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COORDUATION COMPOUNDS. COMPLEXES. Questions. 1. The structure of coordination compounds. Werner’s theory. 2. Classification of complexes. 3. Naming. 4. Isomerism in complexes. 5. Bonding in complex ions. 6. Stability of complexes. 7. Importance of complex compounds in life. Their use in medicine. §1. Historical development and terminology A coordination complex is the product of a Lewis acid-base reaction in which neutral molecules or anions (called ligands) bond to a central metal atom (or ion) by coordinate covalent bonds. d, f-elements form stronger complexes than s, p-elements. Complex compounds of Mn, Fe, Co, Cu, Zn, Mo are very important for vital activity of the organism. Amphoteric p-elements (Al, Sn, Pb) also form complexes. Biogenic s-elements such as Na, K, Ca, and Mg can form complexes with ligands of specific structures only. An ability of complex formation decreases in the following order: f > d> p ≥ s. Coordination compounds of transition metals were studied by Warner. He investigated a series of compounds which contained cobalt, chlorine, and ammonia These colored compounds were thought to have the following empirical formulas: CoCl3 ∙ 6NH3 (orange-yellow), CoCl3 ∙ 5NH3 (violet), CoCl3 ∙ 4NH3 (green), CoCl3 ∙ 3NH3 (green). The dots in these formulas symbolize unspecified bonding between the NH3 molecules and the rest of the compound. Coordination complexes in solution can be studied by measuring the electrical conductivity of the solution. The electrical conductivity of a solution depends on the number of moles of ions in the solution. Electrical conductivity measurements of dilute aqueous solutions of CoCl3 ∙ 6NH3, CoCl3 ∙ 5NH3, CoCl3 ∙4NH3, and CoCl3 ∙ NH3 suggest these compounds contain 4, 3, 2, and 0 mol of ions per mole of the compound, respectively. Thus, three of the compounds are probably ionic and one is molecular. To establish the identities of the components of these compounds and their formulas, the components were dissolved in water and the resulting solutions were treated with excess silver nitrate. Chloride ions in the solution of given cobalt compounds precipitate immediately with silver ions to give solid silver chloride. A mole of CoCl3 ∙ 6NH3 reacts with 3 mol of silver ions; a mole of CoCl3 ∙ 5NH3 reacts with 2 mol of silver ions; a mole of CoCl3 ∙ 4NH3 reacts with 1 mol of silver ions; and CoCl3 ∙ 3NH3 does not immediately react with silver ions. Since a mole of CoCl3 ∙ NH3 in solution reacts with 3 mol of silver ions, there must be 3 mol of chloride ions per mole of the compound. Electrical conductivity measurements show that a mole of CoCl3 3 ∙ 6NH3 produces 4 mol of ions in solution. Werner proposed that the forth ion consists of a central cobalt ion surrounded by six ammonia molecules. These ammonia molecules are ligands that are bonded to the central metal ion by coordinate covalent bonds. The formula for this complex ion is Co(NH3)63+. The central metal ion with its attached ligands is called the coordination sphere. In the neutral compound with the empirical formula CoCl3 ∙ 6NH3, each Co(NH3)63+ ion is associated with three chloride ions. Thus, the formula for the compound is [Co(NH3)6]Cl3. The coordination sphere in a formula of a coordination complex is enclosed in brackets. An aqueous solution of 1 mol of CoCl3 ∙ 5NH3 readily reacts with excess AgNO3 to produce 2 mol of AgCl. Therefore, two Cl- ions must be outside the coordination sphere. A mole of CoCl3 ∙ 5NH3 therefore consists of 1 mol of [Co(NH3)5Cl2]+ ions and 2 mol of Cl- ions. Since a Cl- ion and a Co3+ ion are a part of the coordination sphere, the charge of the complex ion is 2+. The formula of the compound is [Co(NH3)5Cl]Cl2. A mole of CoCl3 ∙ 4NH3 reacts immediately with excess AgNO3 to produce only 1 mol of AgCl. Therefore, only one Cl- ion is outside the coordinates sphere, and the formula of the complex