Transition Metal Chemistry and Coordination Compounds

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Text of Transition Metal Chemistry and Coordination Compounds

  • Transition Metal Chemistry and Coordination CompoundsLecture 18-20

  • The Transition Metals

  • Oxidation States of the 1st Row Transition Metals(most stable oxidation numbers are shown in red)

  • Ionization Energies for the 1st Row Transition Metals

  • Aqueous oxoanions of transition elementsOne of the most characteristic chemical properties of these elements is the occurrence of multiple oxidation states.

  • Effects of the metal oxidation state and of ligand identity on color[V(H2O)6]2+[V(H2O)6]3+[Cr(NH3)6]3+[Cr(NH3)5Cl ]2+

  • Coordination CompoundsA coordination compound typically consists of a complex ion and a counter ion.A complex ion contains a central metal cation bonded to one or more molecules or ions.The molecules or ions that surround the metal in a complex ion are called ligands.A ligand has at least one unshared pair of valence electrons22.3

  • Coordination CompoundsThe atom in a ligand that is bound directly to the metal atom is the donor atom.The number of donor atoms surrounding the central metal atom in a complex ion is the coordination number.Ligands with:one donor atommonodentatetwo donor atomsbidentatethree or more donor atomspolydentateH2O, NH3, Cl-ethylenediamineEDTA

  • Coordination Compoundspolydentate ligand(EDTA)Bidentate and polydentate ligands are called chelating agents

  • EDTA Complex of Lead

  • OH- has charge of -1K+ has charge of +1? Au + 1 + 4x(-1) = 0Au = +3NO3- has charge of -1NH3 has no charge? Cr + 6x(0) + 3x(-1) = 0Cr = +3

  • Naming Coordination Compounds22.3The cation is named before the anion.Within a complex ion, the ligands are named first in alphabetical order and the metal atom is named last.The names of anionic ligands end with the letter o. Neutral ligands are usually called by the name of the molecule. The exceptions are H2O (aqua), CO (carbonyl), and NH3 (ammine).When several ligands of a particular kind are present, the Greek prefixes di-, tri-, tetra-, penta-, and hexa- are used to indicate the number. If the ligand contains a Greek prefix, use the prefixes bis, tris, and tetrakis to indicate the number.The oxidation number of the metal is written in Roman numerals following the name of the metal.If the complex is an anion, its name ends in ate.

  • tetraaquodichlorochromium(III) chloride[Co(en)3]SO4

  • Transition Metal Chemistry and Coordination Compounds

    Lecture 21-22

    My Computer

  • Crystal-Field TheoryModel explaining bonding for transition metal complexes Originally developed to explain properties for crystalline material Basic idea:Electrostatic interaction between lone-pair electrons result in coordination.

  • EnergeticsCFT - Electrostatic between metal ion and donor atomi) Separate metal and ligand high energyii) Coordinated Metal - ligand stabilizediii) Destabilization due to ligand -d electron repulsioniv) Splitting due to octahedral field.iiiiiiiv

  • d-Orbitals and Ligand Interaction(Octahedral Field)Ligands approach metald-orbitals not pointing directly at axis are least affected (stabilized) by electrostatic interaction d-orbitals pointing directly at axis are affected most by electrostatic interaction

  • Ligand-Metal InteractionCrystal Field Theory - Describes bonding in Metal ComplexesBasic Assumption in CFT:Electrostatic interaction between ligand and metald-orbitals align along the octahedral axis will be affected the most.More directly the ligand attacks the metal orbital, the higher the the energy of the d-orbital.In an octahedral field the degeneracy of the five d-orbitals is lifted

  • Splitting of the d-OrbitalsOctahedral field Splitting Pattern:The energy gap is referred to as (10 Dq) , the crystal field splitting energy.The dz2 and dx2y2 orbitals lie on the same axes as negative charges.Therefore, there is a large, unfavorable interaction between ligand (-) orbitals.These orbitals form the degenerate high energy pair of energy levels.The dxy , dyx and dxz orbitals bisect the negative charges.Therefore, there is a smaller repulsion between ligand & metal for these orbitals.These orbitals form the degenerate low energy set of energy levels.

