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Chapter 24 Chemistry of Coordination Compounds

CHEMISTRY The Central Science 9th Edition. Chapter 24 Chemistry of Coordination Compounds. David P. White. Metal Complexes. We know Lewis acids are electron pair acceptors. Coordination complexes: metal compounds formed by Lewis acid-base interactions.

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Chapter 24 Chemistry of Coordination Compounds

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  1. CHEMISTRYThe Central Science 9th Edition Chapter 24Chemistry of Coordination Compounds David P. White Chapter 24

  2. Metal Complexes • We know Lewis acids are electron pair acceptors. • Coordination complexes: metal compounds formed by Lewis acid-base interactions. • Complexes: Have a metal ion (can be zero oxidation state) bonded to a number of ligands. Complex ions are charged. Example, [Ag(NH3)2]+. • Ligands are Lewis bases. • Square brackets enclose the metal ion and ligands. Chapter 24

  3. Metal Complexes • The Development of Coordination Chemistry: Werner’s Theory • Werner discovered that CoCl3·nNH3 (n = 1 – 4) can exist as four different compounds with different numbers of “free” Cl- ions per formula unit. • He deduced that the NH3 ligands were covalently bonded to the central Co3+ ion. • Werner found that a total of six ligands were attached to the central Co. • In the case of CoCl3·4NH3, there are two isomers for the Cl ligands attached to Co. Chapter 24

  4. Metal Complexes The Development of Coordination Chemistry: Werner’s Theory Chapter 24

  5. Metal Complexes • The Metal-Ligand Bond • All ligands have lone pairs that are donated to the metal ion. • The bond between metal and ligand is a 2-electron bond, but both electrons come from the ligand and none from the metal. • The metal-ligand bond alters the physical properties of the metal: • Ag+(aq) + e- Ag(s), E = +0.799 V • [Ag(CN)2]-(aq) + e- Ag(s) + 2CN-(aq), E = -0.031 V Chapter 24

  6. Metal Complexes • The Metal-Ligand Bond • Most metal ions in water exist as [M(H2O)6]n+. • Charges, Coordination Numbers, and Geometries • Charge on complex ion = charge on metal + charges on ligands. • Donor atom: the atom bonded directly to the metal. • Coordination number: the number of ligands attached to the metal. • Most common coordination numbers are 4 and 6. Chapter 24

  7. Metal Complexes • Charges, Coordination Numbers, and Geometries • Some metal ions have constant coordination number (e.g. Cr3+ and Co3+ have coordination numbers of 6). • The size of the ligand affects the coordination number (e.g. [FeF6]3- forms but only [FeCl4]- is stable). • The amount of charge transferred from ligand to metal affects coordination number (e.g. [Ni(NH3)6]2+ is stable but only [Ni(CN)4]2- is stable). • Four coordinate complexes are either tetrahedral or square planar (commonly seen for d8 metal ions). • Six coordinate complexes are octahedral. Chapter 24

  8. Ligands with More than One Donor Atom • Monodentate ligands bind through one donor atom only. • Therefore they occupy only one coordination site. • Polydentate ligands (or chelating agents) bind through more than one donor atom per ligand. • Example, ethylenediamine (en), H2NCH2CH2NH2. • The octahedral [Co(en)3]3+ is a typical en complex. • Chelate effect: More stable complexes are formed with chelating agents than the equivalent number of monodentate ligands. Chapter 24

  9. Chapter 24

  10. Ligands with More than One Donor Atom • [Ni(H2O)6]2+(aq) + 6NH3 [Ni(NH3)6]2+(aq) + 6H2O(l) Kf = 4  108 • [Ni(H2O)6]2+(aq) + 3en [Ni(en)3]2+(aq) + 6H2O(l) Kf = 2  1018 • Sequestering agents are chelating agents that are used to remove unwanted metal ions. • In medicine sequestering agents are used to selectively remove toxic metal ions (e.g. Hg2+ and Pb2+) while leaving biologically important metals. Chapter 24

  11. Ligands with More than One Donor Atom • One very important chelating agent is ethylenediaminetetraacetate (EDTA4-). • EDTA occupies 6 coordination sites, for example [CoEDTA]- is an octahedral Co3+ complex. • Both N atoms (blue) and O atoms (red) coordinate to the metal. • EDTA is used in consumer products to complex the metal ions which catalyze decomposition reactions. Chapter 24

  12. Chapter 24

  13. Ligands with More than One Donor Atom • Metals and Chelates in Living Systems • Many natural chelates are designed around the porphyrin molecule. • After the two H atoms bound to N are lost, porphyrin is a tetradentate ligand. • Porphyrins: Metal complexes derived from porphyrin. • Two important porphyrins are heme (Fe2+) and chlorophyll (Mg2+). • Myoglobin is protein containing a heme unit, which stores oxygen in cells. Chapter 24

