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Coordination chemistry

This document discusses coordination compounds and chemistry. It defines coordination compounds as containing metal complexes, which are central metal atoms bonded to ligands. Coordination compounds form when the ligands, usually anions or polar molecules with lone pairs, bond to the metal via coordinate covalent bonds. Alfred Werner proposed that metals have both a primary oxidation state and a secondary coordination number equal to the number of directly bonded ligands. The document discusses isomers, geometries, ligands, and uses Werner's theory to explain properties of coordination compounds.

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Chemistry of Coordination Compounds Chemistry of Coordination Compounds Represented by- Saurav K. Rawat M.Sc. (Chem.) (Physical Special
Chemistry of Coordination Compounds Complexes • A central metal atom bonded to a group of molecules or ions is a metal complex. • If it’s charged, it’s a complex ion. • Compounds containing complexes are coordination compounds.
Chemistry of Coordination Compounds Complexes • The molecules or ions coordinating to the metal are the ligands. • They are usually anions or polar molecules. • The must have lone pairs to interact with metal
Chemistry of Coordination Compounds A chemical mystery: Same metal, same ligands, different number of ions when dissolved • Many coordination compounds are brightly colored, but again, same metal, same ligands, different colors.
Chemistry of Coordination Compounds Werner’s Theory Co(III) oxidation state Coordination # is 6 • suggested in 1893 that metal ions have primary and secondary valences. Primary valence equal the metal’s oxidation number Secondary valence is the number of atoms directly bonded to the metal (coordination number)
Chemistry of Coordination Compounds Werner’s Theory • The central metal and the ligands directly bonded to it make up the coordination sphere of the complex. • In CoCl3 ∙ 6 NH3, all six of the ligands are NH3 and the 3 chloride ions are outside the coordination sphere.
Chemistry of Coordination Compounds Werner’s Theory In CoCl3 ∙ 5 NH3 the five NH3 groups and one chlorine are bonded to the cobalt, and the other two chloride ions are outside the sphere.
Chemistry of Coordination Compounds Werner’s Theory Werner proposed putting all molecules and ions within the sphere in brackets and those “free” anions (that dissociate from the complex ion when dissolved in water) outside the brackets.
Chemistry of Coordination Compounds 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.
Chemistry of Coordination Compounds What is Coordination? • When an orbital from a ligand with lone pairs in it overlaps with an empty orbital from a metal M L Sometimes called a coordinate covalent bond So ligands must have lone pairs of electrons.
Chemistry of Coordination Compounds Metal-Ligand Bond • This bond is formed between a Lewis acid and a Lewis base. The ligands (Lewis bases) have nonbonding electrons. The metal (Lewis acid) has empty orbitals.
Chemistry of Coordination Compounds Metal-Ligand Bond The metal’s coordination ligands and geometry can greatly alter its properties, such as color, or ease of oxidation.
Chemistry of Coordination Compounds Oxidation Numbers Knowing the charge on a complex ion and the charge on each ligand, one can determine the oxidation number for the metal.
Chemistry of Coordination Compounds Oxidation Numbers Or, knowing the oxidation number on the metal and the charges on the ligands, one can calculate the charge on the complex ion. Example: Cr(III)(H2O)4Cl2
Chemistry of Coordination Compounds Coordination Number • The atom that supplies the lone pairs of electrons for the metal-ligand bond is the donor atom. • The number of these atoms is the coordination number.
Chemistry of Coordination Compounds Coordination Number • Some metals, such as chromium(III) and cobalt(III), consistently have the same coordination number (6 in the case of these two metals). • The most commonly encountered numbers are 4 and 6.
Chemistry of Coordination Compounds Geometries • There are two common geometries for metals with a coordination number of four: Tetrahedral Square planar Tetrahedral Square planar Why square planar? We’ll get to that
Chemistry of Coordination Compounds Geometries By far the most-encountered geometry, when the coordination number is six, is octahedral.
Chemistry of Coordination Compounds Polydentate Ligands • Some ligands have two or more donor atoms. • These are called polydentate ligands or chelating agents. • In ethylenediamine, NH2CH2CH2NH2, represented here as en, each N is a donor atom. • Therefore, en is bidentate.
Polydentate Ligands Ethylenediaminetetraacetate, mercifully abbreviated EDTA, has six donor atoms. Chemistry of Coordination Compounds Wraps around the central atom like an octopus
Chemistry of Coordination Compounds Polydentate Ligands Chelating agents generally form more stable complexes than do monodentate ligands.
