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Transition Metals •! •! Occupy the d-block of periodic table Have d-electrons in valence shell Characteristics: 1.! More than one oxidation state 2.! Many compounds are colored 3.! Interesting magnetic properties 4.! Form Metal complexes or Coordination compounds 5.! Transition metals play important roles in biological systems and modern technology. ! 1! CHEM112 LRSVDS Transition Metals part 1 Electron Configurations and Oxidation States Many transition metals form compounds that have fun colors! – colors are due to oxidation state and electron configuration...more specifics about that later! ! 2 Periodic trends There are periodic trends in the transition metals, but they are often complex (product of several factors, some working in opposite directions – e.g. combining the effects of increasing nuclear charge with the presence of nonbonding d electrons) Lanthanide contraction – similarity in size, behavior, properties 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 ! 3 Electron Configuration: Order of Orbital Filling CHEM112 LRSVDS Transition Metals part 1! ! 4 VALENCE ELECTRON CONFIGURATIONS Orbital Filling in First Row: Sc ! Zn [Ar] = 3s23p6 K [Ar] 4s1 Ca [Ar] 4s2 Sc [Ar] 3d14s2 Ti [Ar] 3d24s2 . . . . . . Zn [Ar] 3d104s2 *Note: 4s is filled before 3d, but when metal is oxidized, 4s electrons are lost before 3d. Ti Ti2+ Ti3+ Ti4+ Ti5+ [Ar]3d24s2 [Ar]3d24s0 [Ar]3d14s0 [Ar]3d04s0 does not exist! CHEM112 LRSVDS Transition Metals part 1! ! 5 Transition Metal Oxidation States Group 3B – 7B: For Sc, Ti, V, Cr, Mn: highest oxidation states common Highest oxidation states = number of valence (4s + 3d) electrons. Sc [Ar]3d14s2 Mn [Ar]3d54s2 Sc3+ [Ar] (maximum oxidation state is +3) Mn7+ [Ar] (maximum oxidation state is +7) Group 8B, 1B, 2B: The maximum oxidation state becomes increasingly unstable and uncommon. Sc3+ Mn7+ Fe8+ Sc2O3 is a stable oxide Exists but is easily reduced (MnO4! strong oxidizing agent) Does not exist; unstable CHEM112 LRSVDS Transition Metals part 1! ! 6 Metal Complexes or Coordination Compounds Transition metal ions are Lewis acids " they accept electron pairs Ligands are Lewis bases " molecules or ions which donate electron pairs CHEM112 LRSVDS Transition Metals part 1! ! 7 Ligands: Donate Lone Pairs of Electrons F!, Cl!, Anionic Ligands CN!, SCN!, NO2!, EDTA4! Br!, Neutral Ligands NH3, H2O, CO, CH3OH, en Mono-dentate Ligands: Only ONE donor atom is bound to the metal (single tooth to hold onto metal d orbital) Examples; NH3, H-O-H , CH3-O-H Bi-dentate Ligands: TWO donor atoms are bound to the metal (has 2 teeth to hold onto metal d orbitals). Example; H2N-CH2-CH2-NH2 (en or ethylenediamine) Poly-dentate Ligands Have more than one functional group with lone pairs (many teeth!) Example; EDTA (ethylenediaminetetraacetic acid) CHEM112 LRSVDS Transition Metals part 1! ! 8 Coordination Compounds [Cu(H2O)2(NH3)2]2+ [Cu(H2O)2(en)2]2+ en =! Coordination number: number of donor atoms attached to the metal. Chelates: “chele or Claw” ligands possessing two or more donor atoms.! CHEM112 LRSVDS Transition Metals part 1! ! 9 TRANSITION METAL COMPLEXES [Cu(NH3)4)]SO4 SO42- + [Cu(NH3)4)]2+ Charge on the complex: Coordination #: Oxidation state of the metal: K2[Ni(CN)4)] 2 K+ + [Ni(CN)4]2- Charge on the complex: Coordination #: Oxidation state of the metal: CHEM112 LRSVDS Transition Metals part 1! ! 10 Werner’s Theory •! This approach correctly predicts there would be two forms of CoCl3 · 4 NH3. –! The formula would be written [Co (NH3)4Cl2]Cl. –! One of the two forms has the two chlorines next to each other. –! The other has the chlorines opposite each other. ! 