspondence and requests for materials should be addressed to K.Y. (email: [email protected]) or to Y.N. (email: [email protected]).
    [Show full text]
  • 540.14Pri.Pdf
    Index Element names, parent hydride names and systematic names derived using any of the nomenclature systems described in this book are, with very few exceptions, not included explicitly in this index. If a name or term is referred to in several places in the book, the most informative references appear in bold type, and some of the less informative places are not cited in the index. Endings and suffixes are represented using a hyphen in the usual fashion, e.g. -01, and are indexed at the place where they would appear ignoring the hyphen. Names of compounds or groups not included in the index may be found in Tables P7 (p. 205), P9 (p. 232) and PIO (p. 234). ~, 3,87 acac, 93 *, 95 -acene, 66 \ +, 7,106 acetals, 160-161 - (minus), 7, 106 acetate, 45 - (en dash), 124-126 acetic acid, 45, 78 - (em dash), 41, 91, 107, 115-116, 188 acetic anhydride, 83 --+, 161,169-170 acetoacetic acid, 73 ct, 139, 159, 162, 164, 167-168 acetone, 78 ~, 159, 164, 167-168 acetonitrile, 79 y, 164 acetyl, III, 160, 163 11, 105, 110, 114-115, 117, 119-128, 185 acetyl chloride, 83, 183 K, 98,104-106,117,120,124-125, 185 acetylene, 78 A, 59, 130 acetylide, 41 11, 89-90,98, 104, 107, 113-116, 125-126, 146-147, acid anhydrides, see anhydrides 154, 185 acid halides, 75,83, 182-183 TC, 119 acid hydrogen, 16 cr, 119 acids ~, 167 amino acids, 25, 162-163 00, 139 carboxylic acids, 19,72-73,75--80, 165 fatty acids, 165 A sulfonic acids, 75 ct, 139,159,162,164,167-168 see also at single compounds A, 33-34 acrylic acid, 73, 78 A Guide to IUPAC Nomenclature of Organic actinide, 231 Compounds, 4, 36, 195 actinoids (vs.
    [Show full text]
  • EI-ICHI NEGISHI Herbert C
    MAGICAL POWER OF TRANSITION METALS: PAST, PRESENT, AND FUTURE Nobel Lecture, December 8, 2010 by EI-ICHI NEGISHI Herbert C. Brown Laboratories of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, IN 47907-2084, U.S.A. Not long ago, the primary goal of the synthesis of complex natural products and related compounds of biological and medicinal interest was to be able to synthesize them, preferably before anyone else. While this still remains a very important goal, a number of today’s top-notch synthetic chemists must feel and even think that, given ample resources and time, they are capable of synthesizing virtually all natural products and many analogues thereof. Accepting this notion, what would then be the major goals of organic synthesis in the twenty-first century? One thing appears to be unmistakably certain. Namely, we will always need, perhaps increasingly so with time, the uniquely creative field of synthetic organic and organometallic chemistry to prepare both new and existing organic compounds for the benefit and well-being of mankind. It then seems reasonably clear that, in addition to the question of what compounds to synthesize, that of how best to synthesize them will become increasingly important. As some may have said, the primary goal would then shift from aiming to be the first to synthesize a given compound to seeking its ultimately satisfactory or “last synthesis”. If one carefully goes over various aspects of organic synthetic methodology, one would soon note how primitive and limited it had been until rather recently, or perhaps even today. For the sake of argument, we may propose here that the ultimate goal of organic synthesis is “to be able to synthesize any desired and fundamentally synthesizable organic compounds (a) in high yields, (b) efficiently (in as few steps as possible, for example), (c) selectively, preferably all in t98–99% selectivity, (d) economically, and (e) safely, abbreviated hereafter as the y(es)2 manner.” with or without catalyst R1M + R2X R1R2 + MX R1, R2: carbon groups.
