Showing papers in "Advances in Organometallic Chemistry in 1988"

H-H, C-H, and Related Sigma-Bonded Groups as Ligands

Abstract: Publisher Summary This chapter discusses H–H, C–H, and sigma-bonded related groups as ligands. Ligands that act by donation of lone pairs (e.g., NH3) or of π-bonding electrons (e.g., C2H4) are common in transition-metal chemistry. Two-electron, three-center bonds seem to form most readily when at least one of the three centers is H. Dihydrogen complexes are the simplest transition-metal species isolobal with H3+, but they have been recognized only in the past three years. Solid-state nuclear magnetic resonance (NMR) spectroscopy has provided further insights into the properties of H2 complexes. The simplest reaction of an H2 complex is dissociation. There is clear evidence in many cases that H2 ligands can undergo rapid exchange with terminal M–H groups. Interactions between a C–H bond, and coordinatively unsaturated metal fragments were first observed in the 1960s by Ibers and by Maitlis. A particularly significant series of agostic complexes is Cp*Co(PR3)-(alkyl). There are a number of cases in which essentially planar methyl groups bridge between two metals. In a recent development, it has been shown that the [HW2(CO)10] anion has a bent W–H–W bridge; previous data had been collected on disordered salts and so were less reliable.

Organometallic Compounds Containing Oxygen Atoms

Abstract: Publisher Summary This chapter discusses the organometallic compounds containing oxygen atoms. At present, the range of organic ligands found in organometallic oxo compounds is quite restricted, the majority being either η-C 5 R 5 or alkyls or aryls having no β-hydrogen atom. Two basic routes to organometallic oxo compounds may be envisaged addition of an organic ligand to an inorganic oxo complex or addition of oxygen to an organometallic compound. Exhaustive decarbonylation with concomitant oxidation of a cyclopentadienyl metal carbonyl has proved to be a useful preparative route to cyclopentadienyl metal oxo compounds having no other ligands. In the ethylene complex, the oxo and ethylene ligands are cis to one another and the C–C axis of the ethylene is perpendicular to the M–O bond, a configuration that maximizes π bonding between the W(IV) (d 2 ) and the ethylene. Complexes containing cyclopentadienyl and oxygen as coligands are of two basic types: those containing a terminal double bond between a metal and oxygen, [M=O], and those containing one or more doubly bridging oxygen atoms, [M(μ 2 -O) n M]. The clusters are held together by M–O bonds and the M–( η 5 -C 5 H 5 ) bonding is of the usual type. Parallel to the development of organometallic clusters containing oxygen atoms has been the preparation of organometallic polyoxometallates. Although the organometallic groups are on the surface of the polyoxometallate, they are strongly and covalently bonded to the peripheral oxygen atoms.

Nucleophilic Activation of Carbon Monoxide: Applications to Homogeneous Catalysis by Metal Carbonyls of the Water Gas Shift and Related Reactions

Abstract: Publisher Summary This chapter discusses the roles played by certain nucleophile adducts of metal–coordinated carbon monoxide in the stoichiometric and catalytic chemistry of metal–carbonyl complexes and relatively recent developments for systems where oxygen or nitrogen bases have been used as the nucleophiles. The syntheses, spectral characterizations, and chemical reaction studies of various nucleophile–carbonyl complexes are described with the goal of drawing attention to how such adduct formation not only activates the carbonyl directly involved for further reaction but also influences the reactivities of the balance of the complex. Preparation by direct addition of a nucleophile anion Nu – to a coordinated CO has been a widely applicable approach. Nuclear magnetic resonance (especially 13 C NMR) and infrared spectroscopy have proved the most used and successful diagnostic tools in characterizing the formation and transformations of various L n M–(CO)Nu complexes and related species. Reactions of uncharged nucleophiles with coordinated CO provide a different situation. Kinetics studies of systems for which characterizable nucleophilecarbonyl adducts have been isolated are relatively few.

