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Transition Metals Transition Metals Chapter 22 Transition Metals 1 Transition Metals: Occupy the d-block of periodic table Have d-electrons in valence shell Some characteristics of Transition Metals and their compounds: 1. 2. 3. 4. 5 5. Exhibit more than one oxidation state They form an extensive series of compounds known as metal complexes l or coordination compounds. di ti d Transition metals exhibit interesting magnetic properties. Many of their compounds are colored. Transition metals play important roles in biological systems and Transition metals play important roles in biological systems and modern technology. Transition Metals 2 There are periodic trends in the transition metals, but they are often complex (product of several factors, some working in opposite directions – e.g. e g combining the effects of increasing nuclear charge with the presence of nonbonding d electrons) Lanthanide contraction – similarity in size, behavior, & properties p p of 4d and 5d transition elements We won’t worry about details of periodic trends in the transition metals or the exact reasons for them Transition Metals 3 Recall the Electron Configurations of the transition metals. Ar 3s23p6 K [Ar]4s1 Ca [Ar]4s2 Sc [Ar]3d14s2 Ti [Ar]3d24s2 Use th U the periodic i di table t bl to t gett electron l t configurations fi ti of the First Row Transition metal ATOMS Sc → Zn V Cr Mn Fe Co Ni Cu Zn [Ar] 3d104s2 Transition Metals 4 Be able to figure out the electron configuration of transition metal ions. Transition Metals can have more than one oxidation state. Ti [Ar]3d24s2 Ni [Ar]3d84s2 Ti2+ Ti3+ Ni2+ Ti4+ Ti5+ Ni4+ Note: 4s is filled before 3d, b t when but h oxidized, idi d 4s electrons are lost before 3d. Transition Metals 5 Some trends in Transition Metal Oxidation States can be identified. Oxidation States: Highest oxidation states of Sc, Ti, V, Cr, Mn = number of valence (4 + 3d) electrons. (4s l t Sc [Ar]3d14s2 Mn [[Ar]3d ] 54s2 Trend from Sc → Mn: The maximum oxidation state becomes increasingly unstable. Sc3+, Ti4+ Are stable. Sc2O3 & TiO4 are stable oxides. 7+ Mn E ists b Exists butt is easil easily red reduced. ced − MnO4 Strong oxidizing agent. Transition Metals 6 Many transition metals form compounds that have fun colors! – colors are due to oxidation state and electron g p configuration...more specifics about that later! Transition Metals 7 Transition metals form COMPLEXES. Transition metal ions are Lewis acids → Ligands are Lewis bases → Transition Metals 8 Metal complex is formed when a metal is bonded to molecules or ions. Complex ion: metal complex that is not neutral Coordination compounds: compounds that contain complexes Coordination number: # of atoms directly bonded to a metal Transition Metals 9 The coordination of the ligand with the metal can greatly alter its physical or chemical properties EXAMPLE: color Transition Metals 10 Chemical properties of Metal Complexes differ from the properties of the metal alone the properties of the metal alone. For example: the ease of oxidation of the metal changes when a metal complex forms metal complex forms. Which is easier to reduce, the metal ion or the complex? Ag+(aq) + e− → Ag(s) E°1/2= +0.80V [Ag(CN)2]−(aq) + e− → Ag(s)+ 2CN−(aq) E°1/2 = −0.31V Transition Metals 11 Complexes are characterized by their Oxidation State and Coordination Number, but the identity of the ligands also matters. • Oxidation State: “primary valence” • Coordination Number: “secondary valence” Early Formula Color # of Ions per Free Cl− Ions formula Unit in Formula Complex ion Formula CoCl3 6NH3 orange 4 3 [Co(NH3)6]Cl3 CoCl3 5NH3 purple 3 2 [Co(NH3)5Cl]Cl2 CoCl3 4NH3 green 2 1 trans‐[Co(NH3)4Cl2]Cl CoCl3 4NH3 violet 2 1 cis‐[Co(NH3)4Cl2]Cl The four compounds in the table below have the same oxidation state and coordination number, but clearly they have different properties. Transition Metals 12 Coordination Sphere is the central atom + ligands bonded to it Use [ ] to set off components in the coordination sphere [] p p Early Formula Color # of Ions per Free Cl− Ions formula Unit in Formula in Formula Complex ion Formula CoCl3 6NH3 orange 4 3 [Co(NH3)6]Cl3 CoCl3 5NH3 purple 3 2 [Co(NH3)5Cl]Cl2 CoCll3 4NH3 green 2 1 trans‐[Co(NH [ ( 3)4Cll2]Cl ] l CoCl3 4NH3 violet 2 1 cis‐[Co(NH3)4Cl2]Cl