ion is [Co(NH3)4C12]. Since there are two Cl ions in the coordination sphere, the complex ion has a charge of 1+. The formula of the compound is [Co(NH3)4Cl2]Cl. An aqueous solution of CoCl3 ∙3NH3 does not conduct electric current, and it does not immediately produce with AgNO3. This means that the compound consists of neutral coordination spheres with the formula [Co(NH3)3Cl3]. Werner explained these observations by suggesting that transition-metal ions such as the Co3+ ion have a primary valence and a secondary valence. The primary valence is the number of negative ions needed to satisfy the charge on the metal ion. In each of the cobalt (III) complexes previously described, three Cl- ions are needed to satisfy the primary valence of the Co3+ ion. The secondary valence is the number of ions of molecules that are coordinated to the metal ion. Werner assumed that the secondary valence of the transition metal in these cobalt(III) complexes is six. The secondary valence is fixed in space. As it was said above, the formulas of these compounds can be written as follows. [Co(NH3)63+][Cl]3 orange-yellow 3+ [Co(NH3)5(H2O) ][Cl ]3 red [Co(NH3)5Cl2+ ] [Cl-]2 purple [Co(NH3)4Cl2+] [Cl-] green The cobalt ion is coordinated to a total of six ligands in each complex, which satisfies the secondary valence of this ion. Each complex also has a total of three chloride ions that satisfy the primary valence. Some of the Cl- ions are free to dissociate when the complex dissolves in water, Others are bound to the Co3+ ion and neither dissociate nor react with Ag+. In a coordination sphere, ligands are bonded to the central metal ion by coordinate covalent bonds. The number of coordinate covalent bonds that link the central atom or ion to its ligands in a coordination sphere is called the coordination number for the complex. This number equals the number of ligands in the coordination sphere if each ligand is attached to the central atom by only one coordinate covalent bond. Thus, the coordination number for Cu in [Cu(NH3)4 ]2+ is 4 and the coordination number for Pt in [Pt(NH3)5Cl3]+ is 6. The most commonly observed coordination numbers are 6, 4, and 2. §2. Classification of complexes. Coordination complexes can be cations such as [Cu(NH3)4]2+, anions such as [Fe(CN)6]3-, or neutral, molecular compounds like [Co(NH3)3Cl3]. Complex cations can combine with simple anions to form salts such as [Cu(NH3)4]Cl2. Simple cations can combine with complex anions to give salts like K3[Fe(CN)6]. Complex cations are also able to combine with complex anions. For example, Cu(NH3)42+ ions can corabine with Fe(CN)63- ion to form [Cu(NH3)4]3[Fe(CN)6]2 This coordination compound consists of positive and negative coordination spheres. In the cation, Cu(NH3)42+, the central Cu (II) ion is surrounded by four ammonia molecules, each attached to the Cu (II) ion by a coordinate covalent bond, Each of these bonds is formed when the nitrogen atom in an ammonia molecule donates its lone electron pair to the Cu (II) ion, The anion in [Cu(NH3)4]3[Fe(CN)6]2 consists of a central Fe (III) ion covalently bonded to six CN- ions. The charge on the complexion is the sum of the oxidation state of the central metal and the charges of all the ligands. Thus, the charge on the complex anion, Fe(CN)63-, is (+3) + 6(-I) = -3. We often refer to the central metal atom in a coordination complex as the central metal “ion” if the complex was formed from the metal ions and ligands. However, when the ion becomes covalently boned to ligands, it loses its character as an ion. For that reason we do not refer to the central metal “ion” in a complex as having a “charge”; instead, we refer to it as being in a certain oxidation state. 