Chap 24. Transition Metals and Coordination Compounds. Hsu Fu-Yin

Transcription

1 Chap 24. Transition Metals and Coordination Compounds Hsu Fu-Yin

2 Gemstones Rubies are deep red and emeralds are brilliant green, yet the color of both gemstones is caused by the same ion Cr 3+ ions. Rubies are crystals of aluminum oxide (Al 2 O 3 ) in which about 1% of the Al 3+ ions are replaced by Cr 3+ ions. Emeralds are crystals of beryllium aluminum silicate [Be 3 Al 2 (SiO 3 ) 6 ] in which a similar percentage of the Al 3+ ions are replaced by Cr 3+. Ruby Emerald

3 Transition Metals

4 24.2 Properties of Transition Metals - Electron Configurations

5 Properties and Electron Configuration of Transition Metals The properties of the transition metals are similar to each other. And very different from the properties of the main group metals High melting points, high densities, moderate to very hard, and very good electrical conductors The similarities in properties come from similarities in valence electron configuration; they generally have two valence electrons.

6 TABLE 24.1 First-Row Transition Metal Orbital Occupancy

7 EXAMPLE 24.1 Writing Electron Configurations for Transition Metals Write the ground state electron configuration for Zr.

8 FIGURE 24.1 Trends in Atomic Radius Mo W Cr The atomic radius of Mo is larger than that of Cr Atomic radius of W is the same as that of Mo Why? 18 electrons are added in progressing from Cr to Mo, and all of them enter s, p, and d subshells. Between Mo and W, however, 32 electrons must be added, and 14 of them enter the 4f subshell. In the series of elements in which 4f the subshell is filled, atomic radii decrease. EX: This phenomenon occurs in the lanthanide series ( Z=58 to 71) and is called the lanthanide contraction.

9 24.2 Properties of Transition Metals - Ionization Energy The first Ionization Energy of the transition metals slowly increases across a series. The first Ionization Energy of the third transition series is generally higher than the first and second series Indicating the valence electrons are held more tightly Trend opposite to main group elements

10 24.2 Properties of Transition Metals - Electronegativity The electronegativity of the transition metals slowly increases across a series. Except for last element in the series Electronegativity slightly increases between first and second series, but the third transition series atoms are about the same as the second. Trend opposite to main group elements

11 24.2 Properties of Transition Metals - Oxidation States Unlike main group metals, transition metals often exhibit multiple oxidation states. The highest oxidation state for a transition metal is +7 for manganese (Mn). Highest oxidation state is the same as the group number for groups 3B to 7B.

12 24.3 Coordination Compounds When a monatomic cation combines with multiple monatomic anions or neutral molecules it makes a complex ion. The attached anions or neutral molecules are called ligands. The charge on the complex ion can then be positive or negative, depending on the numbers and types of ligands attached. [PtCl 4 ] 2- Co(NH 3 ) 6 3+

13 Coordination compound a complex ion combines with one or more counterions (ions of opposite charge that are not acting as ligands), the resulting neutral compound is a coordination compound. EX:

14 Coordination compounds Swiss chemist Alfred Werner studied coordination compounds. He proposed that the central metal ion has two types of interactions that he named primary valence and secondary valence. The primary valence is the oxidation state on the central metal atom the secondary valence is the number of molecules or ions directly bound to the metal atom, called the coordination number. the primary valence is +3 coordination number is 6

15 Coordination compounds CoCl 3 6H 2 O = [Co(H 2 O) 6 ]Cl 3 the primary valence is +3 coordination number is 6

16 Complex Ion Formation Complex ion formation is a type of Lewis acid base reaction. the ligands in coordination complexes is the ability to donate electron pairs to central metal atoms or ions. Ligand act as Lewis bases. In accepting electron pairs, central metal atoms or ions act as Lewis acids. A bond that forms when the pair of electrons is donated by one atom is called a coordinate covalent bond.

17 Ligands Ligands that donate only one electron pair to the central metal are monodentate. Ligands have the ability to donate two pairs of electrons (from two different atoms) to the metal; these are bidentate. Ligands, called polydentate ligands, can donate even more than two electron pairs (from more than two atoms) to the metal.