Chemistry of Coordination Compounds Chelating Agents 5- .. : .. : - - .. : .. : : .. : : .. : - - - .. • Bind to metal ions removing them from solution. • Phosphates are used to tie up Ca2+ and Mg2+ in hard water to prevent them from interfering with detergents.
Chemistry of Coordination Compounds Chelating Agents • Porphyrins are complexes containing a form of the porphine molecule shown at right. • Important biomolecules like heme and chlorophyll are porphyrins.
Chemistry of Coordination Compounds Chelating Agents Porphines (like chlorophyll a) are tetradentate ligands.
Nomenclature of Coordination Chemistry of Coordination Compounds Compounds • The basic protocol in coordination nomenclature is to name the ligands attached to the metal as prefixes before the metal name. • Some common ligands and their names are listed above.
Nomenclature of Coordination Chemistry of Coordination Compounds Compounds • As always the name of the cation appears first; the anion is named last. • Ligands are listed alphabetically before the metal. Prefixes denoting the number of a particular ligand are ignored when alphabetizing.
Nomenclature of Coordination Chemistry of Coordination Compounds Compounds • The names of anionic ligands end in “o”; the endings of the names of neutral ligands are not changed. • Prefixes tell the number of a type of ligand in the complex. If the name of the ligand itself has such a prefix, alternatives like bis-, tris-, etc., are used.
Nomenclature of Coordination Chemistry of Coordination Compounds Compounds • If the complex is an anion, its ending is changed to -ate. • The oxidation number of the metal is listed as a Roman numeral in parentheses immediately after the name of the metal.
Chemistry of Coordination Compounds Isomers Isomers have the same molecular formula, but their atoms are arranged either in a different order (structural isomers) or spatial arrangement (stereoisomers).
Chemistry of Coordination Compounds Structural 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 are formed.
Chemistry of Coordination Compounds Structural Isomers • Some isomers differ in what ligands are bonded to the metal and what is outside the coordination sphere; these are coordination-sphere isomers. • Three isomers of CrCl3(H2O)6 are The violet [Cr(H2O)6]Cl3, The green [Cr(H2O)5Cl]Cl2 ∙ H2O, and The (also) green [Cr(H2O)4Cl2]Cl ∙ 2 H2O.
Chemistry of Coordination Compounds Geometric isomers • With these geometric isomers, two chlorines and two NH3 groups are bonded to the platinum metal, but are clearly different. cis-Isomers have like groups on the same side. trans-Isomers have like groups on opposite sides. # of each atom the same Bonding the same Arrangement in space different
Chemistry of Coordination Compounds Stereoisomers • Other stereoisomers, called optical isomers or enantiomers, are mirror images of each other. • Just as a right hand will not fit into a left glove, two enantiomers cannot be superimposed on each other.
Chemistry of Coordination Compounds Enantiomers A molecule or ion that exists as a pair of enantiomers is said to be chiral.
Chemistry of Coordination Compounds Enantiomers • Most of the physical properties of chiral molecules are the same, boiling point, freezing point, density, etc. • One exception is the interaction of a chiral molecule with plane-polarized light.
Chemistry of Coordination Compounds Enantiomers • If one enantiomer of a chiral compound is placed in a polarimeter and polarized light is shone through it, the plane of polarization of the light will rotate. • If one enantiomer rotates the light 32° to the right, the other will rotate it 32° to the left.
Chemistry of Coordination Compounds
Chemistry of Coordination Compounds
Chemistry of Coordination Compounds
Explaining the properties of transition metal coordination Chemistry of Coordination Compounds complexes 1. Magnetism 2. color
Chemistry of Coordination Compounds Metal complexes and color The ligands of a metal complex effect its color Addition of NH3 ligand to Cu(H2O)4 changes its color
Chemistry of Coordination Compounds Why does anything have color? Light of different frequencies give different colors We learned that elements can emit light of different frequency or color. But these coordination complexes are not emitting light They absorb light. How does that give color?
Light can bounce off an object or get absorbed by object No light absorbed, all reflected get white color All light absorbed, none reflected get Black color What if only one color is absorbed? Chemistry of Coordination Compounds
Chemistry of Coordination Compounds Complimentary color wheel If one color absorbed, the color opposite is perceived. Absorb Orange See Blue Absorb Red See Green
Chemistry of Coordination Compounds [Ti(H2O)6]3+ Absorbs in green yellow. Looks purple.
