Coordination Chemistry I: Structures and Isomers Chapter 9
Coordination Compounds• Coordination compounds – compounds composed of a metal atom or ion and one or more ligands. – [Co(Co(NH3)4(OH2)3]Br6 – Ligands usually donate electrons to the metal – Includes organometallic compounds Werner’s totally inorganic optically active compound.
Werner’s Coordination Chemistry• Performed systematic studies to understand bonding in coordination compounds. – Organic bonding theory and simple ideas of ionic charges were not sufficient.• Two types of bonding – Primary – positive charge of the metal ion is balanced by negative ions in the compound. – Secondary – molecules or ion (ligands) are attached directly to the metal ion. • Coordination sphere or complex ion. • Look at complex on previous slide (primary and secondary)
Werner’s Coordination Chemistry• He largely studied compounds with four or six ligands. – Octahedral and square-planar complexes.• It was illustrated that a theory needed to account for bonds between ligands and the metal. – The number of bonds was commonly more than accepted at that time. • 18-electron rule.• New theories arose to describe bonding. – Valence bond, crystal field, and ligand field.
Chelating Ligands• Chelating ligands trisoxalatochromate(III) ion or just [Cr(ox) ] 3 3- (chelates) – ligands that have two or more points of attachment to the metal atom or ion. – Bidentate, tridentate, tetra.., penta…, hexa… (EDTA).
A Hexadentate Ligand, EDTA• There are six points of attachment to the calcium metal. – Octahedral-type geometry ethylene diamine tetraacetic acid (EDTA) ethylenediaminetetraacetatocalcium ion or just [Ca(EDTA)]2-
Nomenclature• The positive ion (cation) comes first, followed by the name within the coordination sphere, followed by the negative ion (anion). – These ions are not in the coordination sphere. – Diamminesilver(I)chloride and potassium hexacyanoferrate (III).• The inner coordination sphere is enclosed in brackets in the formula. Within this sphere, the ligands are named before the metal, but in formulas the metal ion is written first. – Tetraamminecopper(II) sulfate and hexaamminecobalt(III) chloride.
Nomenclature• The number of ligands is given by the following 2 di bis prefixes. If the ligand name includes prefixes 3 tri tris or is complicated, it is 4 tetra tetrakis set off in parentheses 5 penta pentakis and the second set of prefixes is used. 6 hexa hexakis – [Co(en)2Cl2]+ and 7 hepta heptakis [Fe(C5H4N-C5H4N)3]2+ 8 octa octakis
Nomenclature• Ligands are named in alphabetical order (name of ligand, not prefix) – [Co(NH3)4Cl2]+ and [Pt(NH3)BrCl(CH3NH2)]+2• Anionic ligands are given an ‘o’ suffix. Neutral ligands retain the usual name. – Coordianted water is called ‘aqua’. – Chloro, Cl- – Sulfato, SO42-
Nomenclature• The calculated oxidation number of the metal ion is placed as a Roman numeral in parentheses after the name of the coordination sphere. – [Pt(NH3)4]+2 and [Pt(Cl)4]-2 – A suffix ‘ate’ is added to the metal ion if the charge is negative.• The prefixes cis- and trans- designate adjacent and opposite geometric location, respectively. – trans-diamminedichloroplatinum(III) and cis- tetraamminedichlorocobalt(III)
Nomenclature• Bridging ligands between two metal ions have the prefix ‘µ’. µ-amido-µ-hydroxobis(tetraamminecobalt)(IV)There is an error in this picture. What is it?
Isomerism• Our discussion of isomers will be largely limited to those with the same ligands arranged in different geometries. This is referred to as stereoisomers.
Isomerism• Four-coordinate complexes – Square-planar complexes may have cis and trans isomers. No chiral isomers (enantiomers) are possible when the molecule has a mirror plane. – cis- and trans- diamminedichloroplatinum(II) – How about tetrahedral complexes? – Chelate rings commonly impose a ‘cis’ structure. Why
Chirality• Mirror images are nonsuperimposable.• A molecule can be chiral if it has no rotation-reflection axes (Sn)• Chiral molecules have no symmetry elements or only have an axes of proper rotation (Cn). – CBrClFI, Tetrahedral molecule (different ligands) – Octahedral molecules with bidentate or higher chelating ligands – Octahedral species with [Ma2b2c2], [Mabc2d2], [Mabcd3], [Mabcde2], or [Mabcdef]
Six-Coordinate Octahedral Complexes• ML3L3’ – Fac isomers have three identical ligands on the same face. – Mer isomers have three identical ligands in a plane bisecting the molecule.
Six-Coordinate Octahedral Complexes• The maximum number of isomers can be difficult to calculate (repeats).• Placing a pair of ligands in the notation <ab> indicates that a and b are trans to each other. – [M<ab><cd><ef>], [Pt<pyNH3><NO2Cl><BrI>]• How many diastereoisomers in the above platinum compound (not mirror images)?• Identify all isomers belonging to Ma 3bcd.
