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 ‘’. – -amido--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 Ma3bcd. Determining the Number of Isomers 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 (). – 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 ). • 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 l r – Where l and r 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. l r – 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 (d0 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). • 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.
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