Transition Metals: Properties, Electron Configurations, Oxidation, and Metal Complexes, Study notes of Chemistry

Download Transition Metals: Properties, Electron Configurations, Oxidation, and Metal Complexes and more Study notes Chemistry in PDF only on Docsity! Chemistry 112-002 Transition Metals-22 Chemistry of the transition metals 6:8-9, 23:7, 24:1-3,5-6, p. 745-750 Dr. Raymond Schaak Penn State University SPRING 2009 • Transition metals • Periodic trends • Electron configurations • Oxidation states • Magnetic properties • Metal complexes • Ligands • Examples of metal complexes • Coordination compounds • Chelating ligands • Chelate effect • Applications • Color and magnetism in transition metals • Crystal field theory • Example: TM3+ ions • Crystal field splitting energy • Spectrochemical series • Example: [CoX6]3– • Colors of metal complexes • Examples OUTLINE Transition metals Occupy the d-block of the periodic table Have d-electrons in valence shell Some general characteristics of transition metals and their compounds Exhibit more than one oxidation state Many of their compounds are colored They exhibit interesting magnetic properties They form an extensive series of compounds known as metal complexes or coordination compounds Solids containing transition metals play some of the most important roles in modern technology 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 Metal complexes Fe3+ NH4SCN [Fe(H2O)5NCS]2+ Metal complexes Transition metal ions accept electron pairs Ligands molecules/ions that donate electron pairs Metal complexes (or coordination compounds) are ligands bonded to metal ions Number of electron donor atoms attached to a metal: Ligands possessing two or more donor atoms: Ligands Ligand molecules typically have at least one lone pair of electrons Anions F–, Cl–, Br–, CN–, SCN–, NO2 – Neutral ligands NH3, H2O, CO Ligands Mono-dentate (single “tooth” to hold onto metal d-orbital) NH3, H2O, CH3OH Bi-dentate (two “teeth” to hold onto metal d-orbitals) Ligand has two or more functional groups that have lone pairs Example: ethylenediamine (en) Ligands Poly-dentate (chelating) (multiple “teeth” to hold onto metal d-orbitals) Example: ethylenediaminetetraacetic acid (EDTA) Ligands Coordination compounds Coordination number = 6 e.g. [Co(en)3]3+ Co F F F F F F Co N N N N N N Octahedral, e.g. [CoF6]3- Chelating ligands Polydentate ligands (grasp the metal between two or more donor atoms) Chelating ligands form more stable complexes than related monodentate ligands [Ni(H2O)6] 2+ + 6 NH3 [Ni(NH3)6] 2+ + 6 H2O Kf = 1.2 × 10 9 [Ni(H2O)6] 2+ + 3 en [Ni(en)3] 2+ + 6 H2O Kf = 6.8 × 10 17 What is the formation constant for this reaction? Chelate effect Chelating ligands (> 1 points of attachment) form more stable compounds than monodentate ligands WHY? [Cu(H2O)4]2+ + 2 NH3 [Cu(H2O)2(NH3)2]2+ + 2 H2O ∆Hº = – 46 kJ ∆Sº = – 8.4 J/K ∆Gº = – 43 kJ [Cu(H2O)4]2+ + en [Cu(H2O)2en]2+ + 2 H2O ∆Hº = – 54 kJ ∆Sº = + 23 J/K ∆Gº = – 61 kJ The en ligand occupies two coordination sites (NH3 only one), releasing two H2O molecules upon binding one en. Three molecules on the right, two on the left in the same aqueous solution: positive entropy change Important chelating ligands EDTA Porphine Applications Myoglobin Protein that stores oxygen in cells Applications Chlorophyll Mg-containing porphyrins (family of chemically similar molecules), key components in photosynthesis Crystal field theory In an octahedral ligand field, the d-orbitals that are degenerate in a uniform spherical field split into two sets: e (dz2, dx2-y2) and t2 (dxy, dxz, dyz) (Crystal field splitting) ∆ = crystal field splitting energy Crystal field splitting energy The magnitude of ∆ depends on: ______________________________ ______________________________ ______________________________ ______________________________ P = spin pairing energy (does not depend on the ligands) Low-spin complexes: ____________________ High-spin complexes: ____________________ Spectrochemical series List showing the relative order of the ability of a ligand to increase the crystal field splitting energy (∆) The magnitude of ∆ increases by roughly a factor of 2 from the far left to the far right – and the pairing energy is somewhere in the middle Cl– < F– < H2O < NH3 < en < NO2 – < CN– Example: TM3+ ions How many d electrons are there in each transition metal cation below, and in which orbitals would they reside? Sc3+ Ti3+ V3+ Cr3+ Mn3+ Fe3+ Example: [CoX6] 3– Which of the diagrams below corresponds to [CoF6]3– and which corresponds to [Co(CN)6]3–? WHY? Would you predict that the colors of these two complexes are the same or different? WHY? Would you predict that the magnetic properties of these two complexes are the same or different? WHY? Colors of metal complexes Color depends on the identity of the ligands WHY? [Fe(H2O)5NCS]2+[Fe(H2O)6]3+