Determining the Number of Isotopes• Bailar method• With restrictions (such as chelating agents) some isomers may be eliminated.• Determine and identify the number if isomers. – [Ma2b2cd] and [M(AA)bcde]
Combinations of Chelate Rings• Propellers and helices – Left- and right-handed propellers• Examine the movement of a propeller required to move it in a certain direction. – For a left-handed propeller, rotating it ccw would cause it to move away (Λ). – For a right-handed propeller, rotating it cw would cause it to move away (∆). This is called ‘handedness’. Many molecules possess it.
Tris(ethylenediamine)cobalt(III)• This molecule can be treated like a three- bladed propeller.• Look down a three fold axis to determine the ‘handedness’ of this complex ion. – The direction of rotation required to pull the molecule away from you determines the handedness (∆ or Λ).• Do this with you molecule set and rubber bands.
Determining Handedness for Chiral Molecules• Complexes with two or more nonadjacent chelate rings may have chiral character. – Any two noncoplanar and nonadjacent chelate rings can be used. – Look at Figure 9-14 (Miessler and Tarr).• Molecules with more than one pair of rings may require more than one label. – Ca(EDTA)2+ • Three labels would be required. • Remember that the chelate rings must be noncoplanar, nonadjacent, and not connected at the same atom.
Linkage (ambidentate) Isomerism• A few ligands may bond to the metal through different atoms. – SCN- and NO2-• How would you expect hard acids to bond to the thiocyanate ligand?• Solvents can also influence bonding. – High and low dielectric constants.• Steric effects of linkage isomerism• Intramolecular conversion between linkages. – [Co(NH3)5NO2]+2, Figure 9-19.
Separation and Identification of Isomers• Geometric isomers can be separated by fractional crystallization with different counterions. – Due to the slightly different shapes of the isomers. – The ‘fit’ of the counterion can greatly influence solubility. • Solubility is the lowest when the positive and negative charges have the same size and magnitude of charges (Basolo).
Separation and Identification of Chiral Isomers• Separations are performed with chiral counterions. The resulting physical properties will differ allowing separation.• Rotation of polarized light will be opposite for two chiral isomers at a specific wavelength. – The direction of optical rotation can change with wavelength.
Circular Dichroism Meaurement• The difference in the absorption of right and left circularly polarized light is measured. Circular dichroism = ε l − ε r – Where εl and εr are the molar absorption coefficients for left and right circularly polarized light.• The light received by the detector is presented as the difference between the absorbances.Figure 9-20.
Plane-Polarized Light Measurement• The plane of polarization is rotated when passing through a chiral substance. – Caused by a difference in the refractive indices of the right and left circularly polarized light. ηl − η r α= λ – The optical rotation illustrates positive value on one side of the adsorption maximum and negative side on the other. This is termed as the Cotton effect.
Coordination Numbers and Structures• Factors considered when determining structures. – The number of bonds. Bond formation is exothermic; the more the better. – VSEPR arguments – Occupancy of d orbitals. – Steric interference by large ligands. – Crystal packing effect. It may be difficult to predict shapes.
Low Coordination Numbers (C.N.)• C.N. 1 is rare except in ion pairs in the gas phase.• C.N. 2 is also rare. – [Ag(NH3)2]+, Ag is d10 (how?) – VSEPR predicts a linear structure. – Large ligands help force a linear or near-linear arrangment. • [Mn(N[SiMePh2]2)2] in Figure 9-22.• C.N. 3 is more likely with d10 ions. – Trigonal-planar structure is the most common. – [Cu(SPPh3)3]+, adopts a low C.N. due to ligand crowding.
Coordination Number 4• Tetrahedral and square planar complexes are the most common. – Small ions and/or large ligands prevent high coordination numbers (Mn(VII) or Cr(VI)).• Many d0 or d10 complexes have tetrahedral structures (only consider bonds). – MnO4- and [Ni(CO)4] – Jahn-Teller distortion (Chapter 10)
Coordination Number 4• Square-planar geometry – d8 ions (Ni(II), Pd(II), and Pt(III)) • [Pt(NH3)2Cl2] – The energy difference between square-planar and tetrahedral structures can be quite small. • Can depend on both the ligand and counterion. • More in chapter 10.
Coordination Number 5• Common structures are trigonal bipyramid and square pyramid. – The energy difference between the two is small. In many measurements, the five ligands appear identical due to fluxional behavior. – How would you modify the experiment to differentiate between the two structures?• Five-coordinate compounds are known for the full range of transition metals. – Figure 9-27.
Coordination Number 6• This is the most common C.N. with the most common structure being octahedral. – If the d electrons are ignored, this is the predicted shape. • [Co(en)3]3+• This C.N. exists for all transition metals (d 0 to d10).
Distortions of Complexes Containing C.N. 6• Elongation and compression (Fig. 9-29). – Produces a trigonal antiprism structure when the angle between the top and bottom triangular faces is 60°. – Trigonal prism structures are produced when the faces are eclipsed. • Most trigonal prismatic complexes have three bidentate ligands (Figure 9-30). ∀ π interactions may stabilize some of these structures. The Jahn-Teller effect (Ch. 10) is useful in predicting observed distortions.
Higher Coordination Numbers• C.N. 7 is not common• C.N. 8 – There are many 8-coordinate complexes for large transition elements. • Square antiprism and dodecahedron• C.N.’s up to 16 have been observed.