Organopalladium and Platinum Compounds with Pentahalophenyl Ligands

Abstract: Publisher Summary This chapter discusses the existing synthetic methods that permit introduction of up to four C 6 X 5 groups in the coordination sphere of palladium or platinum as well as the reactivity of the different complexes obtained. It presents much more numerous compounds with the metals in oxidation state (II), because these provide the gateway for the preparation of other complexes involving oxidation state (I), (III), or (IV), whose chemistry is more reduced in scope. All the complexes show infrared bands characteristic of the presence of the C 6 X 5 groups, most readily observed for C 6 F 5 compounds. Some of these internal absorptions are related to the skeletal symmetry of the molecule and to the oxidation state of the metal center and have therefore been used for structural diagnosis. The information obtained from infrared (IR) data is also discussed. Because the carbanions C 6 X 15 – are not strong nucleophiles, they do not displace neutral Group Vb ligands in the starting compounds MCI 2 L 2 , and bisaryl derivatives M(C 6 X 5 ) 2 L 2 are obtained. A general method that gives the four possible complexes in yields over 85% involves the addition of aqueous HCI to the corresponding tetrakis(pentahalophenyl) compound in methanolic solution (2:1 ratio) albeit half of the pentahalophenyl groups are lost as C 6 X 5 H. The title compounds have interesting possibilities for a variety of syntheses. Platinum(II) complexes cis-Pt(C 6 X 5 ) 2 L 2 undergo redox condensation with Pt(η-C 2 H 4 )(PPh 3 ) 2 in oxygen-free refluxing tetrahydrofuran to give binuclear Pt(I) complexes. The C 5 F 5 groups show strong absorptions in the 1650, 1500, 1300, 1050, 950, and 800 cm –1 regions.

Interaction of Ketenes with Organometaliic Compounds: Ketene, Ketenyl, and Ketenylidene Complexes

Abstract: Publisher Summary This chapter discusses the interaction of ketenes with organometallic compounds and four separate but related aspects of the interaction of ketenes with metals; the reactions of ketenes with organometallic complexes; the preparation, structures, and reactions of stable ketene complexes; the chemistry of ketenyl complexes that have a metal as one of the ketene substituents; and the chemistry of ketenyfidene complexes in which the ketene carbon has only metals as substituents. Also discussed are reactions in which ketene complexes are suspected as intermediates, but proof of their structure and formulations is lacking. There are numerous examples of reactions of ketenes with coordinated ligands with little or no direct interaction of the ketene with the metal. A number of organometallic complexes have been prepared that contain ketene and substituted ketene ligands, but these have a variety of different ketene coordination modes. A variety of synthetic procedures have been used to prepare ketene complexes, with the most useful involving direct addition of ketenes to unsaturated organometallic complexes, carbonylation of carbene complexes, and deprotonation of acyl complexes. A number of ketene complexes have been prepared by the addition of carbon monoxide to preformed metal–carbene complexes. Similar proton abstraction from bridging acetyl ligands would give μ-ketene complexes. A synthetic method that has often been used for preparing new ketene complexes involves the modification of the coordination sphere of existing ketene complexes.

Graphite metal compounds

Abstract: Publisher Summary This chapter discusses the most recent and important developments in the field of graphite and the structural properties and use of these compounds in organic and organometallic chemistry. Graphite has a lamellar structure, with an interlayer distance of 0.335 nm. Besides the large number of reagents that can be intercalated, a number of different types of graphites are used. The most commonly used potassium–graphite intercalation compound, C 8 K, shows an interlayer distance of 5.34 A, and all carbon layers are separated by a layer of K. Differences among the alkali metals in behavior toward graphite have been explained in terms of electron transfer. Pure yellow, first-stage compounds of Ba and Sr with graphite were prepared by the direct action of metal vapor on graphite in metallic tubes sealed under vacuum. C 8 K in toluene, benzene, or isopropylbenzene can act as an alkylation catalyst in the presence of ethylene to yield the corresponding nuclear and side chain-alkylated aromatic hydrocarbons. Graphite intercalation compounds can act as mild anionic polymerization initiators offering some benefits in comparison to analogous reactions in homogeneous medium or to metal dispersions.

Recent Developments in NMR Spectroscopy of Organometallic Compounds

Abstract: Publisher Summary This chapter discusses the recent developments in nuclear magnetic resonance (NMR) spectroscopy of organometallic compounds. The experimental aspects and some results from multinuclear NMR spectroscopy and some examples from solid-state NMR spectroscopy are examined. Modern NMR spectrometers have powerful routines to modify the free induction decay and hence the resolution and signal/noise ratio. Homonuclear decoupling has long been established as a valuable tool in determining which nuclei are coupled. The Nuclear Overhauser Enhancement (NOE) is one of the most valuable tools in determining the connectivity among protons and, to a lesser extent, between protons and other nuclei. The sensitivity of an NMR experiment depends on the population difference among the energy levels. The Incredible Natural Abundurice Double Quantum Transfer Experiment (in adequate) pulse sequence was originally designed to detect 13 C – 13 C coupling without the confusion of the 13 C singlets due to the species without adjacent 13 C atoms. The normal π /2 pulse excites nuclei over a wide range of frequencies. Broadband decoupling has been applied for many years to obtain X-nuclei NMR spectra without 1 H coupling. Until very recently, variable-temperature solid-state NMR spectroscopy with magic angle spinning has been restricted close to room temperature because of the great difficulty of spinning the sample at high speed.