What happens to these complexes when dissolved in water? Transition Metals 13 Knowing the charge on a complex ion and the charge on each ligand, one can determine the oxidation number for the metal one can determine the oxidation number for the metal. [[Cu(NH ( 3)4]]SO4 K2[[Ni(CN) ( )4]] Charge on the complex: Charge on the complex: Coordination #: Coordination #: Coordination #: Oxidation state of the metal: Oxidation state of the metal: Oxidation state of the metal: Transition Metals 14 Or, knowing the oxidation number on the metal and the charges on the ligands, one can calculate the charge on the complex ion. Fe3+ surrounded by six water molecules Pt(II) surrounded by two ammonia molecules and two bromide ions molecules and two bromide ions Charge on the complex: Charge on the complex: Coordination number: Coordination number: Transition Metals 15 Geometries of Transition Metal Complexes Geometry for Coordination # = 4 • Tetrahedral • Square Planar Square Planar [Ni(CN)4]22 [Zn(NH3)4]2+ [PtCl2(NH3)2] Transition Metals 16 Geometries of Transition Metal Complexes Geometryy for Coordination # = 5 • Trigonal Bipyramidal [Fe(CO)5] [Re(SCH2C6H4OCH3-p)3(PPh3)2] ReL3(PR3)2 Transition Metals 17 Geometries of Transition Metal Complexes Geometryy for Coordination # = 6 • Octahedral chromium(III) and cobalt(III) consistently have h i (III) d b lt(III) i t tl h the same coordination number of 6 3 [CoF6]3− [Co(en)3]3+ Transition Metals 18 Special Ligands: Chelating Agents Chelates are ligands possessing two or more donor atoms. atoms • Mono‐dentate Ligands Examples: [Cu(H2O)2(NH3)2]2+ • Bi‐dentate Ligands Example: ethylenediamine = en :NH2−CH2−CH2− :NH2 [Cu(H2O)2(en)]2+ Transition Metals 19 These are examples of Bi-dentate Ligands Transition Metals 20 Poly‐dentate ligands bind to the metal in more than 2 sites. Example: ethylenediaminetetraacetic acid (EDTA) Transition Metals 21 Below are representations of metal complexes with ethylenediamine and EDTA as the ligands. Transition Metals 22 EDTA is used to sequester metal ions. ethylenediaminetetraacetic acid (EDTA) Applications: Applications: Removes trace metals ions that catalyze food decomposition Used in poison control Used by lichen to obtain y minerals from rock. Transition Metals 23 Chelating Agents can be used to soften water Used in shampoo to remove Used in shampoo to remove trace metals from hard water (Ca2+ and Mg2+): EDTA Na5P3O10 Used in detergents to remove trace amounts of dissolved metals: Transition Metals 24 Chelating Agents play important roles in biological applications. applications Porphine is a flat molecule Complexes with this molecule Complexes with this molecule are called porphyrins CH2 CH3 CH CH2CH3 H3C N N Mg N N CH3 H3C C20H39OOCH2CH2C H3COOC O Transition Metals 25 These are the Important Chelating Agents you need to know. Chelate # of Coordination Sites Charge Ethylenediamine (en) P Porphine hi EDTA Oxalate (C2O42−) Carbonate (CO322−) Transition Metals 26 METAL COMPLEXES are very stable. The formation reactions have large values of K g f. Cu(OH2)42+ + 4NH3 → Cu(NH3)42+ + 4H2O Cu2+(aq) + 4NH3 → Cu(NH3)42+ + 4H2O Kf VALUES OF SOME COMPLEXES [Ag(NH3)2] + [Cu(NH3)4] 2+ [Cu(CN)4 ] 2− [Ag(CN)2 ] − [Ag(S2O3)2 ] 3− 1.7 × 107 5 × 1012 1 × 1025 1 × 1021 2.9 × 1013 Transition Metals 27 THE CHELATE EFFECT: Chelating ligands form exceptionally stable metal complexes when compared to related monodentate ligands. [Ni(H2O)6]2+ + 6 NH3 [Ni(H2O)6]2+ + 3 en [Ni(NH3)6]2+ + 3 en [Ni(NH3)6]2+ + 6H2O [Ni(en)3]2+ + 6H2O Kf = 4×108 Kf = 2×1018 [Ni(en)3]2+ + 6 NH3 What is the expression for the formation constant for this reaction? Transition Metals 28 THE CHELATE EFFECT: Chelating ligands form exceptionally THE CHELATE EFFECT stable metal complexes when compared to related monodentate d t t ligands. li d This “chelate effect” is due to : 1) Probability 2) Entropy Effects Probability Effect: Cd+2 H2 N NH2CH3 Cd+2 NH2 NH2CH3 Transition Metals 29 ΔS for the formation of a chelate is positive. 1) Cd 1) Cd2+ + 4CH 4CH3NH2 [Cd(CH3NH2)4]2+ 2) Cd2+ + 2en 1 ΔH° = −57.3 kJ 57 3 kJ [Cd(en)2]2+ + 4H2O ΔH° = −56.5 kJ Ligand ΔH°(kJ) Methyl amine y CH3NH2 −57.3 −37.2kJ −56.5 −60.7kJ 2 ΔS°(J/K) ΔG° Why is ΔS° so much larger? [Cd(H2O)4]2+ + 4 CH3NH2 [Cd(CH3NH2)4]2+ + 4H2O [Cd(H2O)4]2+ + 2 en Transition Metals [Cd(en)2]2+ + 4H2O 30