1) According to the charge of inner sphere: a) cationic (positive) [Ni(H2O)4]SO4 b) anionic (negative) K2[CuCl4] c) neutral [Pt(CO)4] 2) According to the nature of ligands: a) L = NH3 - aminocomplexes or ammiacates b) L = H2O - aquacomplexes c) L = acid radicals (Cl-, NO3-, CO3-2 and so on) - àcidocomplexes d) L = OH- - hydroxocomplexes e) L various particles, mixed complexes 3) According to the ionization ability: a) complex electrolytes. They are complex salts K2[Cu(CN)4] , complex acids H[AuCl4], complex bases [Zn(NH3)4](OH)2. b) complex non-electrolytes [PdCl4]. 4) According to the dentate number. The dentate number is a number of bonds that each ligand forms with the central atom (ion). All ligands have one or more-lone pairs of electrons that can be donated for bonding. a) monodentate complexes. Monodentate ligands are Lewis bases that donate a single pair (“mono”) of electrons to a metal atom. Monodentate ligands can be either ions (usually anions) or neutral molecules. §3. Naming metal complexes. Metal complexes are named by placing the name(s) of the ligand(s) (in alphabetical order if there are more than one and ignoring prefixes like di-, tri-, etc. in deciding the alphabetical order) before the name of the metal atom. If the overall complex has a negative charge (anionic complex) this is indicated by the suffix “-ate” When the suffix “-ate” is used, the Latin version of the metal name is used, so we have “cuprate” and “ferrate”. The oxidation number of the metal is given in brackets at the end. The name used for ligands ate given in table1. TABLE 1 Names of some ligands LIGAND ammonia carbon monooxidate water cyanide ion rodanide ion chloride ion hydroxide ion sulfate ion nitrate ion carbonate ion tiosulfate ion FORMULA NH3 CO H2O CNSCNClOHSO4-2 NO3CO3-2 S2O3-2 NAME ammine-/amine carbonylaquacyanorodanochlorohydroxosulfatonitrocarboxotiosulfato- For example: K2[ Fe(CN)4] potassium tetracyanoferrate(II) Na[Ag(CNS)2] sodium dirodanoargentate(I) [Pt(H2O)4]SO4 tetraaquaplatinum(II) su1fate [Ni(NH3)2(CO)2]Cl2 diaminedicarbonylnickel(II) chloride §4. Isomerism in complexes. Isomers: • are two or more molecules or ions that contain the same number and kind of atoms, but the atoms are arranged differently (i.e., the structures are non-superimposable). • have the same molecular formula • have different physical and/or chemical properties. • do not exist for all coordination compounds Structural isomers: contain the same number and kinds of atoms, but one or more bonds is/are different (i.e., the connectivity between atoms is different). They are coordination isomers. Stereoisomers: contain the same number and kinds of atoms, and the same number and kinds of bonds (i.e., the connectivity between atoms is the same), but the atoms are arranged differently in space. There are two types of stereoisomers: • geometric isomers, and • optical isomers. Note that coordination isomers and linkage isomers can exist only with coordination compounds. Geometric isomers and optical isomers occur not only with coordination compounds but also with many organic compounds. Coordination isomerism. Here ligands vary in their bonding. Coordination isomers are two or more coordination compounds in which the composition within the coordination sphere (i.e., the metal atom plus the ligands that are bonded to it) is different (i.e., the connectivity between atoms is different). Not all coordination compounds have coordination isomers. Coordination isomers have different physical and chemical propel-ties. For example, three compounds exist with the overall formula CrCl3 ∙ 6H20. They are: [Cr(H2O)6]Cl3 violet hexaaquachromium(III) chloride [Cr(H2O)5Cl]Cl2 ∙ H2O light green pentaaquachlorochromium(III) chloride [Cr(H2O)4Cl2]Cl ∙ 2H20 dark green tettaaquadichlorochromium(III) chloride As well as by their colors, these complexes can be distinguished by addition of silver nitrate. Only the free Cl- ions will react to form an AgCl precipitate, those bonded to the chromium (as ligands) will not. So l mole of compound 1 will react with 3 moles of AgNO3, 1 mole of compound 2 with 2 moles of silver nitrate and