11 Geometry of Transition Metal Complexes: Coordination Number of 4 Tetrahedral e.g. [Zn(NH3)4]2+ Square Planar e.g. [Ni(CN)4]2- e.g. [PtCl2(NH3)2] Cl Pt Cl CHEM112 LRSVDS Transition Metals part 1! NH3 NH3 ! 12 Geometries of Transition Metal Complexes Geometry for Coordination # = 5 •! Trigonal Bipyramidal [Re(SCH2C6H4OCH3-p)3(PPh3)2] ReL3(PR3)2! [Fe(CO)5] ! 13 Six Coordinate Complexes: Octahedral (six vertices, eight FACES) e.g. [CoF6]3F F F Co F F F e.g. [Co(en)3]3+! N N N Co N N N CHEM112 LRSVDS Transition Metals part 1! ! 14 Poly-dentate Ligands: Form Metal CHELATES! ethylenediamine (“en”)! = ethylenediaminetetraacetate ion! ! CHEM112 LRSVDS Transition Metals part 1! 15 IMPORTANT CHELATING LIGANDS O HOCCH2 O NCH2CH2N : HOCCH2 : EDTA CH2COH CH2COH O O CHEM112 LRSVDS Transition Metals part 1! ! 16 Uses of Chelating Agents •! Used to “sequester” or “seize” metal ions •! Used in detergents to remove trace amounts of dissolved metals: Na5P3O10 •! Complex trace metal ions that catalyze food decomposition: EDTA •! Used in poison control: EDTA •! Used in shampoo and cleaning products to remove trace metals from hard water (Ca2+ and Mg2+): EDTA ! CHEM112 LRSVDS Transition Metals part 1! 17 IMPORTANT CHELATING LIGANDS Porphine forms metal complexes called Phorphyrins + Fe + protein ! ! !"#$%#&'() Fe! CHEM112 LRSVDS Transition Metals part 1! ! 18 Chemistry and Life! !"#$%$&"'##()! http://fr.academic.ru/dic.nsf/frwiki/29449! *+%%,-"%$.+! http://en.wikipedia.org/wiki/File:Ferrichrome.png! ! CHEM112 LRSVDS Transition Metals part 1! 19 Important Chelating Agents Chelate # of Coordination Sites Charge Ethylenediamine Porphine EDTA4Oxalate (C2O42-) Carbonate (CO32-) CHEM112 LRSVDS Transition Metals part 1! ! 20 Metal Complexes and Isomers ! 21 Structural Isomers •! Coordination Sphere Isomers –! 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 –! differ in what ligands are bonded to the metal and what is outside the coordination sphere Three isomers of CrCl3(H2O)6 are: violet [Cr(H2O) 6]Cl3 green [Cr(H2O) 5Cl]Cl2·H2O green [Cr(H2O) 4Cl2]Cl·H2O ! 22 Stereoisomers •! Geometric Isomers •! Optical Isomers ! 23 METAL COMPLEX STABILITY Cu(OH2)42+ + 4NH3 [Cu(NH3)4]2+ + 4H2O(l) Cu2+(aq) + 4NH3 [Cu(NH3)4]2+ + 4H2O(l) Kf = ! [H2O] = constant Formation Constant: Kf is very large! [Ag(NH3)2]+ [Cu(NH3)4]2+ [Ag(S2O3)2]3! [Ag(CN)2] ! [Cu(CN)4]2! 1.7 x 107 5.0 x 1012 2.9 x 1013 1.0 x 1021 1.0 x 1025 CHEM112 LRSVDS Transition Metals part 1! ! 24 THE CHELATE EFFECT Chelating ligands form exceptionally stable metal complexes. [Ni(H2O)6]2+ + 6NH3 [Ni(NH3)6]2+ + 6H2O Kf = 4x108 [Ni(H2O)6]2+ + 3en [Ni(en)3]2+ + 6H2O Kf = 2x1018 *DUE TO: 1) PROBABILITY 2) ENTROPY EFFECTS! Probability Effect:! Cd+2 H2 N NH2CH3 Cd+2 NH2 NH2CH3 ! CHEM112 LRSVDS Transition Metals part 1! 25 Entropy and the Chelate Effect 1) Cd2+ + 4CH3NH2 2) Cd2+ + 2en [Cd(CH3NH2)4]2+ [Cd(en)2]2+! Ligand #H°(kJ) #S°(J/K) #G° 1 methyl amine -37.2kJ 2 en -60.7kJ Why is #S° so much larger? [Cd(H2O)4]2+ + 4CH3NH2 [Cd(H2O)4]2+ + 2en [Cd(CH3NH2)4]2+ + 4H2O [Cd(en)2]2+ + 4H2O CHEM112 LRSVDS Transition Metals part 1! ! 26 Magnetic Properties of Transition Metals Diamagnetic: unaffected by a magnetic field Paramagnetic: influenced by a magnetic field The magnetic properties depend on the number of unpaired electrons Na+ Mn2+ Ti2+ Co3+ CHEM112 LRSVDS Transition Metals part 1! ! 