18 TABLE 24.2 Common Ligands

19 Chelating agent A chelate is a complex ion containing a multidentate ligand. The ligand is called the chelating agent.

20 Geometries in Complex Ions Metal ions with a d 8 electron configuration (such as [PdCl 4 ] 2- ) exhibit square planar geometry, Metal ions with a d 10 electron configuration (such as [Zn(NH 3 ) 4 ] 2+ ) exhibit tetrahedral geometry.

21 Naming Coordination Compounds Anions as ligands are named by using the ending o. normally -ide endings change to -o, -ite to -ito, and -ate to -ato.

22 Naming Coordination Compounds Neutral molecules as ligands generally carry the unmodified name. the name ethylenediamine is used both for the free molecule and for the molecule as a ligand. Aqua, ammine, carbonyl, and nitrosyl are important exceptions

23 Naming Coordination Compounds The number of ligands of a given type is denoted by a prefix. Mono, di, tri, tetra, penta, hexa Ex: pentaaqua signifies five molecules. If the ligand name is a composite name that itself contains a numerical prefix, such as ethylenediamine, place parentheses around the name and precede it with bis, tris, tetrakis, pentakis EX: presence of two ethylenediamine ligands, bis(ethylenediamine)

24 Names of Common Metals when Found in Anionic Complex Ions

25 Naming Coordination Compounds When we name a complex, ligands are named first, in alphabetical order followed by the name of the metal center. The oxidation state of the metal center is denoted by a Roman numeral If the complex is an anion, the ending -ate is attached to the name of the metal EX: Tetraaquadichlorochromium (III)

26 EXAMPLE 24.3 Naming Coordination Compounds Pentaaquachlorochromium(III) chloride. Potassium hexacyanoferrate(iii)

27 24.4 Structure and Isomerization Structural isomers are molecules that have the same number and type of atoms, but they are attached in a different order. Stereoisomers are molecules that have the same number and type of atoms, and that are attached in the same order, but the atoms or groups of atoms point in a different spatial direction.

28 Types of Isomers

29 Linkage Isomers Linkage isomers are structural isomers that have ligands attached to the central cation through different ends of the ligand structure.

30 Ligands Capable of Linkage Isomerization

31 Geometric Isomers Geometric isomers are stereoisomers that differ in the spatial orientation of ligands. cis trans isomerism in square-planar complexes MA 2 B 2

32 Geometric Isomers In cis trans isomerism, two identical ligands are either adjacent to each other (cis) or opposite to each other (trans) in the structure. cis trans isomerism in octahedral complexes MA4B2

33 Geometric Isomers In fac mer isomerism three identical ligands in an octahedral complex either are adjacent to each other making one face (fac) or form an arc around the center (mer) in the structure. fac mer isomerism in octahedral complexes MA3B3

34 EXAMPLE 24.5 Identifying and Drawing Geometric Isomers Draw the structures and label the type of all the isomers of Sol: The ethylenediamine (en) ligand is bidentate, Cl - is monodentate The total coordination number is 6, so this must be an octahedral complex. MA 4 B 2

35 Optical Isomers Optical isomers are stereoisomers that are nonsuperimposable mirror images of each other. Superimposable and nonsuperimposable objects an open-top box Superimposable nonsuperimposable ( 可重疊 ) ( 不可重疊 )

36 Optical Isomers Structures that are nonsuperimposable mirror images of each other are called enantiomers ( 鏡像異構物 ) and are said to be chiral ( 對掌 ) Structures that are superimposable are achiral.

37 24.5 Bonding in Coordination Compounds Valence Bond Theory

38 Crystal Field Theory Crystal Field Theory: Bonding in a complex ion is considered to be an electrostatic attraction between the positively charged nucleus of the central metal ion and electrons in the ligands. Repulsions also occur between the ligand electrons and electrons in the central ion. Crystal field theory focuses on the repulsions between ligand electrons and d electrons of the central ion.

39 d orbitals six anions to a metal ion to form a complex ion with octahedral structure

40 Repulsions between ligand electrons and d-orbital electrons are strengthened in the direct, head-to-head approach of ligands to the d z 2 orbitals and orbitals d x 2 -y 2 These two orbitals have their energy raised with respect to an average d-orbital energy for a central metal ion in the field of the ligands.

