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MS101 Metal Catalysts C.Nevado 1 MS101 TM‐OC: What is special? C.Nevado TM‐OC = Transition Metal Organometallic Complex ‐) A convenient classification of organometallics is based on the bond type: 2 MS101 Ionization Potentials ‐) ‐) ‐) ‐) C.Nevado More electronegative element means lower energies of the orbitals Early metals more easily oxidized, more basic, more electron‐rich Mid to late TM’s: third‐row metals can form high oxidation state compounds more easily In many cases, 3rd row metals are more basic than 2nd row metals which are more basic than 1st row metals. more basic than 1 row metals 3 MS101 Trends in size ‐) ‐) C.Nevado Second‐row metals larger than first‐row (higher quantum number of the valence orbitals) Third‐row metals comparable in size to second row (“lanthanide contraction”, nuclear charges of third‐row much higher than second row and balance the effect of higher quantum number 4 MS101 Historical Perspective 1760 1827 1849 1852 1881 1890 1909 1917 1931 1938 Cadet [(CH3)2As]2O, first organometallic compound Zeise’s salt Na[PtCl3(C2H4)], first olefin complex Frankland, Zn(Et)2. Very air‐sensitive, Hydrogen gas was used as a protective atmosphere! protective atmosphere! Löwig and Schweizer make Pb(Et)4, Bi(Et)3 and Sb(Et)3. Grignard, organomagnesium halides. Mond, Ni(CO)4. First homoleptic metal carbonyl. Pope (CH3)3PtI, first sigma Pope, (CH PtI first sigma‐organotransition‐metal organotransition metal compound. compound Schlenk, lithium alkyls. Hieber, Fe(CO)4H2 , first TM‐hydride complex. Roelen, discovers hydroformylation. 1951 Pauson and Miller discover ferrocene. 1955 1964 1965 1983 Ziegler, Natta, polyolefins from ethylene and propylene. Fischer (CO)5W=C(OMe)Me, first carbene complex. Fischer, (CO) W C(OMe)Me first carbene complex Wilkinson, Rh(PPh3)3Cl, catalyst for hydrogenation. Bergman, Graham, C‐H activation of alkanes with TM’s. C.Nevado 5 MS101 Metal‐Ligand complexes C.Nevado Some definitions: Some definitions: Ligand: A ligand is a molecule or ion that is bonded covalently to a metal center. Monodentate: Monodentate ligands have only one point of attachment to the metal center and occupy only one coordination site Ambidentate: Ambidentate ligands can bind to a metal center through more than one kind of atom. Classic examples include NO2‐, SCN‐ or R2S=O. These ligands are examples of linkage isomerism. Ambidentate can also refer to a polydentate ligand that is not fully These ligands are examples of linkage isomerism. Ambidentate can also refer to a polydentate ligand that is not fully coordinated to a transition metal. Bidentate: Bidentate ligands have two points of attachment to the metal center and occupy 2 coordination sites. Examples: 2,2’‐bipyridine (bipy), ethylenediamine (en), diphenylphosphinoethane (dppe), acetylacetonate (acac), etc. Polydentate: Polydentate (or multidentate) ligands have more than one point of attachment to the metal center and occupy more than one coordination site. Examples include edta, PCP or TP ligands. Chelating: A chelating ligand involves a polydentate ligand that forms a ring that includes the metal. Also called a chelate. Homoleptic Complexes: Homoleptic Complexes: contain only one kind of ligand. Primary Coordination sphere The primary coordination sphere of a metal involves the set of ligands closest to the metal that are directly attached. Mobile cations or counterions are said to be in the outer or secondary coordination sphere. 6 MS101 Metal‐Ligand complexes C.Nevado The coordination number: For simple monodentate and chelating ligands, the coordination number can be defined as p g g , the number of atoms or ligands directly bonded to the metal atom. For example, [Fe(NH3)6]3+ and [Fe(en)3]3+ are both 6‐coordinate complexes. Complication: C li ti Cp2Fe is considered to be a 6‐coordinate complex, although it should be called a 10‐ coordinate complex. Alkoxide, imido and oxo ligands can donate one, two or three pairs of electrons to a , g , p metal, but only occupy one coordination site. Cp2TiCl2 is not considered to be an 8‐coordinate complex. Therefore; the coordination number of Cp lies between 1 and three. Coordinative unsaturation: The term coordinative unsaturation is used to describe a complex that has one or more open coordination sites where another ligand can be accommodated or more open coordination sites where another ligand can be accommodated. Typically, most complexes with a CN of less than 6 fall into this category. Coordinative unsaturation is an important concept in organotransition metal chemistry. For example, for an olefin to undergo insertion, metathesis or polymerization, there must be room for the olefin to approach or bond to the metal. 7 MS101 TM‐OC: Metal‐Ligand complexes C.Nevado Electron counting: The 18 electron rule Because most metals possess d orbitals, they can accommodate 18 valence electrons. And just as the octet rule is often violated, so is the 18 electron rule. Nonetheless, both serve a useful purpose the octet rule is often violated, so is the 18 electron rule. Nonetheless, both serve a useful purpose in predicting reactivity. Counting electrons in organometallic complexes Knowing how many valence electrons belong to a transition metal complex allows us to make predictions about the mechanisms of reactions and the possible modes of reactivity. There are two methods for counting electrons: M th d 1 Th i i ( h Method 1: The ionic (charged) model d) d l We ‘remove’ all the ligands from the metal and, if necessary, add the proper number of electrons to each ligand to bring it to a closed shell state, for example NH3, but CH3‐. Method 2: The covalent (neutral) model Method 2: The covalent (neutral) model We ‘remove’ the ligands from the metal, but rather than take them to a closed shell state, we do whatever is necessary to make them neutral, for example NH3, but CH3.. Notice that this method does not give us any immediate information about the formal oxidation state of the metal, but is useful when dealing with larger metallic networks such as the ones found in clusters. 8 MS101 C.Nevado TM‐OC: Geometries of Metal‐Ligand complexes Sterically preferred geometries: ‐) trigonal planar ‐) tetrahedral ‐) trigonal bipyramidal ‐) octahedral Sometimes, electronics override steric preferences: ‐) d8 metals, especially of the second and third row, are almost always square planar (dx2‐y2 orbital high in energy) 9 MS101 TM‐OC: Counting electrons The two methods compared: 10 MS101 TM‐OC: Ligand electron count 11 MS101 TM‐OC: Ligand electron count II 12