27 Magnetic Behavior # unpaired e" ! prior alignment alignment in magnetic field !attracted to magnetic field ! Diamagnetic! Paramagnetic! Ferromagnetic! CHEM112 LRSVDS Transition Metals part 1! ! 28 CRYSTAL FIELD THEORY The presence of metal-ligand bonding electrons raises the energy of metal d orbitals due to electrostatic repulsion. E d orbitals in uniform, “spherical” field of negative charge; all orbitals raised in energy equally d orbitals in free metal ion (all degenerate)! CHEM112 LRSVDS Transition Metals part 1! ! 29 Octahedral Metal Complexes; Effect of metalligand bonding electrons on metal d orbitals Which metal d orbitals are most affected by the metalligand bonding electrons?! Metal ion in Octahedral charge Field CHEM112 LRSVDS Transition Metals part 1! ! 30 CRYSTAL FIELD SPLITTING Crystal Field Energy Splitting of d orbitals in octahedral ligand field! e set (dz2, dx2–y2) E #$ “delta octahedral” t2 set (dxy, dxz, dyz)! d orbitals in uniform, “spherical” field of negative charge! d orbitals in octahedral field of negative charge! #!o= Crystal field splitting energy !o depends on: 1. 2. 3. CHEM112 LRSVDS Transition Metals part 1! ! 31 Spin Pairing Energy P = spin pairing energy The energy required to place two electrons of opposite spin in the same orbital *magnitude of P is independent of the ligands If # is large (P < #) " Low Spin Complex If # is small (P > #) " High Spin Complex For a transition metal with 5 d electrons;! First 3 electrons fill lower E orbitals! CHEM112 LRSVDS Transition Metals part 1! ! 32 Cl! < SPECTROCHEMICAL SERIES: Ability of L to increase the energy gap F! < H2O < NH3 < en < NO2! < CN! Increasing # % CN! CO NO2! en NH3 H 2O Oxalate OHF! SCN! Cl! BrI! Strong field! ligands! Weak field ! ligands! absorbs! observed! CHEM112 LRSVDS Transition Metals part 1! ! 33 MAGNETIC PROPERTIES Which diagram corresponds to CoF63- and which corresponds to Co(CN)63- and why? ! E High spin Paramagnetic! Low spin (spin-paired) Diamagnetic CHEM112 LRSVDS Transition Metals part 1! ! 34 Practice Problems 1. Ammonia is a strong field ligand. Is [Mn(NH3)6]3+ high spin or low spin? 2. The oxalate complex [Co(C2O4)3]4& has 3 unpaired electrons. Is it high spin or low spin? CHEM112 LRSVDS Transition Metals part 1! ! 35 OPTICAL PROPERTIES; COLOR OF COMPLEXES Observed color is related to the amount of energy required to promote an electron. Compare # to energy absorbed. CHEM112 LRSVDS Transition Metals part 1! ! 36 OPTICAL PROPERTIES OF TRANSITION METALS Color depends on identity of the ligands [Ni(H2O)6]2+ + 6 NH3 ! [Ni(NH3)6]2+ + 6 H2O CHEM112 LRSVDS Transition Metals part 1! ! 37 COLOR ABSORPTION BY METAL COMPLEXES When light of a certain wavelength is absorbed by a complex, the complex will appear the complementary color of the wavelength absorbed Observed Color: 1) OR 2) CHEM112 LRSVDS Transition Metals part 1! ! 38 Visible Absorption Spectra:! What color is Ti(H2O)63+?! CHEM112 LRSVDS Transition Metals part 1! ! 39 Color of Metal Complexes SCN"! [Fe(H2O)6]3+! [Fe(SCN)(H2O)5]2+! 1. Which of these complexes absorbs light at a shorter wavelength? 2. Which complex has the larger #o? 3.! Which is a weaker field ligand; water or thiocyanate? 4. What color will TiO2 be? What color will ZnO be? CHEM112 LRSVDS Transition Metals part 1! ! 40 Reduction Potential of Metal Complexes Stability of Transition Metal Complexes depends on reduction potential of the metal complex. 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 CHEM112 LRSVDS Transition Metals part 1! ! 41 USES OF TRANSITION METALS Titanium Vanadium Chromium Manganese CHEM112 LRSVDS Transition Metals part 1! ! 42