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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. The term can also be used to describe instances where a ligand can behave as monodentate or a chelating ligand. Bite angle is the ligand–metal–ligand angle formed when a polydentate ligand coordinates to a metal centre. 1 A Chelate (Greek word for “claw” is a polydentate ligand that forms a ring that includes a metal. Examples are EDTA, acac–, en. Heterobimetallic describes a complex in which there are two different metal centres. Homobimetallic complexes have two metal centres that are the same elements. These need not have identical ligands or coordination number, but are usually found as symmetric dimers. Homoleptic complexes are compounds in which all the ligands that bound to the metal centre are identical. A ligand is a molecule or ion that is bonded directly to a metal centre, usually by a covalent or coordinate bond. Monodentate ligands have only one point of attachment to the metal centre and occupy one coordination site only. Examples of monodentate ligands are NH3, CO, NMe3, H2O and PMe3. Polydentate ligands have more than one point of attachment to the centre and occupy more than one coordination site. By definitions above bidentate ligands and ambidentate ligands are special cases of polydentate ligands. The primary coordination sphere of a metal involves the set of ligands closest to the metal that are directly bonded. Mobile cations and anions are said to be in the outer or secondary sphere. HISTORICAL OVERVIEW OF ORGANOMETALLIC CHEMISTRY Many examples of organometallic compounds involving alkyls, alkenes, alkynes, carbonyls and aromatics are known. Organometallic chemistry is a relatively recent area of chemistry (40-50 years), even though organometallic compounds have been known since 1760; e.g. As2Me4O was made in 1760. Organometallic Chemistry has really been around for millions of years Naturally occurring Cobalimins contain Co—C bonds Vitamin B12: Some other historical landmarks are: 1827 Zeise’s salt: Na[PtCl3(C2H4)] 1849 Frankland: [(C2H5)2Zn] 1890 Mond: [Ni(CO)4] – the first binary metal carbonyl, and useful in purifying Ni 1931 Hieber: [Fe(CO)4H2] – the first hydride to be made. 2 1951 Ferrocene: [Fe(cp)2] – the first sandwich complex. Nobel -Prize Winners in the area of Organometallic: Victor Grignard and Paul Sabatier (1912) Grignard reagent K. Ziegler, G. Natta (1963) Zieglar-Natta catalyst E. O. Fisher, G. Wilkinson (1973) Sandwich compounds K. B. Sharpless, R. Noyori (2001) Hydrogenation and oxidation Yves Chauvin, Robert H. Grubbs, Richard R. Schrock (2005) Metal-catalyzed alkene metathesis BONDING Bonding is essentially covalent and can be Metal–Carbon σ (as in metal carbonyls and alkyls) or Metal–Carbon bonds (as in metal alkenes, alkynes and arenes). -– + Ionic compounds are often formed with electropositive metals, e.g. (C6H5)3C Na and – 2+ (C5H5 )2Ca . These ionic compounds are insoluble in hydrocarbon solvents and are very sensitive to air and moisture. ELECTRON COUNTING The 18-electron rule The 18-electron rule is like the octet rule to the organometallic chemists. It is useful in predicting the reactivity of organomettalic compounds. It is also often violated 18-electron rule is also known as the effective atomic number rule (EAN rule). The rule is used to determine the number of valence electrons in a complex to determine whether that complex is likely to be stable or not. 18 electrons arise from electrons from completely filled five d, three p, and one s orbitals. “A stable complex is obtained when the sum of metal d-electrons, the electron donated from the ligands, and the overall charge of the complex equals 18.” 3 Counting electrons in organometallic complexes Two distinct methods are used to count electrons: the neutral or covalent method and the effective atomic number or ionic method. These are just two different methods of an accounting system that give the same final answer. All you need to do is to understand one and keep to that to avoid confusion. What are d-electrons The configuration of transition metal as [Ar]4s23dn. This representation is only true for isolated metal atoms. When a metal ion is put into an electronic field (surrounded with ligands), the d-orbitals drop in energy and fill first. The organometallic chemist therefore, considers the transition metal valence electrons to be all the d-electrons. Thus a transition metal such as Ti in the zero oxidation state attains a d4, and not d2 configuration. Thus for zero-valent metals, the electron count simply corresponds to the column it occupies in the periodic table. Fe, for example, in the 8th column is d8 and not d6; Re3+ is d4 (seventh column for Re, and then add 3 position charges or subtract three negative ones). That is, d-occupancy = group number – oxidation state Electronic contribution of ligands Method 1: The ionic (charged) model The basic premise of this method is that we remove all of the ligands from the metal and, if necessary, add the proper number of electrons to each ligand to bring it to a closed valence shell state. For example, if we remove ammonia from a complex, NH3, has a completed octet and acts as a neutral molecule. When it bonds to the metal centre if does so through its lone pair (in a classic Lewis acid-base sense) and there is no need to change the oxidation state of the metal to balance charge. Ammonia is called a neutral 2-electron donor. In contrast, if we remove a methyl group from the metal and complete its octet, then we – formally have CH3 . 4 If we bond this methyl anion to the metal, the lone pair forms one metal-carbon bond and the methyl group acts as a 2-electron donor ligand. To keep charge neutrality we must oxidize the metal by one electron (i.e. assign a positive charge to the metal). This, in turn, reduces the d-electron count of the metal centre by one. Method 2: the covalent (neutral) model The major premise of this method is that we remove all of the ligand from the metal, but rather than take them to a closed shell state, we do whatever is necessary to make them neutral. Consider ammonia again: when we remove it from the metal, it is a neutral molecule with one lone pair of electrons. Therefore, as with ionic, ammonia is a neutral 2-electron donor. For methyl, we diverge from the ionic model. When we remove it from the metal and make the fragment neutral we have a neutral radical. Both the metal and methyl radical must donate one electron each to form a metal-carbon bond. The methyl group is, therefore, a one-electron donor, not a two-electron donor as it is under the ionic formation. Notice that this method does not give us any immediate information about the oxidation state of the metal, so we must go back and assign that later. Some commonly encountered ligands donate the following numbers of valence electrons. 1-electron donor: H• (in any bonding mode), and terminal Cl•, Br•, I•, R• (e.g. R = alkyl or Ph) or RO•; 2 2-electron donor: CO, PR3, P(OR)3, R2C=CR2 (η -alkene), R2C: (carbine); 3 • • • • • 3-electron donor: η -C3H5 (allyl radical), RC (carbine), μ-Cl , μ-Br , -I , μ-R2P ; 4 4 4-electron donor: η -diene, η -C4R4 (cyclobutadienes); 5 • • • • • 5-electron donor: η -C5H5 , μ3-Cl , μ3-Br , μ3-I , μ3-R2P ; 6 6 6 6-electron donor: η -C6H6 (and other η -arenes, e.g. η -C6H5Me); 1- or 3-electron donor: NO Counting electrons provided by bridging ligands, metal-metal bonds and net charges requires care. • • When X (X = CI, Br, I) or R2P the ligand uses the unpaired electron and a lone pair to bridge two metal centres, i.e. one electron is donated to M, and two to the second metal, M´ In a doubly bridged species such as (CO)2Rh(μ-Cl)2Rh(CO)2 the μ-Cl atoms are equivalent as are the Rh atoms, and the two Cl bridges together contribute three electrons to each Rh.
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