Chapter 10 Coordination Chemistry II: Bonding

Chapter 10 Coordination Chemistry II:Bonding10-1 Experimental Evidence for Electronic Structures10-2 Theories of Electronic Structure10-3 Ligand Field Theory10-4 Angular Overlap10-5 The Jahn-Teller Effect10-6 Four- and Six-Coordinate Preferences10-7 Other Shapes“Inorganic Chemistry” Third Ed. Gary L. Miessler, Donald A. Tarr, 2004, Pearson Prentice Hallhttp://en.wikipedia.org/wiki/Expedia

Experimental Evidence for Electronic StructuresThermodynamic DataMagnetic SusceptibilityElectronic SpectraCoordination Numbers and MolecularShapes

Experimental Evidence for Electronic Structures;Thermodynamic DataOne of the primary goal of a bonding theory is toexplain the energy of compound.The energy is openly not determined directly byexperiment.Thermodynamic measurements of enthalpies andfree energies of reaction are used to compare.Bonding strength Stability constants(formation constants)

Experimental Evidence for Electronic Structures;Thermodynamic DataWhat is the stability constants?The equilibrium constants for formation ofcoordination complex.

Experimental Evidence for Electronic Structures;Thermodynamic DataStability constants HSAB conceptsThermodynamic values Prediction of properties, structures

Experimental Evidence for Electronic Structures;Thermodynamic DataHSAB conceptsThe gist of this theory is that soft acids react faster and formstronger bonds with soft bases, whereas hard acids reactfaster and form stronger bonds with hard bases, all other factorsbeing equal.The classification in the original work was mostly based onequilibrium constants for reaction of two Lewis bases competingfor a Lewis acid.Hard acids and hard bases tend to have:small sizehigh oxidation statelow polarizabilityhigh electronegativityenergy low-lying HOMO (bases) or energy high-lying LUMO(acids).

Experimental Evidence for Electronic Structures;Thermodynamic DataHSAB concepts

Experimental Evidence for Electronic Structures;Thermodynamic DataChelating LigandsEntropy Effecten vs methyl amineChelate EffectFive or six membered ringFigure in head .Stability .

Experimental Evidence for Electronic Structures;Magnetic SusceptibilityThe magnetic properties of a coordinationcompound can provide indirect evidence of theorbital energy level.Hund’s rule the max. # of unpaired e-.Diamagnetic: all e- paried repelled by a magnetic fieldParamagnetic: all e- paried attracted into a magnetic fieldMagnetic Susceptibility: Measuring Magnetism

Experimental Evidence for Electronic Structures;Magnetic SusceptibilityMagnetic SusceptibilityGouy methodA sample that is to be tested is suspendedfrom a balance between the poles of amagnet. The balance measures theapparent change in the mass of thesample as it is repelled or attracted by themagnetic field.

Experimental Evidence for Electronic Structures;Magnetic SusceptibilityIn physics and applied disciplines such as electricalengineering, the magnetic susceptibility is the degree ofmagnetization of a material in response to an appliedmagnetic field.Electron spin Spin magnetic moment (ms)Total spin magnetic moment Spin quantum # S (sum of ms)Isolated oxygen atom 1s22s2p4S 1/2 1/2 1/2 -1/2 1Electron spin Orbital magnetic moment (ml)Total orbital magnetic moment Orbital quantum # L (sum of ml)Max. L for the p4L 1 0 -1 1 1

Experimental Evidence for Electronic Structures;Magnetic SusceptibilityTwo sources of magnetic moment – spin (S) and Angular (L) motions of electronsSpin quantum numberAngular momentum quantum numberThe equation for the magnetic momentContribution from L is small in first transition series2.00023 2

Experimental Evidence for Electronic Structures;Electronic SpectraGive a direct evidence of orbital energy levelGive an information for geometry of complexes

Theories of Electronic StructureValence bond theoryCrystal field theoryLigand field theoryAngular overlap method

Theories of Electronic Structure;Valence bond theoryHybridization ideasOctahedral: d2sp3d orbitals could be 3d or 4d for the first-rowtransition metals. (hyperligated, hypoligated)