41 d xy, d xz, and d yz orbital energies are lowered with respect to the average d-orbital energy. The difference in energy between the two groups of d orbitals is called crystal field splitting (represented by the symbol o )

42 Splitting of d Orbital Energies Due to Ligands in an Octahedral Complex The size of the crystal field splitting energy, D, depends on the kinds of ligands and their relative positions on the complex ion, as well as the kind of metal ion and its oxidation state.

43 The Color of Complex Ions and Crystal Field Strength The color of an object is related to the absorption of light energy by its electrons. If a substance absorbs all of the visible wavelengths, it appears black. If it transmits (or reflects) all the wavelengths (absorbs no light), it appears colorless.

44 Complex Ion Color The observed color is the complementary color of the one that is absorbed. A substance that absorbs green light (the complement of red) will appear red.

45 Complex Ion Color To measure the energy difference between the d orbitals in a complex ion is to use spectroscopy to determine the wavelength of light absorbed when an electron makes a transition from the lower energy d orbitals to the higher energy ones.

46 Consider the [Ti(H 2 O) 6 ] 3+ absorption spectrum shown in Figure. The maximum absorbance is at 498 nm.

47 High spin & Low spin Whether the fourth electron enters the lowest level and becomes paired or, instead, enters the upper level with the same spin as the first three electrons depends on the magnitude of o o is less than the pairing energy, greater stability is obtained by keeping the electrons unpaired. (high spin) o 有電子排斥力 (pairing energy) 高能階

48 Ligands and Crystal Field Strength Ligands such as H 2 O and F - produce only a small crystal field splitting, leading to high-spin complexes; such ligands are said to be weak-field ligands. Ligands, such as NH 3 and CN - produce large crystal field splitting, leading to low-spin complexes; such ligands are said to be strong-field ligands. The size of the energy gap depends on what kind of ligands are attached. Strong field ligands include CN > NO 2 > en > NH 3 Weak field ligands include H 2 O > OH > F > Cl > Br > I. The size of the energy gap also depends on the type of cation. Increases as the charge on the metal cation increases Co 3+ > Cr 3+ > Fe 3+ > Fe 2+ > Co 2+ > Ni 2+ > Mn 2+

49 Magnetic Properties and Crystal Field Strength consider these two complexes of Co(III): sp 3 d 2 d 2 sp 3 paramagnetic (unpaired electrons) diamagnetic (paired electrons)

50 Tetrahedral Complexes For a tetrahedral complex, the d orbital splitting pattern is the opposite of the octahedral splitting pattern: three d orbitals (d xy, d xz, and d yz ) are higher in energy, and two d orbitals (d x 2 -y 2 and d z 2) are lower in energy

51 Square Planar Complexes A square planar complex gives us the most complex splitting pattern of the three geometries Z 軸的配位基跑至無窮遠處, 形成 Square-planar complex 造成有 z 的 d 軌域能量變低

52 EX: The complex ion [Ni(CN) 4 ] 2- is diamagnetic. Use ideas from the crystal field theory to speculate on its probable structure. Sol: The electron configuration of Ni is [Ar]3d 8 4s 2 and that of Ni(II) is [Ar]3d 8. Because the complex ion is diamagnetic, all 3d electrons must be paired. (a) if the structure were tetrahedra paramagnetic (b) if the structure were square-planar. diamagnetic

53 Applications of Coordination Compounds Extraction of metals from ores Silver and gold as cyanide complexes Nickel as Ni(CO) 4 (g) Use of chelating agents in heavy metal poisoning EDTA for Pb poisoning Chemical analysis Qualitative analysis for metal ions Blue = CoSCN + Red = FeSCN 2+ Ni 2+ and Pd 2+ form insoluble colored precipitates with dimethylglyoxime.

54 Biomolecules

55 Applications of Coordination Compounds Commercial coloring agents Prussian blue = mixture of hexacyanofe(ii) and Fe(III) Inks, blueprinting, cosmetics, paints

56 Applications of Coordination Compounds Cisplatin: A Cancer-Fighting Drug

57 Biological Applications: Porphyrins porphyrin structure metal porphin complex is called a porphyrin.

58 Biological Applications: Cytochrome C & Hemoglobin