mole of compound 3 with l mole of AgNO3. [Cr(H2O)6]Cl3 + 3AgNO3 → 3AgCl↓ + [Cr(H2O)6](NO3)3 [Cr(H2O)5Cl]Cl2 + 2AgNO3 → 2AgCl↓+ [Cr(H2O)5Cl](NO3)2 [Cr(H2O)4Cl2]Cl + AgNO3 → AgCl ↓ + [Cr(H2O)4Cl2]NO3 Geometric (cys — trans) isomerism Here ligands differ in their position in space relative to one another. Geometric isomers are two or more coordination compounds which contain the same number and types of atoms, and bonds (i.e., the connectivity between atoms is the same), but which have different spatial arrangements of the atoms. Not all coordination compounds have geometric isomers For example, in the square planar molecule, Pt(NH3)2Cl2, the two ammonia ligands (or the two chloride ligands) can be adjacent to one another or opposite one another For example: copper(ll) tetraamine. b) bidentate and polydentate complexes. Bidentate ligands are Lewis bases that donate two pairs (“bi”) of electrons to a metal atom. Bidentate ligands are oflen referred to as chelating ligands (“chelat” is derived from the Greek word for “claw) because they can “grab” a metal atom in two places. A complex that contains a chelating ligand is called a chelate. For example: copper (II) ethane-l,2-diamine (it is abbreviated to [Cu(en)2]). Polydentate or chelate complexes are hemoglobin, chlorophyll (One of the green pigments in plants) and Vit B12 (Vitamin B12 is one of the essential vitamins required for living organisms). Cl NH3 | | H3N – Pt – Cl Cl – Pt – Cl | | NH3 NH3 Note that these two structures contain the same number and kinds of atoms and bonds but are non-superimposable. The isomer in which like ligands are adjacent to one another is called the cis isomer. The isomer in which like ligands are opposite one another is called the trans isomer. Optical isomerism. Here two isomers are non-identical mirror images of one another. This only occurs when a metal ion is coordinated with two or more bidentate ligands. Optical isomers are two compounds which contain the same number and kinds of atoms, and bonds (i.e., the connectivity between atoms is the same), and different spatial arrangements of the atoms, but which have nonsuperimposable mirror images. Each non-superimposable mirror image structure is called an enantiomer. Molecules or ions that exist as optical isomers are called chiral. Not all coordination compounds have optical isomers. Optical isomers are said to be chiral (pronounced kiy —ral), which means “handed”. A solution of one isomer will rotate the plane of polarization of polarized light clockwise. A solution of equal concentration of the other isomer will rotate the plane of polarization of polarized light by the same amount anticlockwise. §5. Bonding in complex ions. In metal complexes, dative covalent bonds are formed by lone pairs on the ligands donating electrons into the partly empty orbitals of the metal ion. [Cu(NH3)4]+2 Coordinate number is 4 (There are 4 bonds between metal and ligands). In this case Cu+2 is acceptor (it has empty orbitals) and 4NH3 are donors (they have electron pairs). Cu0 4s1 3d10 — the structure of the atom. Cu+2 4s0 3d9 — the structure of the km. Cu+2 4 necessary empty orbitals sp -hybridisation, spatial structure — tetrahedral. The angle is 109°28’. [Ag(CN)2]-1 Coordination number is 2 (2bonds). Ag° 5s1 4d10 Ag+15s04d10 3 2 necessary empty orbitals sp-hybridisation, the geometry of the ion is linear. The angle is 1800. [Cr(H2O)6]+3 Coordination number is 6 (6 bonds). Cr° 3d54s1 Cr+3 3d3 4s0 6 necessary empty orbitals d2sp3-hybridisation, the geometry of the ion is octahedral. §6. Stability and non-stability constants. The strong covalent bonds are between central atom and ligands. The weak ionic bond is between inner and external spheres, which is ionized by water. [Cu(NH3)4]Cl2 ↔ [Cu(NH3)]4+2 + 2Cl-1 The characteristic of complexes stability is dissociation constant of dissociation reaction of the inner sphere. [Cu(NH3)4]+2 ↔ Cu+2 + 4NH3 The