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Ch. 23: Transition Metals and Coordination Chemistry

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Ch. 23: Transition Metals and Coordination Chemistry Ch. 23: Transition metals and Coordination Chemistry Learning goals and key skills: Determine the oxidation number and number of d electrons for metal ions in complexes Name coordination compounds given their formula and write the formula given their name Recognize and draw geometric isomers of a complex Recognize and draw isomers of a complex Use crystal-field theory to explain the colors and to determine the number of unpaired electrons in a complex. 1 2 Co2+ Ni2+ Cu2+ Zn2+ 3 4 Coordination compounds • A central metal atom bonded to a group of molecules or ions is a metal complex. • If the complex bears a charge, it is a complex ion. • Compounds containing complexes are coordination compounds. • The molecule or ions that bond to the metal ion are known as ligands. • The central metal and its ligands constitute the coordination sphere. • The donor atom is the atom directly bound to the metal. • The coordination number is the number of atoms directly bonded to the central atom. Metal-Ligand Bond • Metal: Lewis acid (has empty orbitals) • Ligand: Lewis base (has nonbonding electrons) • The metal complex has distinct physical and chemical properties (color, reduction potentials, etc.) from the metal ion and ligand ions. 5 Oxidation Number Knowing the charge on a complex ion and the charge on each ligand, one can determine the oxidation number for the metal. Knowing the oxidation number on the metal and the charges on the ligands, one can calculate the charge on the complex ion. Find the charge on a complex ion with Cr, four water molecules and two chloride ions. + [Cr(H2O)4Cl2] 6 Geometry and Coordination Number Coordination number General trends: Lower coordination numbers are found with • Larger ligands – 3– [FeCl4] vs [FeF6] • Higher charged ligands 2– 2+ [Ni(CN)4] vs [Ni(NH3)6] 7 Polydentate Ligands • Monodentate: “one tooth” • Bidentate: “two teeth” • Polydentate: “many-toothed” • Chelating agents: “claw” EDTA4- 8 ethylenediamine ethylenediamine (en) is a bidentate ligand. ethylenediaminetetraacetate ion EDTA is a polydentate ligand with six donor atoms. 9 Porphyrins: Important biomolecules like chlorophyll and heme are porphyrins. Nomenclature of Coord. Comp. • Cation is 1st; anion is 2nd • Prefixes: di-, tri-, tetra-, penta-, hexa- [or bis-, tris-, tetrakis-, pentakis-, for complex ligands in parenthesis]. • Within the complex ion: – ligands 1st then metal – list ligands (not prefixes) alphabetically • anions end in “o”; neutral ones use normal names – exceptions: aqua, ammine, carbonyl • complex anions end in –ate • oxidation number of the metal is in Roman numerals Pentaaquachlorochromium (III) chloride Potassium hexacyanoferrate (III) 10 Nomenclature Example: Isomers: same composition, different arrangement of atoms 11 Structural isomers: different bonds Coordination sphere isomer CrCl3·6H2O chromium (III) chloride hexahydrate [Cr(H2O)6]Cl3 hexaaquachromium(III) chloride [Cr(H2O)5Cl]Cl2·H2O pentaaquachlorochromium(III) chloride monohydrate [Cr(H2O)4Cl2]Cl·2H2O tetraaquadichlorochromium(III) chloride dihydrate Linkage isomers 2+ [Co(NH3)5(NO2)] If a ligand (like the NO2 group at the bottom of the complex) can bind to the metal with one or another atom as the donor atom, linkage isomers are formed. 12 Common Ligands in Linkage Isomers nitrito nitro cyano isocyano isocyanato cyanato isothiocyanato thiocyanato Stereoisomers: same bonds, different spatial arrangements geometric isomers: cis-Isomers have like groups on the same side. trans-Isomers have like groups on opposite sides. 13 Stereoisomers: optical isomers/entaniomers: mirror images Two enantiomers cannot be superimposed on each other. These are chiral molecules. Enantiomers are optically active • The physical properties of chiral molecules are the same except in instances where the spatial placement of atoms matters. • One example is the interaction of a chiral molecule with plane-polarized light. 14 Enantiomers: D and L D – dextrorotatory (to the right) L – levorotatory (to the left) racemic: a mixture of the two Amino acids in proteins are L-isomers. D-Amino acids have been found in small peptides of bacterial cell walls and some peptide antibiotics. Complexes and color The color arises from the fact that the complex absorbs some wavelengths of visible light and reflects others. 15 Complexes and color •A substance appears black if it absorbs all the visible light that strikes it, whereas if a substance absorbs no visible light it is white or colorless. •An object appears green if it reflects all colors except red, the complementary color of green. •Similarly, this concept applies to transmitted light that pass through a solution. Complexes and color 3+ [Ti(H2O)6] appears red-violet in color because it absorbs light at the center (yellow and green) parts of the spectrum. 16 Metal–ligand bond formation, the ligand acts as a Lewis base and the metal acts as a Lewis acid. Crystal field splitting When negative charges are brought up to the ion, the average energy of the d orbitals increases. • Because the repulsion felt by the dz2 and dx2–y2 orbitals is greater than that felt by the dxy, dxz, and dyz orbitals, the five d orbitals split into a lower- energy set of three (the t2 set) and a higher-energy set of two (the e set). 17 3+ The d-d transition in [Ti(H2O)6] is produced by the absorption of 495-nm light. Complexes and color Interactions between electrons on a ligand and the orbitals on the metal cause differences in energies between orbitals in the complex. 18 Electron Configurations in Octahedral Complexes The magnitude of the energy gap, D, increases by a factor of two from the left to the right of the spectrochemical series below. increasing D → weak-field strong-field - - - - ligands ligands Cl < F < H2O < NH3 < en < NO2 < CN 19 Tetrahedral complexes Square planar complexes d8 20 Charge-transfer transitions 21.
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