A precise measurement of the absorption Chemistry of Coordination Compounds spectrum of Compounds is critical
Chemistry of Coordination Compounds Metal complexes and color But why do different ligands on same metal give Different colors? Why do different ligands change absorption? Addition of NH3 ligand to Cu(H2O)4 changes its color
Model of ligand/metal bonding. Electron pair comes from ligand Bond very polarized. Assumption: interaction pure electrostatic. Chemistry of Coordination Compounds
Now, think of point charges being attracted to metal nucleus Positive charge. What about electrons in d orbitals? Chemistry of Coordination Compounds Ligand negative charge Is repelled by d electrons, d orbital energy goes up
Ligands will interact with some d orbitals more than others Depends on relative orientation of orbital and ligand Ligands point right at lobes Chemistry of Coordination Compounds
Chemistry of Coordination Compounds In these orbitals, the ligands are between the lobes Interact less strongly
Splitting due to ligand/orbirtal orientation. Chemistry of Coordination Compounds
Chemistry of Coordination Compounds = 495 nm
Different ligands interact more or less, change E spacing Of D orbitals. Chemistry of Coordination Compounds
Spectrochemical series (strength of ligand interaction) Chemistry of Coordination Compounds Cl- < F- < H2O < NH3 < en < NO2 - < CN-Increasing D Increasing D
Electron configurations of some octahedral complexes Chemistry of Coordination Compounds
As Energy difference increases, electron configuration changes Chemistry of Coordination Compounds “High spin” “Low spin” Co(III) is d6
Chemistry of Coordination Compounds
In tetrahedral complexes, orbitals are inverted. Again because of orientation of orbitals and ligands D is always small, always low spin (less ligands) Chemistry of Coordination Compounds Tetrahedral Complexes
Square planar complexes are different still Chemistry of Coordination Compounds
Chemistry of Coordination Compounds
Chemistry of Coordination Compounds
Chemistry of Coordination Compounds Intense color can come from “charge transfer” Ligand electrons jump to metal orbitals KMnO4 KCrO4 KClO4 No d orbitals in Cl, orbitals higher In energy
Chemistry of Coordination Compounds
Chemistry of Coordination Compounds MO theory for coordination compounds MO theory: Rules: • 1. The number of MO’s equals the # of Atomic orbitals • 2. The overlap of two atomic orbitals gives two molecular orbitals, 1 bonding, one antibonding • 3. Atomic orbitals combine with other atomic orbitals of similar energy. • 4. Degree of overlap matters. More overlap means bonding orbital goes lower in E, antibonding orbital goes higher in E. • 5. Each MO gets two electrons • 6. Orbitals of the same energy get filled 1 electron at a time until they are filled.
Chemistry of Coordination Compounds Difference between pi and sigma orbitals End on Side to side.
A typical MO diagram, like the one below. For 2p and 2s atomic orbital mixing. Chemistry of Coordination Compounds
Oxygen O2 is Paramagnetic, why? Chemistry of Coordination Compounds
Chemistry of Coordination Compounds Show me why.
For exam concentrate on these topics more Chemistry of Coordination Compounds Concentrate on the homeworks and the quiz! Terms: 1. Coordination sphere 2. Ligand 3. Coordination compound 4. Metal complex 5. Complex ion 6. Coordination 7. Coordination number Same ligands different properties? Figuring oxidation number on metal
Chemistry of Coordination Compounds Polydentate ligands (what are they)? Isomers. structural isomers (formula same, bonds differ) geometric isomers (formula AND bonds same, structure differs) Stereoisomers: Chirality, handedness,
Chemistry of Coordination Compounds
Chemistry of Coordination Compounds Stereoisomers
Chemistry of Coordination Compounds Explaining the properties of metal complexes Magnetism and color How does seeing color work? Absorb Orange See Blue Absorb Red See Green
Different ligands on same metal give different colors Addition of NH3 ligand to Cu(H2O)4 changes its color Chemistry of Coordination Compounds
Chemistry of Coordination Compounds Splitting of d orbitals in an oxtahedral ligand field dz 2 dx 2 -y 2 dxy dyz dxz
Spectrochemical series (strength of ligand interaction) Chemistry of Coordination Compounds Cl- < F- < H2O < NH3 < en < NO2 - < CN-Increasing D Increasing D Know low spin versus high spin
Chemistry of Coordination Compounds There is also splitting from tetrahedral And square planar. Know they are different, don’t remember exactly what they are like.
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