dissociation constant can be written: Its constant is called non-stability constant. The reverse meaning is called stability constant. The stability constant for a complex is a measure of the stability of the complex. The larger the Kstab the more stable is the complex. Some stability constants for complexes of copper(ll) are given in Table 2. Table 2 Stability constants of copper(II) complexes. LIGAND FORMULA OF COMPLEX STABILITY CONSTANT Kstab -2 Cl-1 [CuCl4] 4 ∙105 NH3 [Cu(NH3)4(H2O)2]+2 1 ∙1013 Edta [Cu(Edta)]-2 6 ∙1018 The table shows that the [Cu(NH3)4(H2O)2]+2 complex is about 20 000 000 times more stable than the [CuCl4]-2 complex. §7. Application. The dark-blue, square planar [Cu(NH3)4]2+ complex ion is present in some brands of waterbed conditioners. It is responsible for inhibiting the growth of fungi and bacteria. The square planar [RhI2(CO)2]- complex ion is used as a catalyst in the “Monsanto Process” for making acetic acid, the active ingredient in vinegar. Some cleansers, which contain oxalic acid, are used to remove rust deposits. Rust reacts with oxalic acid to produce a colorless, water-soluble complex ion (i.e., [Fe(C2O4)3]3-) which contains the bidentate ligand, oxalate ion. Because the complex ion is water-soluble it can be washed away. The sodium salt of EDTA4- (i.e., Na4EDTA) can be found in many commercial products including: ● soap ● beer ● mayonnaise This hexadentate ligand forms very stable complexes (usually octahedral structures) with most of the transition metals. The donor atoms in EDTA4- are the two N atoms, and the four, negatively charged 0 atoms. EDTA4- is used to “trap” trace amounts of transition metals that could potentially catalyze the decomposition of the product. §8. The use of complex compounds in medicine. As known, the environment pollution by heavy metals (Hg, Pb, Cr, Cd, Ni) causes poisoning. The toxic properties of these metals are explained by an interaction of heavy metal ions with bioorganic complexes. MbioL + Mheavy ↔ Mbio + MheavyL Here M, is the complex of biogenic metal ion (Mbio is Fe, Co, Zn, Cu) with bioorganic ligands (porphyrin) and Mheavy is a heavy metal ion. If the stability of MheavyL is more than MbioL, the displacement of equilibrium will take place to the right, In this case, MheavyL is concentrated in the organism. As a result, normal vital activity is disturbed and toxicosis takes place. Antidotes and complexions are used for the treatment of toxicosis. Antidotes are sulfurcontaining compounds, which can form insoluble untoxic complexes with heavy metals. Complexons are organic compounds, which form complexes with many metals. For example, Edta, ethylenediaminetetraacetate, complexon II, trilon A. This can act as a hexadentate ligand using lone pairs of the four –COO- ions and both the nitrogen to form complexonates. Na2 Edta, complexon III, trilon B. It is known in medicine as Calsol, Titriplex, and Questrex. They are used for the treatment of Hg+2, Cd+2 poisoning. Complexonates are weak toxic and have the great values of Ksp and are not destroyed in the organism. They are gone out with urine. §9. Problems. 1. Name the complex compounds given below. Indicate the central atom and its charge, ligands. the internal coordination sphere and the external sphere. Write the dissociation equilibrium of these compounds in aqueous solutions. a) K3[Cu(CN)4] b) Na2[Pb(OH)4] c) [Co(NH3)6]Cl2 d) [Al(H2O)6]2(SO4)3 e) Na3[AlF6] f) (NH4)3[Co(CN)6] g) [Ni(NH3)6](NO3)2 h) K3[BiI6] i) [Hg(NH3)6]Br2 j) Na2[Zn(CN)4] k) [Ni(H2O)6](NO3)2 l) Na2[Sn(OH)4] m) [Cr(H2O)5OH]Cl2 n) [Au(NH3)4]Cl3 o) K3[FeF6] p) (NH4)2[Pb(CH3COO)4] q) Na2[Fe(SCN)3H20] r) Na2[Cd(CN)3Cl] s) Na3[FeF4Cl2] t) K[Ag(CN)2] u) K2[Co(SCN)4] v) K6[Pb(S2O3)4] w) K4[Mn(C2O4)3] x) K2[MoI4] y) K2[Pt(OH)5Cl] z) Cs[Fe(SCN)4(H2O)2] 2. Write the formulas ofihe following complexes: a) barium dithiosulfatocuprate(II); b) triaminetrinitrocobalt(III); c) pentaamminethiocyanatocobalt(III) nitrate; d) potassium