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Chapter 25 : Complex Ions and Coordination Compounds

Chapter 25 : Complex Ions and Coordination Compounds . 25-1 Werner’s Theory of Coordination Compounds: An Overview. 25-2 Ligands . 25-3 Nomenclature. 25-4 Isomerism. 25-5 Bonding in Complex Ions: Crystal Field Theory. 25-6 Color and the Colors of Complexes.

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Chapter 25 : Complex Ions and Coordination Compounds

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  1. Chapter 25 : Complex Ions and Coordination Compounds 25-1 Werner’s Theory of Coordination Compounds: An Overview 25-2 Ligands 25-3 Nomenclature 25-4 Isomerism 25-5 Bonding in Complex Ions: Crystal Field Theory 25-6 Color and the Colors of Complexes 25-7 Aspects of Complex Ion Equilibria 25-8 Acid-Base Reactions of Complex Ions 25-9 Some Kinetic Considerations 25-10 Applications of Coordination Chemistry

  2. 25-1 Werner’s Theory of Coordination Compounds: An Overview Complex Ion - is a polyatomic cation or anion composed of a central metal ion to w/c are bonded other groups (molecules or ions) Substances containing complex ions belong to a category of compounds called coordination compounds

  3. The following formulas describe a series of three coordination compounds: CoCl3.6NH3 (a) CoCl3.5NH3 (b) CoCl3.4NH3 (c) In 1893, Swiss chemist Alfred Werner proposed that certain metal atoms, primarily those of the transition metals, have two types of valence. 1. The primary valence, is based on the number of electrons the atom loses in forming the metal ion. 2. Secondary or auxiliary, valence is responsible for the bonding of other groups, called ligands, to the central metal ion.

  4. [Co(NH3)6]Cl3 (a) [CoCl(NH3)5]Cl2 (b) [CoCl2(NH3)4]Cl (c) Ionization of coordination compound (a) can be represented as H2O [Co(NH3)6]Cl3(s) [Co(NH3)6]3+(aq) + 3 Cl-(aq) In the presence of an excess of AgNO3(aq) (a) yield three moles of AgCl(s) per mole of compound (b) yield only two moles of AgCl(s) per mole of compound (c) and yield only one moles of AgCl(s) per mole of compound

  5. Werner’s theory also accounts for 1. The coordination compound [CoCl3(NH3)3], w/c is a nonelectrolyte and yields no precipitate with AgNO3(aq). 2. The compound Na[CoCl4(NH3)2], in w/c the complex ion has a net negative charge, that is [CoCl4(NH3)2]- [Co(NH3)6]3+ are all complex ions, and because they carry a positive charge, they are cations. [CoCl(NH3)5]2+ [CoCl2(NH3)4]+ [CoCl4(NH3)2]- is also a complex ion, but it is an anion. [CoCl3(NH3)3], is a neutral molecule.

  6. Coordination number - is the number of points around the metal center at w/c bonds to ligands can form. - ranging from 2 to 12 have been observed in complexes, although the number 6 is by far the most common, follow by 4. - 2 is most limited to complexes of Cu(I), Ag(I), and Au(I). - greater than 6 are not often found in members of the first transition series but are more common in those of the second and third series. - stable complexes w/ coordination no. 3 and 5 are rarely encountered. - observed in a complex depends on a number of factors, such as the ratio of the radius of the central metal atom or ion to those of the attached ligands.

  7. Table 25.1 Some Common Coordination Number of Metal Ions Cu+ 2, 4 Ag+ 2 Au+ 2, 4 Al3+ 4, 6 Sc3+ 6 Cr3+ 6 Fe2+ 6 Fe3+ 6 Co2+ 4, 6 Co3+ 6 Ni2+ 4, 6 Au3+ 4 Cu2+ 4, 6 Zn2+ 4

  8. Figure 25-1Structure of complex ions. Attachment of the NH3 molecules occurs through the lone pair electrons on the N atoms.