  14. Ligands with More than One Donor Atom • Metals and Chelates in Living Systems • A five membered nitrogen containing ring binds the heme unit to the protein. • When oxygen is attached to the iron(II) in heme, oxymyoglobin is formed. • The protein has a molecular weight of about 18,000 amu. • The Fe2+ ion in oxyhemoglobin or oxymyoglobin is octahedral. • Four N atoms from the porphyrin ring (red disk) are attached to the Fe2+ center. Chapter 24

  15. Ligands with More than One Donor Atom • Metals and Chelates in Living Systems • The fifth coordination site is occupied by O2 (or H2O in deoxyhemoglobin or CO in carboxyhemoglobin). • The sixth coordination site is occupied by a base, which attaches the structure to the protein. • Photosynthesis is the conversion of CO2 and water to glucose and oxygen in plants in the presence of light. • One mole of sugar requires 48 moles of photons. • Chlorophyll absorbs red and blue light and is green in color. Chapter 24

  16. Chapter 24

  17. Ligands with More than One Donor Atom • Metals and Chelates in Living Systems • Chlorophyll a is the most abundant chlorophyll. • The other chlorophylls differ in the structure of the side chains. • Mg2+ is in the center of the porphyrin-like ring. • The alternating double bonds give chlorophyll its green color (it absorbs red light). • Chlorophyll absorbs red light (655 nm) and blue light (430 nm). Chapter 24

  18. Ligands with More than One Donor Atom • Metals and Chelates in Living Systems • The reaction • 6CO2 + 6H2O  C6H12O6 + 6O2 • is highly endothermic. • Plant photosynthesis sustains life on Earth. Chapter 24

  19. Nomenclature of Coordination Compounds • Rules: • For salts, name the cation before the anion. Example in [Co(NH3)5Cl]Cl2 we name [Co(NH3)5Cl]2+ before Cl-. • Within a complex ion, the ligands are named (in alphabetical order) before the metal. Example [Co(NH3)5Cl]2+ is tetraamminechlorocobalt(II). Note the tetra portion is an indication of the number of NH3 groups and is therefore not considered in the alphabetizing of the ligands. • Anionic ligands end in o and neutral ligands are simply the name of the molecule. Exceptions: H2O (aqua) and NH3 (ammine). Chapter 24

  20. Nomenclature of Coordination Compounds • Rules: • Greek prefixes are used to indicate number of ligands (di-, tri-, tetra-, penta-, and hexa-). Exception: if the ligand name has a Greek prefix already. Then enclose the ligand name in parentheses and use bis-, tris-, tetrakis-, pentakis-, and hexakis. • Example [Co(en)3]Cl3 is tris(ethylenediamine)cobalt(III) chloride. • If the complex is an anion, the name ends in -ate. • Oxidation state of the metal is given in Roman numerals in parenthesis at the end of the complex name. Chapter 24

  21. Isomerism • Isomers: two compounds with the same formulas but different arrangements of atoms. • Coordination-sphere isomers and linkage isomers: have different structures (i.e. different bonds). • Geometrical isomers and optical isomers are stereoisomers (i.e. have the same bonds, but different spatial arrangements of atoms). • Structural isomers have different connectivity of atoms. Chapter 24

  22. Isomerism • Stereoisomers have the same connectivity but different spatial arrangements of atoms. Chapter 24

  23. Isomerism Chapter 24

  24. Isomerism • Structural Isomerism • Some ligands can coordinate in different ways. • That is, the ligand can link to the metal in different ways. • These ligands give rise to linkage isomerism. • Example: NO2- can coordinate through N or O (e.g. in two possible [Co(NH3)5(NO2)]2+ complexes). • When nitrate coordinates through N it is called nitro. • Pentaamminenitrocobalt(III) is yellow. • When ONO- coordinates through O it is called nitrito. • Pentaamminenitritocobalt(III) is red. Chapter 24

  25. Isomerism • Structural Isomerism • Similarly, SCN- can coordinate through S or N. • Coordination sphere isomerism occurs when ligands from outside the coordination sphere move inside. • Example: CrCl3(H2O)6 has three coordination sphere isomers: [Cr(H2O)6]Cl3 (violet), [Cr(H2O)5Cl]Cl2.H2O (green), and [Cr(H2O)4Cl2]Cl.2H2O (green). • Structural Isomerism • Consider square planar [Pt(NH3)2Cl2]. • The two NH3 ligands can either be 90 apart or 180 apart. Chapter 24

