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Coordination Chemistry: Nomenclature, Isomerism, and Structure

Coordination Chemistry: Nomenclature, Isomerism, and Structure. Chapter 9. 1. Nomenclature Rules. For charged molecules, the cation comes first followed by the anion. The following rules apply to both neutral and charged molecules:

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Coordination Chemistry: Nomenclature, Isomerism, and Structure

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  1. Coordination Chemistry: Nomenclature, Isomerism, and Structure Chapter 9

  2. 1. Nomenclature Rules • For charged molecules, the cation comes first followed by the anion. • The following rules apply to both neutral and charged molecules: • B. The elemental formulation has the inner coordination sphere in brackets. • [Pt(NH3)4]Cl2 • When writing the name, the ligands within the coordination sphere are written before the metal. • tetraammineplatinum(II) chloride • C. Ligand names (refer to handout). • Monodentate: Ligands with one point of attachment • Chelates (Bidentate…multidentate): Ligands with two or more points of attachment

  3. 1. Nomenclature Rules C. The number of ligands of each kind is indicated by prefixes using the following table. • Use prefixes in column A for simple cases. • Use prefixes in column B for ligands with names that already use prefixes from column A. • [Co(en)2Cl2]+ • Dichlorobis(ethylenediamine)cobalt(III) A B

  4. 1. Nomenclature Rules • D. Ligands are written in alphabetical order-according to the ligand name, not the prefix. • E. Anionic ligands are given an o suffix. • Neutral ligands retain their usual name • Coordinated water is called aqua • Coordinated ammonia is called ammine

  5. 1. Nomenclature Rules F. Designate the metal oxidation state after the metal. [PtClBr(NH3)(H2O)] Ammineaquabromochloroplatinum(II) [Pt(NH3)4]2+ Tetraammineplatinum(II) If the molecule is negatively charged, the suffix –ate is added to the name [Pt(NH3)Cl3]- Amminetrichloroplatinate(II)

  6. 1. Nomenclature Rules Special names for metals when in a negatively charged molecule: Copper (Cu): Cuprate Iron (Fe): ferrate Silver (Ag): argentate Lead (Pb): Plumbate Tin(Sn): Stannate Gold(Au): Aurate

  7. 1. Nomenclature Rules • G. Prefixes designate adjacent (cis-) and opposite (trans-) geometric locations • cis-diamminedichloroplatinum(II) is an anticancer agent. • The trans isomer is not.

  8. 1. Nomenclature Rules H. Bridging ligands between two metal ions have the prefix μ μ-amido-μ-hydroxobis(tetraaminecobalt)(IV)

  9. 2. Isomerism Ligand isomers

  10. 2. Isomerism Conformational Isomers Geometric Isomers cis-trans fac-mer

  11. A. Constitutional Isomers • Linkage (Ambidentate) Isomers • A ligand can bind in more than one way • [Co(NH3)5NO2]2+ • Co-NO2 Nitro isomer; yellow compound • Co-ONO Nitrito isomer; red compound • The binding at different atoms can be due to the hard/soft-ness of the metal ions • SCN- • Hard metal ions bind to the N • Soft metal ions bind to the S • Solvent can influence the point of attachment • For SCN-: M-S bonds favored in solvents of high dipole moment • M-N bonds favored in solvents of low dipole moment

  12. A. Constitutional Isomers • Ligand Isomers • III. Ionization Isomers • Difference in which ion is included as a ligand and which is present to balance the overall charge • [Co(NH3)5Br]SO4vs[Co(NH3)5SO4]Br • IV. Solvate (Hydrate) Isomers • The solvent can play the role of ligand or as an additional crystal occupant • [CrCl(H2O)5]Cl2· H2O vs[Cr(H2O)6]Cl3

  13. A. Constitutional Isomers V. Coordination Isomers Same metal Formulation- Pt(NH3)2Cl2 [Pt(NH3)2Cl2] [Pt(NH3)3Cl][Pt(NH3)Cl3] [Pt(NH3)4][PtCl4] Same metal but different oxidation states [Pt(NH3)4][PtCl6] +2 +4 [Pt(NH3)4Cl2][PtCl4] +4 +2 Different Metals [Co(NH3)6][Cr(CN)6] [Co(CN)6][Cr(NH3)6]

  14. B. Stereoisomers • Enantiomers • Optical isomers (chiral) • Non-superimposable mirror image • Recall from group theory, something is chiral if • a. Has no improper rotation axis (Sn) • b. Has no mirror plane (S1) • c. Has no inversion center (S2) Square planar complex If it were tetrahedral, it would not be chiral.