  • Do3/5 Do2/5 DoDo is the crystal field splittingt2gegE(t2g) = -0.4Do x 3 = -1.2DoE(eg) = +0.6Do x 2 = +1.2DoSplitting of d orbitals in an octahedral field

  • The magnitude of the splitting(ligand effect)StrongfieldWeakfieldThe spectrochemical seriesCO, CN- > phen > NO2- > en > NH3 > NCS- > H2O > F- > RCO2- > OH- > Cl- > Br- > I-

  • The magnitude of the splitting(metal ion effect)StrongfieldWeakfield increases with increasing formal charge on the metal ion increases on going down the periodic table

  • The spectrochemical seriesFor a given ligand, the color depends on the oxidation state of the metal ion.For a given metal ion, the color depends on the ligand.Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

  • Electron Configuration in Octahedral FieldElectron configuration of metal ion:s-electrons are lost first. Ti3+ is a d1, V3+ is d2 , and Cr3+ is d3 Hund's rule:First three electrons are in separate d orbitals with their spins parallel.Fourth e- has choice:Higher orbital if is small; High spinLower orbital if is large: Low spin.Weak field ligandsSmall , High spin complexStrong field LigandsLarge , Low spin complex

  • Placing electrons in d orbitalsStrong field Weak fieldStrong field Weak field

  • d4Strong field =Low spin(2 unpaired)Weak field =High spin(4 unpaired)P < Do P > Do When the 4th electron is assigned it will either go into the higher energy eg orbital at an energy cost of D0 or be paired at an energy cost of P, the pairing energy.Notes: the pairing energy, P, is made up of two parts. 1) Coulombic repulsion energy caused by having two electrons in same orbital

  • Pairing Energy, PThe pairing energy, P, is made up of two parts. Coulombic repulsion energy caused by having two electrons in same orbital. Destabilizing energy contribution of Pc for each doubly occupied orbital.Exchange stabilizing energy for each pair of electrons having the same spin and same energy. Stabilizing contribution of Pe for each pair having same spin and same energy P = sum of all Pc and Pe interactions

  • Placing electrons in d orbitals

  • To sum upElectron Configuration for Octahedral complexes of metal ion having d1 to d10 configuration [M(H2O)6]+n. Only the d4 through d7 cases have both high-spin and low spin configuration.Electron configurations for octahedral complexes of metal ions having from d1 to d10 configurations. Only the d4 through d7 cases have both high-spin and low-spin configurations.

  • Bonding in Coordination Compounds

  • Splitting of d orbitals in a tetrahedral fieldt2eDtDt = 4/9DoAlways weak field (high spin)

  • Bonding in Coordination Compounds

  • A crystal-field aproach: from octahedral to tetrahedralLess repulsions along the axeswhere ligands are missing

  • Octahedral, Tetrahedral & Square PlanarCF Splitting pattern for various molecular geometryOctahedralTetrahedralSquare planarPairing energy Vs. Weak field < PeStrong field > PeSmall High SpinMostly d8(Majority Low spin)Strong field ligandsi.e., Pd2+, Pt2+, Ir+, Au3+

  • Applications of coordination compoundsColour of transition metal complexesNumber of unpaired electrons and magnetic propertiesCoordination compounds in living systems Coordination compounds in medicine

  • Colour in Coordination Compounds22.5DE = hn

  • D = hn= 4.23 x 10-19 JD (kJ/mol) =4.23 x 10-19 J/atom x 6.022 x 1023 atoms/mol= 255 kJ/mol22.5

  • Color Absorption of Co3+ ComplexesThe Colors of Some Complexes of the Co3+ IonThe complex with fluoride ion, [CoF6]3+ , is high spin and has one absorption band. The other complexes are low spin and have two absorption bands. In all but one case, one of these absorptionsis in the visible region of the spectrum. The wavelengths refer to the center of that absorption band.

    Complex IonWavelength of Color of Light Color of Complex light absorbed Absorbed[CoF6] 3+ 700 (nm)RedGreen[Co(C2O4)3] 3+ 600, 420Yellow, violetDark green[Co(H2O)6] 3+ 600, 400Yellow, violetBlue-green[Co(NH3)6] 3+ 475, 340Blue, violetYellow-orange[Co(en)3] 3+470, 340 Blue, ultravioletYellow-orange[Co(CN)6] 3+310 UltravioletPale Yellow

  • Colors & How We Perceive it800430650580560490Artist color wheelshowing the colors whichare complementary to oneanother and the wavelengthrange of each color.400

  • Complex Influence on ColorCompounds of Transition metal complexes solution.[Fe(H2O)6]3+[Co(H2O)6]2+[Ni(H2O)6]2+[Cu(H2O)6]2+[Zn(H2O)6]2+

  • If a sample absorbs all wavelength of visible light, none reaches our eyes from that sample. Consequently, it appears black.When a sample absorbs light, what we see is the sum of the remaining colors that strikes our eyes.If the sample absorbs novisible light, it is white or colorless.Black and White

  • Absorption and ReflectionIf the sample absorbsall but orange, thesample appears orange.Further, we also perceive orange color when visible light of all colors except blue strikes our eyes. In a complementary fashion, if the sample absorbed only orange, it would appear blue; blue and orange are said to be complementary colors.

  • Light absorption Properties of Metal ComplexesRecording the absorption Spectrum

  • Chemistry In Action: Coordination Compounds in Living Systems Hemoglobin

  • Chlorophyll a & bChl a has a methyl group Chl b has a carb