  26. Isomerism Stereoisomers Chapter 24

  27. Isomerism • Structural Isomerism • The spatial arrangement of the atoms in the cis and trans isomers is different. • This is an example of geometrical isomerism. • In the cis isomer, the two NH3 groups are adjacent. The cis isomer (cisplatin) is used in chemotherapy. • The trans isomer has the two NH3 groups across from each other. • It is possible to find cis and trans isomers in octahedral complexes. Chapter 24

  28. Isomerism • Structural Isomerism • For example, cis-[Co(NH3)4Cl2]+ is violet and trans-[Co(NH3)4Cl2]+ is green. • The two isomers have different solubilities. • In general, geometrical isomers have different physical and chemical properties. • It is not possible to form geometrical isomers with tetrahedra. (All corners of a tetrahedron are identical.) Chapter 24

  29. Isomerism • Structural Isomerism • Optical isomers are mirror images which cannot be superimposed on each other. • Optical isomers are called enantiomers. • Complexes which can form enantiomers are chiral. • Most of the human body is chiral (the hands, for example). Chapter 24

  30. Isomerism Structural Isomerism Chapter 24

  31. Isomerism Structural Isomerism Chapter 24

  32. Isomerism • Structural Isomerism • Enzymes are the most highly chiral substances known. • Most physical and chemical properties of enantiomers are identical. • Therefore, enantiomers are very difficult to separate. • Enzymes do a very good job of catalyzing the reaction of only one enantiomer. • Therefore, one enantiomer can produce a specific physiological effect whereas its mirror image produces a different effect. Chapter 24

  33. Isomerism • Structural Isomerism • Enantiomers are capable of rotating the plane of polarized light. • Hence, they are called optical isomers. • When horizontally polarized light enters an optically active solution. • As the light emerges from the solution, the plane of polarity has changed. • The mirror image of an enantiomer will rotate the plane of polarized light in the opposite direction. Chapter 24

  34. Isomerism • Structural Isomerism • Dextrorotatory solutions rotate the plane of polarized light to the right. This isomer is called the d-isomer. • Levorotatory solutions rotate the plane of polarized light to the left. This isomer is called the l-isomer. • Chiral molecules are optically active because of their effect on light. • Racemic mixtures contain equal amounts of l- and d-isomers. They have no overall effect on the plane of polarized light. Chapter 24

  35. Isomerism • Structural Isomerism • Pasteur was the first to separate racemic ammonium tartarate (NaNH4C4H9O6) by crystallizing the solution and physically picking out the “right-handed” crystals from the mixture using a microscope. • Optically pure tartarate can be used to separate a racemic mixture of [Co(en)3]Cl3: if d-tartarate is used, d-[Co(en)3]Cl3 precipitates leaving l-[Co(en)3]Cl3 in solution. Chapter 24

  36. Color and Magnetism • Color • Color of a complex depends on: (i) the metal and (ii) its oxidation state. • Pale blue [Cu(H2O)6]2+ can be converted into dark blue [Cu(NH3)6]2+ by adding NH3(aq). • A partially filled d orbital is usually required for a complex to be colored. • So, d0 metal ions are usually colorless. Exceptions: MnO4- and CrO42-. • Colored compounds absorb visible light. Chapter 24

  37. Color and Magnetism • Color • The color perceived is the sum of the light not absorbed by the complex. • The amount of absorbed light versus wavelength is an absorption spectrum for a complex. • To determine the absorption spectrum of a complex: • a narrow beam of light is passed through a prism (which separates the light into different wavelengths), • the prism is rotated so that different wavelengths of light are produced as a function of time, Chapter 24

  38. Color and Magnetism • Color • the monochromatic light (i.e. a single wavelength) is passed through the sample, • the unabsorbed light is detected. Chapter 24

  39. Color and Magnetism • Color • The plot of absorbance versus wavelength is the absorption spectrum. • For example, the absorption spectrum for [Ti(H2O)6]3+ has a maximum absorption occurs at 510 nm (green and yellow). • So, the complex transmits all light except green and yellow. • Therefore, the complex is purple. Chapter 24

  40. Chapter 24

  41. Color and Magnetism • Magnetism • Many transition metal complexes are paramagnetic (i.e. they have unpaired electrons). • There are some interesting observations. Consider a d6 metal ion: • [Co(NH3)6]3+ has no unpaired electrons, but [CoF6]3- has four unpaired electrons per ion. • We need to develop a bonding theory to account for both color and magnetism in transition metal complexes. Chapter 24

  42. Crystal-Field Theory • Crystal field theory describes bonding in transition metal complexes. • The formation of a complex is a Lewis acid-base reaction. • Both electrons in the bond come from the ligand and are donated into an empty, hybridized orbital on the metal. • Charge is donated from the ligand to the metal. • Assumption in crystal field theory: the interaction between ligand and metal is electrostatic. Chapter 24

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