  9. Example 25-1 Relating the Formula of a Complex to the Coordination Number and Oxidation State of the Central Metal. What are the coordination number and oxidation state of Co in the complex ion [CoCl(NO2)(NH3)4]+? charge of 1- on Cl - total negative charge: 2- oxidation state = x charge of 1- on NO2 - [CoCl(NO2)(NH3)4]+ net charge on complex ion x - 2 = +1 x = +3 coordination number = 6

  10. Practice Example A: What are the coordination number and oxidation state of nickel in the ion [Ni(CN)4I]3 - ? Ans.Coordination Number is 5 Oxidation State is +2 Practice Example B: Write the formula of a complex with cyanide ion ligands, an iron ion with an oxidation state of +3, and a coordination number of 6. Ans.[Fe(CN)6]3 -

  11. 25-2 Ligands A common feature shared by the ligands in complexes is the • ability to donate electrons pairs to central metal atoms or ions act as Lewis bases. • ability in accepting electrons pairs, central atoms or ions act as Lewis acids. A ligand that uses one pair of electrons to form one point of attachment to the central metal atom or ion is called monodentate ligand. Some ligands are capable of donating more than a single electron pair from different atoms in the ligand and to different sites in the geometry structure of a complex are called polydentate ligands.

  12. Table 25.2 Some Common Monodentate Ligands Formula Name as Ligand Formula Name as Ligand Neutral Anions molecules H2O aqua F - fluoro NH3 ammine Cl - chloro CO carbonyl Br - bromo NO nitrosyl I - iodo CH3NH2 methylamine O 2- oxo C5H5N pyridine OH - hydroxo CN - cyano SO42- sulfato S2O32- thiosulfato NO2- nitroa ONO - nitritoa SCN - thiocyanatob NCS - isothiocyanatob

  13. The molecule ethylenediamine (en) can donate two electron pairs, one from each N atom. Bidentate ligand attaches itself to the central atom of a complex at two points in the coordination sphere.

  14. Table 25.3 Some Common Polydentate Ligands (Chelating Agents) Abbreviation Name Formula en ethylenediamine ox oxalato EDTA ethylenediaminetetraacetato

  15. Figure 25-2Two representations of the chelate [Pt(en)2]2+. The ligands attach at adjacent corners along an edge of the square. They do not bridge the square by attaching to opposite corners. Bonds are shown in red, and the square planar shape is indicated by the black parallelogram. Chelate results from the attachment of polydentate ligands to the central atom of a complex ion. Chelating agent is a polydentate ligand. It simultaneously attaches to two or , more position in the coordination sphere of the central atom of a complex ion.

  16. 25-3 Nomenclature 1.In names and formulas of coordination compounds, cations come first, followed by anions. 2.Anions as ligands are named by using the ending “o”. Normally, “ide” ending change to “o,” “ite” to “ito,” and “ate” to “ato” 3.Neutral molecules as ligands generally carry the unmodified name. For ex., the name ethylenediamine is used both for the free molecule and for the molecule as a ligand. Aqua, ammine, carbonyl, and nitrosyl are important exceptions. 4.The number of ligands of a given type is denoted by a prefix. - the usual prefixes are mono = 1, di = 2, tri = 3, tetra = 4 and so on...

  17. - if the ligand name is a composite name that itself contains a prefix, such as ethylenediamine, place parentheses around the name and precede it w/ bis = 2, tris = 3, tetrakis = 4, and so on... 5.In naming a complex, ligands are named first, in alphabetical order, followed by the name of the metal center. The oxidation state of the metal center is donated by a Roman numeral. If the complex is an anion, the ending “ate” is attached to the name of the metal. - prefixes (di,tri,bis,tris,…) are ignored in establishing the alphabetical order. For instance, [CrCl2(H2O)4]+ is called tetraaquadichlorochromium(III) ion [CoCl2(en)2]+ is dichlorobis(ethylenediamine)cobalt(III) ion [Cr(OH)4]- is tetrahydroxochromate(III) ion

  18. 6.In writing the formula of a complex, the chemical symbol of the metal center is written first, followed by the formulas of anions and then neutral molecules. - if there are two or more different anions or neutral molecules as ligands, they are written in alphabetical order according to the first chemical symbols of their formulas. For instance, the formula of the tetraaminechloronitrocobalt(III) ion, Cl- precedes NO2-, and both are placed ahead of the neutral NH3 molecules: [CoCl(NO2)(NH3)4]+

  19. Example 25-2 Relating Names and Formulas of Complexes. (a) What is the name of the complex [CoCl3(NH3)3]? (b) What is the formula of the compound pentaaquachlorochromium(III) chloride? (c) What is the name of the compound K3[Fe(CN)6]? Sol. (a)[CoCl3(NH3)3] consist of three ammonia molecules and three chloride ions attached to a central Co3+ ion; it is electrically neutral. The name of this neutral complex is triamminetrichlorocobalt(III). (b) The central metal ion is Cr3+. There are five H2O molecules and one Cl - ion as ligands. The complex ion carries a net charge of 2+. Two Cl - ions are required to neutralize the charge on this complex cation. The formula of the coordination compound is [CrCl(H2O)5]Cl2.