Theories of Electronic Structure;Valence bond theoryFe(III)Isolated ion; 5 unpaired eIn Oh compound; 1 or 5 unpaired eCo(II)Low spinLow spinHigh spinHigh spin

Theories of Electronic Structure;Crystal field theoryCrystal field theory (CFT) is a model that describes theelectronic structure of transition metal compounds, all ofwhich can be considered coordination complexes.CFT successfully accounts for some magnetic properties,colours, hydration enthalpies, and spinel structures oftransition metal complexes, but it does not attempt todescribe bonding.CFT was developed by physicists Hans Bethe and JohnHasbrouck van Vleck in the 1930s.CFT was subsequently combined with molecular orbitaltheory to form the more realistic and complex ligand fieldtheory (LFT), which delivers insight into the process ofchemical bonding in transition metal complexes.

Theories of Electronic Structure;Crystal field theoryRepulsion between d-orbital electrons and ligand electrons Splitting of energy levels of d-orbitals

Theories of Electronic Structure;Crystal field theory

Theories of Electronic Structure;Crystal field theory

Theories of Electronic Structure;Crystal field theoryElectrostatic approachIn an Octahedral field of ligand e- pairs; any ein them are repelled by the field.Crystal field stabilization energy (CFSE);the actual distribution vs the uniform field.Good for the concept of the repulsion oforbitals by the ligands but no explanationfor bonding in coordination complexes.

Theories of Electronic Structure;Crystal field theory

Theories of Electronic Structure;Crystal field theory

Theories of Electronic Structure;Crystal field theory

Theories of Electronic Structure;Crystal field theory

Theories of Electronic Structure;Crystal field theory

Theories of Electronic Structure;Crystal field theory

Theories of Electronic Structure;Crystal field theoryWhy are complexes formed in crystal field theory?Crystal Field Stabilization Energy (CFSE)Or Ligand Field Stabilization Energy (LFSE) the stabilization of the d orbitals because ofmetal-ligand environments

Theories of Electronic Structure;Crystal field theory E strong field – weak field E 0 weak field E 0 strong field

Theories of Electronic Structure;Crystal field theoryWhat determine ?Depends on the relative energiesof the metal ions and ligandorbitals and on the degree ofoverlap.

Theories of Electronic Structure;Crystal field theorySpectrochemical Series for Metal IonsOxidation # Only low spin aqua complexSmall size & higher chargePt4 Ir3 Pd4 Ru3 Rh3 Mo3 Mn4 Co3 Fe3 V2 Fe2 Co2 Ni2 Mn2

Ligand field theory;Molecular orbitals for Octahedral complexesCFT & MO were combinedThe dx2-y2 and dz2 orbitals can form bonding orbitalswith the ligand orbitals, but dxy, dxz, and dyz orbitalscannot form bonding orbitals

Ligand field theory;Molecular orbitals for Octahedral complexesThe combination ofthe ligand and metalorbitals (4s, 4px, 4py,4pz, 3dz2, and 3dx2-y2)form six bonding andsix antibonding witha1g, eg, t1u symmetries.The metal T2g orbitalsdo Electronnot havein bondingorbitals providetheappropriatesymmetrypotential energy that holds- nonbondingmolecules together

Ligand field theory;Orbital Splitting and Electron SpinStrong-field ligand – Ligands whose orbitalsinteract strongly with the metal orbitals large oWeak-field ligand.d0 d3 and d8 d10 – only one electronconfiguration possible no difference in thenet spinStrong fields lead to low-spin complexesWeak fields lead to high-spin complexes

Ligand field theory;Orbital Splitting and Electron SpinWhat determine ?Depends on the relativeenergies of the metal ionsand ligand orbitals and onthe degree of overlap.