hexacyanoferrate(III); e) potassium bromopentanitroplatinate(IV); f) aquatetraaminedichlorocobalt(III) chloride; g) sodium hexacyanochromate(III); h) tetraaminephosphatochromium(III); i) diamminetetrachloroplatinum(IV); j) potassium octacyanomolybdate(IV); k) sodium tetrahydroxoplumbate(II); l) ammonium tetrafluoroargentate(I); m) potassium tetracyanoaurate(III) §10. Practical work “Preparation and properties of complexes” Experiment 1. Formation of silver thiosulphate complex For this purpose add drop by drop dilute silver nitrate solution to the tube with sodium thiosulfate solution until silver thiosulphate precipitate disappears. 2AgNO3 + Na2S2O3 = Ag2S2O3↓ + 2NaNO3 Ag2S2O3↓ + Na2S2O3 = Na3[Ag(S2O3)2] Name this complex. Experiment 2. Formation of copper coordination compound Add ammonium hydroxide solution to the tube with copper sulfate solution until the precipitate disappears. Note the color of complex and name it. CuSO4 + 4NH4OH = [Cu(NH3)4]SO4 + 4H2O Experiment 3. Interaction of hexacyanoferrate (II) with copper sulphate Place hexacyanoferrate(II) solution to the tube with copper sulfate solution. Note the color of the precipitate formed. K4Fe(CN)6] + 2CuSO4 = Cu2[Fe(CN)6]↓ + 2K2SO4 Experiment 4. Interaction of potassium hexacyanoferrate (II) with Fe (III) salt. Add potassium hexacyanoferrate(II) solution to the tube with iron(III) chloride solution. Observe the precipitate formation. 3K4[Fe(CN)6] + 4FeCl3 = Fe4[Fe(CN)6]3↓ + 12KCl . Then add excess of alkali solution and stir the content. Observe the grayish- brown precipitate and yellow alkaline metal hexacyanoferrate(II) solution. Fe4[Fe(CN)6]3 + I2MeOH = 3Me4[Fe(CN)6] + 4Fe(OH)3↓ §11. TEST 1. Which of these metals is not a transition element? A. Ti B. Cr C. Ca D. Fe 2. What is the ground state electron configuration of Cr2+? A. [Ar]4s13d3 B. [Ar]4s23d2 C. [Ar]3d4 D. [Ar]3d3 3. What is the coordination number of the central atom in the coordination compound [Pt(NH3)4]SO4? A. 6 B. 2 C. 3 D. 4 4. Which of the statements are true of ligands? A.1igands are Lewis bases. B. ligands are always anions. C. ligands only bond to a metal through a single atom. D. ligands accept electron pairs from the metal when forming bonds. 5. The formula of potassium pentacyanocarbonylcobaltate (III) is: A. K3[Co(CN)5CO] B. K2[Co(CN)5CO] C. K2[Co(CN)5CO] D. K4[Co(CN)3CO3] 6. What is the oxidation state of cobalt in the compound [Co(NH3)4Cl2]Cl? A.+1 B. +2 C. +3 D. +6 7. Which of the following ligands might form chelates with a metal ion? A. Cl-, chloride B. NH3, ammonia C. SCN-, thiocyanate D. H2NCH2CH2NH2, ethylene diamine 8. The paramagnetism in complex ions of transition metals such as CoF63- or NiCl42 is a result of: A. the charge on the ions. B. electron pair donation by the ligands. C. unpaired d-orbital electrons on the metal ions. D. the square planar geometry possible with these metals. 9. Magnetic measurements on an octahedral complex of a transition metal show that it contains three unpaired electrons. Which of the following d-electron configurations of the metal ion are consistent with this fact? Select the most complete answer. A. d3 B. d3 and d7 C. d3, d5, and d7 D. d3, d5, d7, and d9 10. Chlorophyll is a metal porphyrin complex of Mg2+. Judging by its color, approximately what wavelength of light corresponds to the maximum absorption for the complex? A. 200 nm B. 400 nm C. 550 nm D. 700 nm 11. a. Draw and label the cis- and trans- isomers of Pt(NH3)2Cl2. (10) b. Give an example of two compounds that exhibit linkage isomerism. (5) c. A coordination compound of titanium(IV) contains four ammonia molecules, one sulfate ion and two chloride ions. When aqueous barium chloride is added to a solution of the titanium complex, a white precipitate forms. When aqueous silver nitrate is added to a solution of the titanium complex, no precipitate forms. Propose a structure for the compound. (5)