    [Show full text]
  • Aromaticity Sem- Ii
    AROMATICITY SEM- II In 1931, German chemist and physicist Sir Erich Hückel proposed a theory to help determine if a planar ring molecule would have aromatic properties .This is a very popular and useful rule to identify aromaticity in monocyclic conjugated compound. According to which a planar monocyclic conjugated system having ( 4n +2) delocalised (where, n = 0, 1, 2, .....) electrons are known as aromatic compound . For example: Benzene, Naphthalene, Furan, Pyrrole etc. Criteria for Aromaticity 1) The molecule is cyclic (a ring of atoms) 2) The molecule is planar (all atoms in the molecule lie in the same plane) 3) The molecule is fully conjugated (p orbitals at every atom in the ring) 4) The molecule has 4n+2 π electrons (n=0 or any positive integer Why 4n+2π Electrons? According to Hückel's Molecular Orbital Theory, a compound is particularly stable if all of its bonding molecular orbitals are filled with paired electrons. - This is true of aromatic compounds, meaning they are quite stable. - With aromatic compounds, 2 electrons fill the lowest energy molecular orbital, and 4 electrons fill each subsequent energy level (the number of subsequent energy levels is denoted by n), leaving all bonding orbitals filled and no anti-bonding orbitals occupied. This gives a total of 4n+2π electrons. - As for example: Benzene has 6π electrons. Its first 2π electrons fill the lowest energy orbital, and it has 4π electrons remaining. These 4 fill in the orbitals of the succeeding energy level. The criteria for Antiaromaticity are as follows: 1) The molecule must be cyclic and completely conjugated 2) The molecule must be planar.
    [Show full text]
  • 1 5. Chemical Bonding
    5. Chemical Bonding: The Covalent Bond Model 5.1 The Covalent Bond Model Almost all chemical substances are found as aggregates of atoms in the form of molecules and ions produced through the reactions of various atoms of elements except the noble-gas elements which are stable mono-atomic gases. Chemical bond is a term that describes the attractive force that is holding the atoms of the same or different kind of atoms in forming a molecule or ionic solid that has more stability than the individual atoms. Depending on the kinds of atoms participating in the interaction there seem to be three types of bonding: Gaining or Losing Electrons: Ionic bonding: Formed between many ions formed by metal and nonmetallic elements. Sharing Electrons: Covalent bonding: sharing of electrons between two atoms of non-metals. Metallic Bonding: sharing of electrons between many atoms of metals. Ionic Compounds Covalent Compounds Metallic Compounds 1. Metal and non-meal Non-metal and non-meal Metal of one type or, element combinations. elements combinations. combinations of two or metal elements combinations. 2. High melting brittle Gases, liquids, or waxy, low Conducting, high melting, crystalline solids. melting soft solids. malleable, ductile crystalline solids. 3. Do not conduct as a solid Do not conduct electricity at Conduct electricity at solid but conducts electricity any state. and molten states. when molten. 4. Dissolved in water produce Most are soluble in non-polar Insoluble in any type of conducting solutions solvents and few in water. solvents. (electrolytes) and few These solutions are non- are soluble in non-polar conducting (non- solvents.
    [Show full text]
  • Fundamental Studies of Early Transition Metal-Ligand Multiple Bonds: Structure, Electronics, and Catalysis
    Fundamental Studies of Early Transition Metal-Ligand Multiple Bonds: Structure, Electronics, and Catalysis Thesis by Ian Albert Tonks In Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy CALIFORNIA INSTITUTE OF TECHNOLOGY Pasadena, California 2012 Defended December 6th 2011 ii 2012 Ian A Tonks All Rights Reserved iii ACKNOWLEDGEMENTS I am extremely fortunate to have been surrounded by enthusiastic, dedicated, and caring mentors, colleagues, and friends throughout my academic career. A Ph.D. thesis is by no means a singular achievement; I wish to extend my wholehearted thanks to everyone who has made this journey possible. First and foremost, I must thank my Ph.D. advisor, Prof. John Bercaw. I think more so than anything else, I respect John for his character, sense of fairness, and integrity. I also benefitted greatly from John’s laissez-faire approach to guiding our research group; I’ve always learned best when left alone to screw things up, although John also has an uncanny ability for sensing when I need direction or for something to work properly on the high-vac line. John also introduced me to hiking and climbing in the Eastern Sierras and Owens Valley, which remain amongst my favorite places on Earth. Thanks for always being willing to go to the Pizza Factory in Lone Pine before and after all the group hikes! While I never worked on any of the BP projects that were spearheaded by our co-PI Dr. Jay Labinger, I must also thank Jay for coming to all of my group meetings, teaching me an incredible amount while I was TAing Ch154, and for always being willing to talk chemistry and answer tough questions.