  15. B. Stereoisomers • II. Diastereomers • Geometric isomers • 4-coordinate complexes • Cis and trans isomers of square-planar complexes (cis/transplatin) • Chelate rings can enforce a cis structure if the chelating ligand is too small to span the trans positions

  16. B. Stereoisomers • II. Diastereomers • Geometric isomers • 6-coordinate complexes • Facial(fac) arrangement of ligands • Meridional(mer) arrangement of ligands • Two sets of ligands segregated into two perpendicular planes. • Two sets of ligands segregated to two different faces.

  17. B. Stereoisomers • II. Diastereomers • Geometric isomers • 6-coordinate complexes • Different arrangements of chelating ring

  18. B. Stereoisomers • Conformational isomers • Because many chelate rings are not planar, they can have different conformations in different molecules, even in otherwise identical molecules.

  19. B. Stereoisomers • Conformational isomers • Ligands as propellers

  20. B. Stereoisomers • Conformational isomers • Ligand symmetry can be changed by coordination. Coordination may make ligands chiral as exhibited by the four-coordinate nitrogens.

  21. C. Separation of Isomers • Fractional crystallization can separate geometric isomers. • Strategy assumes isomers have different solubilities in a specific solvent mixture and will • not co-crystallize. • b. Ionic compounds are least soluble when the positive and negative ions have the same size and magnitude of charge. • Large cations will crystallize best with large anions of the same charge. • Chiral isomers can be separated using • a. Chiral counterions for crystallization • b. Chiral magnets

  22. C. Identification of Isomers • X-ray crystallography • Spectroscopic methods • In general, crystals of different handedness rotate light differently. • a. Optical rotatory dispersion (ORD): Caused by a difference in the refractive indices of the right and left circularly polarized light resulting from plane-polarized light passing through a chiral substance. • b. Circular dichroism(CD): Caused by a difference in the absorption of right-and left-circularly polarized light.

  23. 3. Coordination Numbers and Structures • Common Structures • Factors involved: • VSEPR considerations • Occupancy of metal d orbitals • Sterics • Crystal packing effects

  24. 3. Coordination Numbers and Structures • a. Low coordination numbers • Making bonds makes things more stable. • Coordination number = 1 • Rare for complexes in condensed phases (solids and liquids). • Often solvents will try to coordinate.

  25. 3. Coordination Numbers and Structures • ii. Coordination number = 2 • Also rare • Ag(NH3)2+; d10 metal • Linear geometry • iii. Coordination number = 3 • [Au(PPH3)3]+; d10 metal • Trigonal planar geometry

  26. 3. Coordination Numbers and Structures • b. Coordination Number = 4 • Avoid crowding large ligands around the metal. • Tetrahedral geometry is quite common • Favored sterically • Favored for L = Cl-, Br-, I- and • M = noble gas or pseudo noble gas configuration • Ones that don’t favor square planar geometry by ligand field stabilization energy. • Square planar • Ligands 90° apart • d8 metal ions; M(II) • Smaller ligands, strong field ligands that π-bond well to compensate for no six-coordination • Cisand trans isomers

  27. 3. Coordination Numbers and Structures • c. Coordination Number = 5 • Trigonalbipyramidalvs square pyramidal • Highly fluxional • Isolated complexes tend to be a distorted form of one or the other D3h C4v • TBP Geometry favored by: • d1, d2, d3, d4, d8, d9, d10 metal ions • Electronegative ligands prefer axial position • Big ligands prefer equatorial position • SqPyr Geometry favored by: • d6 (low spin)metal ions

  28. 3. Coordination Numbers and Structures • c. Coordination Number = 6 • Mostly octahedral geometry (Oh) • Favored by relatively small metals • Isomers • ii. Distortions from Oh • Tetragonal distortions: Elongations or compressions along Z axis • Symmetry becomes D4h

  29. 3. Coordination Numbers and Structures • Trigonal distortions (Elongation or compression along C3 axis) • Trigonal prism (D3h) • Favored by chelates with small • bite angles or specific types of • ligands • Trigonalantiprism(D3d) • Rhombic distortions (Changes in two C4 axes so that no two are equal; D2h)

  30. 3. Coordination Numbers and Structures • Coordination Number = 7 • Not common • Pentagonal bipyramid • Capped octahedron • 7th ligand added @ triangular face • Capped trigonal prism • 7th ligand added @ rectangular face

  31. 3. Coordination Numbers and Structures • Coordination Number = 8 • Not common • Cube • CsCl • Trigonal dodecahedron • Square antiprism

  32. 3. Coordination Numbers and Structures • II. Rules of thumb • Factors favoring low coordination numbers: • Soft ligands and metals in low oxidation states • Large bulky ligands • Counterions of low basicity • “Least coordinating anion” to make low coordinate • BArF

  33. 3. Coordination Numbers and Structures • II. Rules of thumb • Factors favoring high coordination numbers: • Hard ligands and metals in high oxidation states • Small ligands • Large nonacidic cations

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