Ligand field theory;Orbital Splitting and Electron SpinSpectrochemical Series for Metal IonsOxidation # Small size & higher chargePt4 Ir3 Pd4 Ru3 Rh3 Mo3 Mn4 Co3 Fe3 V2 Fe2 Co2 Ni2 Mn2

Ligand field theory;Ligand field Stabilization Energy

Ligand field theory;Orbital Splitting and Electron SpinOrbital configuration of the complex isdetermined by o, πc, and πeIn general o for 3 ions is larger than o for 2 ionswith the same # of e-. o π low-spin o π high-spinFor low-spinconfigurationRequire a strongfield ligand

Ligand field theory;Ligand field Stabilization Energy

Ligand field theory;Orbital Splitting and Electron SpinThe position of the metal in the periodictableSecond and third transition series form lowspin more easily than metals form the firsttransition series-The greater overlap between the larger 4dand 5d orbitals and the ligand orbitals-A decreased pairing energy due to thelarger volume available for electrons

Ligand field theory;Pi-BondingThe reducible representation is

Ligand field theory;Pi-BondingLUMO orbitals:can be usedfor π bonding with metalHOMO

Ligand field theory;Pi-Bondingmetal-to-ligand π bondingor π back-bonding-Increase stability-Low-spin configuration-Result of transfer ofnegative charge away fromthe metal ionLigand-to metal π bonding-decrease stability-high-spin configuration

Ligand field theory;Square planar Complexes; Sigma bonding

Ligand field theory;Square planar Complexes; Sigma bondingll e- from metal16 e-8 e-

Ligand field theory;Tetrahedral Complexes; Sigma bondingThe reducible representation isA1 and T2

Ligand field theory;Tetrahedral Complexes; Pi bondingThe reducible representation isE, T1 and T2

Angular OverlapLFT No explicit use of the energy change that resultsDifficult to use other than octahedral, squareplanar, tetrahedral.Deal with a variety of possible geometries andwith a mixture of ligand. Angular OverlapModelThe strength of interaction between individual ligandorbitals and metal d orbitals based on the overlapbetween them.

Angular Overlap:Sigma-Donor InteractionsThe strongest σ interactionThere are no examples of complexes with e- inthe antibonding orbitals from s and p orbitals,and these high-energy antibonding orbitals arenot significant in describing the spectra ofcomplexes. we will not consider them further.

Angular Overlap:Sigma-Donor Interactions

Angular Overlap:Sigma-Donor InteractionsStabilization is 12eσ

Angular Overlap:Pi-Acceptor InteractionsThe strongest π interaction is considered tobe between a metal dxy orbitals and a ligand π*orbital.Because of the overlap for these orbitals issmaller than the σ overlap, eπ eσ.

Angular Overlap:Pi-Acceptor Interactions

Angular Overlap:Pi-Acceptor Interactions

Angular Overlap:Pi-Donor InteractionsIn general, in situations involving ligands that canbehave as both π acceptors and π donors (suchas CO, CN-), the π acceptor nature predominates.

Angular Overlap:Pi-Donor Interactions

Angular Overlap:Pi-Acceptor Interactions

Angular Overlap:Types of the ligands and the spectrochemical seriesSpectrochemical Series for LigandsCO CN- PPh3 NO2- phen bipy en σ donor onlyNH3 py CH3CN NCS- H2O C2O42OH- RCO2- F- N3- NO3- Cl- SCNS2- Br- Iπ acceptor (strong field ligand)π donor(weak field ligand)

Angular Overlap:Magnitudes of eσ eπ and Metal and ligand

Angular Overlap:Magnitudes of eσ eπ and Angular overlapparameters derivedfrom electronicspectraeσ is always largerthan eπ. overlapisoelectronicThe magnitudes ofboth the σ and πparameters with size and electronegativity ofthe halide ions.overlap

Angular Overlap:Magnitudes of eσ eπ and Can describe theelectronic energyof complexes withdifferent shapes orwith combinationsof different liagnds.The magnitude of o Magneticproperties andvisible spectrum.

Angular Overlap:The Jahn-Teller EffectThere cannot be unequal occupation of orbitals with identical orbitals.To avoid such unequal occupation, the molecule distorts so thatthese orbitals no longer degenerate.In other words, if the ground electron configuration of a nonlinearcomplex is orbitally degenerate, the complex will distort to removethe degeneracy and achieve a lower energy.

Angular Overlap:The Jahn-Teller Effect

Angular Overlap:Four- and Six-Coordinate PreferenceAngular overlap calculationsOnly σ bonding is considered.Low-spin square planarLarge # of bonds formed inthe octahedral complexes.