    [Show full text]
  • Chemical Forces Understanding the Relative Melting/Boiling Points of Two
    Chapter 8 – Chemical Forces Understanding the relative melting/boiling points of two substances requires an understanding of the forces acting between molecules of those substances. These intermolecular forces are important for many additional reasons. For example, solubility and vapor pressure are governed by intermolecular forces. The same factors that give rise to intermolecular forces (e.g. bond polarity) can also have a profound impact on chemical reactivity. Chemical Forces Internuclear Distances and Atomic Radii There are four general methods of discussing interatomic distances: van der Waal’s, ionic, covalent, and metallic radii. We will discuss the first three in this section. Each has a unique perspective of the nature of the interaction between interacting atoms/ions. Van der Waal's Radii - half the distance between two nuclei of the same element in the solid state not chemically bonded together (e.g. solid noble gases). In general, the distance of separation between adjacent atoms (not bound together) in the solid state should be the sum of their van der Waal’s radii. F F van der Waal's radii F F Ionic Radii – Ionic radii were discussed in Chapter 4 and you should go back and review that now. One further thing is worth mentioning here. Evidence that bonding really exists and is attractive can be seen in ionic radii. For all simple ionic compounds, the ions attain noble gas configurations (e.g. in NaCl the Na+ ion is isoelectronic to neon and the Cl- ion is isoelectronic to argon). For the sodium chloride example just given, van der Waal’s radii would predict (Table 8.1, p.
    [Show full text]
  • Catalytic Systems Based on Cp2zrx2 (X = Cl, H), Organoaluminum
    catalysts Article Catalytic Systems Based on Cp2ZrX2 (X = Cl, H), Organoaluminum Compounds and Perfluorophenylboranes: Role of Zr,Zr- and Zr,Al-Hydride Intermediates in Alkene Dimerization and Oligomerization Lyudmila V. Parfenova 1,* , Pavel V. Kovyazin 1, Almira Kh. Bikmeeva 1 and Eldar R. Palatov 2 1 Institute of Petrochemistry and Catalysis of Russian Academy of Sciences, Prospekt Oktyabrya, 141, 450075 Ufa, Russia; [email protected] (P.V.K.); [email protected] (A.K.B.) 2 Bashkir State University, st. Zaki Validi, 32, 450076 Ufa, Russia; [email protected] * Correspondence: [email protected]; Tel.: +7-347-284-3527 i i Abstract: The activity and chemoselectivity of the Cp2ZrCl2-XAlBu 2 (X = H, Bu ) and [Cp2ZrH2]2- ClAlEt2 catalytic systems activated by (Ph3C)[B(C6F5)4] or B(C6F5)3 were studied in reactions with 1-hexene. The activation of the systems by B(C6F5)3 resulted in the selective formation of head- to-tail alkene dimers in up to 93% yields. NMR studies of the reactions of Zr complexes with organoaluminum compounds (OACs) and boron activators showed the formation of Zr,Zr- and Zr,Al-hydride intermediates, for which diffusion coefficients, hydrodynamic radii, and volumes were estimated using the diffusion ordered spectroscopy DOSY. Bis-zirconium hydride clusters of type x[Cp ZrH ·Cp ZrHCl·ClAlR ]·yRnAl(C F ) − were found to be the key intermediates of alkene 2 2 2 2 6 5 3 n dimerization, whereas cationic Zr,Al-hydrides led to the formation of oligomers. Citation: Parfenova, L.V.; Kovyazin, P.V.; Bikmeeva, A.K.; Palatov, E.R.
    [Show full text]
  • The Chemistry of Alkynes
    14_BRCLoudon_pgs4-2.qxd 11/26/08 9:04 AM Page 644 14 14 The Chemistry of Alkynes An alkyne is a hydrocarbon containing a carbon–carbon triple bond; the simplest member of this family is acetylene, H C'C H. The chemistry of the carbon–carbon triple bond is similar in many respects toL that ofL the carbon–carbon double bond; indeed, alkynes and alkenes undergo many of the same addition reactions. Alkynes also have some unique chem- istry, most of it associated with the bond between hydrogen and the triply bonded carbon, the 'C H bond. L 14.1 NOMENCLATURE OF ALKYNES In common nomenclature, simple alkynes are named as derivatives of the parent compound acetylene: H3CCC' H L L methylacetylene H3CCC' CH3 dimethylacetyleneL L CH3CH2 CC' CH3 ethylmethylacetyleneL L Certain compounds are named as derivatives of the propargyl group, HC'C CH2 , in the common system. The propargyl group is the triple-bond analog of the allyl group.L L HC' C CH2 Cl H2CA CH CH2 Cl L L LL propargyl chloride allyl chloride 644 14_BRCLoudon_pgs4-2.qxd 11/26/08 9:04 AM Page 645 14.1 NOMENCLATURE OF ALKYNES 645 We might expect the substitutive nomenclature of alkynes to be much like that of alkenes, and it is. The suffix ane in the name of the corresponding alkane is replaced by the suffix yne, and the triple bond is given the lowest possible number. H3CCC' H CH3CH2CH2CH2 CC' CH3 H3C CH2 C ' CH L L L L L L L propyne 2-heptyne 1-butyne H3C CH C ' C CH3 HC' C CH2 CH2 C' C CH3 L L L L 1,5-heptadiyneLL L "CH3 4-methyl-2-pentyne Substituent groups that contain a triple bond (called alkynyl groups) are named by replac- ing the final e in the name of the corresponding alkyne with the suffix yl.