  20. (c) This compound consist of K+ cation and complex anions having the formula [Fe(CN)6]3 -. Each cyanide ion carries a charge of 1-, so the oxidation state the iron must be +3. The Latin-based name “ferrate” is used because the complex ion is an anion. The name of the anion is hexacyanoferrate(III) ion. The coordination compound is potassium hexacyanoferrate(III). Practice Example A: What is the formula of the compound potassium hexachloroplatinate(IV)? Ans.K2[Pt(Cl)6] Practice Example A: What is the name of the compound [Co(SCN)(NH3)5]Cl2? Ans.Pentaamminethiocyanatocobalt(III) chloride

  21. 25-4 Isomerism Isomers are substances that have the same formulas but differ in their structures and in their properties. Kinds of isomerism can be lumped into two broad categories: Structural isomers have the same number and kinds of atoms but they differ in their structural formulas. types are, Ionization Isomerism, Coordination Isomerism, & Linkage Isomerism Stereoisomers the number and types of atoms and bonds in molecules are the same, but certain atoms are oriented differently in space. types are, Geometric Isomerism, and Optical Isomerism

  22. Ionization Isomerism [CrSO4(NH3)5]Cl pentaamminesulfatochromium(III) chloride (a) [CrCl(NH3)5]SO4 pentaamminechlorochromium(III) sulfate (b) Coordination Isomerism [Co(NH3)6][Cr(CN)6] hexaamminecobalt(III) hexacyanochromate(III) (a) [Cr(NH3)6][Co(CN)6] hexaamminechromium(III) hexacyanocobaltate(III) (b)

  23. Linkage Isomerism Some ligands may attach to the central metal ion of a complex ion in different ways. For example, [CoCl(NO2)(NH3)4]+ tetraaminechloronitrocobalt(III) ion (a) [CoCl(ONO)(NH3)4]+ tetraaminechloronitritocobalt(III) ion (b)

  24. Geometry Isomerism • in organic compounds refers to the existence of nonequivalent structures (cis and trans) that differ in the positioning of substituent groups relative to a double bond. • in complexes the nonequivalent structures are based on the positions at which ligands are attached to the metal ceter. [PtCl2(NH3)2] cis-diamminedichloroplatinum(II) or trans- diamminedichloroplatinum(II)

  25. (a) No isomerism Substitute third Cl - for NH3 cis- trans- (b)Cis and Trans isomerism Substitute third Cl - for NH3 (c) No isomerism

  26. Optical Isomerism - also called enantiomers(nonsuperimposable mirror images), are isomers that differ only in the way they rotate the plane of polarized light. • Structures that are nonsuperimposable mirror images of each other are called enantiomers and are said to be chiral. • Structures that are superimposable are achiral. Enantiomer rotates the plane of polarized light to the right (clockwise) is said to be dextrorotatory (designated + or d). Enantiomer rotates the plane of polarized light to the left (clockwise) is said to be levorotatory (designated - or l).

  27. 25-5 Bonding in Complex Ions: Crystal Field Theory Crystal field theory describes bonding in complex ions in terms of electrostatic attractions between ligands and the nucleus of the central metal ion. - particular attention is focused on the splitting of the d energy level of the central metal ion that results from electron repulsion. Different ligands can be arranged in order of their abilities to produce a splitting of the d energy levels, this arrangement is known as the spectrochemical series.

  28. The d-orbital energy levels of the free central ion are raised in the presence of ligands to the average level shown, but the five levels are split into two groups. d d x2 x2 - y2  Average energy of d orbitals in field of ligands dxy dxz dyz Figure 25-3 Splitting of d energy levels in the formation of an octahedral complex ion.

  29. Thank you for your listening!!!

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