    [Show full text]
  • Organometrallic Chemistry
    CHE 425: ORGANOMETALLIC CHEMISTRY SOURCE: OPEN ACCESS FROM INTERNET; Striver and Atkins Inorganic Chemistry Lecturer: Prof. O. G. Adeyemi ORGANOMETALLIC CHEMISTRY Definitions: Organometallic compounds are compounds that possess one or more metal-carbon bond. The bond must be “ionic or covalent, localized or delocalized between one or more carbon atoms of an organic group or molecule and a transition, lanthanide, actinide, or main group metal atom.” Organometallic chemistry is often described as a bridge between organic and inorganic chemistry. Organometallic compounds are very important in the chemical industry, as a number of them are used as industrial catalysts and as a route to synthesizing drugs that would not have been possible using purely organic synthetic routes. Coordinative unsaturation is a term used to describe a complex that has one or more open coordination sites where another ligand can be accommodated. Coordinative unsaturation is a very important concept in organotrasition metal chemistry. Hapticity of a ligand is the number of atoms that are directly bonded to the metal centre. Hapticity is denoted with a Greek letter η (eta) and the number of bonds a ligand has with a metal centre is indicated as a superscript, thus η1, η2, η3, ηn for hapticity 1, 2, 3, and n respectively. Bridging ligands are normally preceded by μ, with a subscript to indicate the number of metal centres it bridges, e.g. μ2–CO for a CO that bridges two metal centres. Ambidentate ligands are polydentate ligands that can coordinate to the metal centre through one or more atoms. – – – For example CN can coordinate via C or N; SCN via S or N; NO2 via N or N.
    [Show full text]
  • AROMATIC COMPOUNDS Aromaticity
    AROMATICITY AROMATIC COMPOUNDS Aromaticity Benzene - C6H6 H H H H H H H H H H H H Kekulé and the Structure of Benzene Kekule benzene: two forms are in rapid equilibrium 154 pm 134 pm • All bonds are 140 pm (intermediate between C-C and C=C) • C–C–C bond angles are 120° • Structure is planar, hexagonal A Resonance Picture of Bonding in Benzene resonance hybrid 6 -electron delocalized over 6 carbon atoms The Stability of Benzene Aromaticity: cyclic conjugated organic compounds such as benzene, exhibit special stability due to resonance delocalization of -electrons. Heats of hydrogenation + H2 + 120 KJ/mol + 2 H2 + 230 KJ/mol calc'd value= 240 KJ/mol 10 KJ/mol added stability + 3 H 2 + 208 KJ/mol calc'd value= 360 KJ/mol 152 KJ/mol added stability + 3 H2 + 337 KJ/mol 1,3,5-Hexatriene - conjugated but not cyclic Resonance energy of benzene is 129 - 152 KJ/mol An Orbital Hybridization View of Bonding in Benzene • Benzene is a planar, hexagonal cyclic hydrocarbon • The C–C–C bond angles are 120° = sp2 hybridized • Each carbon possesses an unhybridized p-orbital, which makes up the conjugated -system. • The six -electrons are delocalized through the -system The Molecular Orbitals of Benzene - the aromatic system of benzene consists of six p-orbitals (atomic orbitals). Benzene must have six molecular orbitals. Y6 Y4 Y5 six p-orbitals Y2 Y3 Y1 Degenerate orbitals: Y : zero nodes orbitals that have the 1 Bonding same energy Y2 and Y3: one node Y and Y : two nodes 4 5 Anti-bonding Y6: three node Substituted Derivatives of Benzene and Their Nomenclature
    [Show full text]