Chapter 22: Metal Complexes

1
Chapter 22 Metal Complexes

• Chemistry The Molecular Nature of Matter, 6E
• Jespersen/Brady/Hyslop

2
Transition Metal Complexes

• Ligands
• Neutral molecules or ions that have lone pair of
  electrons that can be used to form bond to metal
• Lewis base electron pair donor
• Metal
• Lewis Acid electron pair acceptor
• Can accept more than one Ligand (Lewis base)
• ML bond
• Coordinate covalent bond
• Lewis acid base adduct formation

3
Transition Metal Complexes

• Coordinate Covalent Bond
• Both electrons in shared pair come from same atom
• Coordination Complexes
• Central metal atom surrounded by set of ligands
• Complex ion Co(NH3)63 , PtCl42?
• Coordination compound Ni(CO)4

4
Common Ligands

• 1. Monodentate M ?L
• 1 donor atom
• 1 lone pair
• 1 bond to metal
• Anions
• F, Cl, Br, I, O2, S2, NO2, C, OH, SCN,
  S2O32
• Molecules
• H2O, NH3, CO

5
Common Ligands

• 2. Chelate or Polydentate Ligands
• Have two or more atoms on one molecule with lone
  pairs
• Each of which can simultaneously form 2 e bonds
  to Mn
• Usually 5 or 6-membered rings with M
• Sometimes form 4-membered rings
• Must be nonlinear molecules

6
Bidentate Ligands

• Two possible sites of attachment
• Two Lewis base sites that can attach to Mn
• Part of larger class of chelate

Ethylenediamine (en)
2,2-bipyridine (bpy)
Oxalate (ox2?)
1,10-phenanthroline (phen)
7
Important Polydentate Ligands
EDTA4
Porphyrin
Corrin ring
8
Structure of EDTA Ligand and Complex

• H4EDTA common polydentate ligand
• EDTA4 used to complex metal ions
• 6 donor atoms
• Wraps around metal ion
• Forms very stable complexes

9
Complex Ions of Nickel (II)

• Left Green
• Ni2 with
• Six water ligands
• Ni(H2O)62
• Right Blue
• Ni2 with
• One water and five NH3 ligands
• Ni(NH3)5(H2O)2

10
Formulas of Complex Ions

• The symbol for the metal ion is always given
  first, followed by ligands.
• When more than one kind of ligand is present,
• Anionic ligands are first (in alphabetical order)
• Neutral ligands are next (in alphabetical order)
• Charge on complex is algebraic sum of charge on
  metal ion and charges on ligands
• The formula is placed inside of square brackets
  with the charge of the complex as a superscript
  outside the brackets, if it is not zero.

11
Ex. Co2 with 6 H2O and 2 Cl

• Ligands
• The six H2O molecules are bound to the Co2 ion
• Counterions
• The two Cl ions are there only to balance the
  charge
• Electrical neutrality

12
Learning Check

• 1. What is the formula for the complex made by
  Cu2 and four ammonia (NH3) molecules? Decide if
  the complex could be isolated as a chloride salt
  or a potassium salt. Write the formula of the
  appropriate salt.
• Cu2 has 2 charge
• NH3 is neutral
• So overall charge of ion is 2 4(0) 2
• Cu(NH3)42
• Need two Cl to make neutral complex
• Cu(NH3)4Cl2

13
Learning Check

• 2. What is the formula for the complex made by
  Ag and two cyanide (CN) ions? Decide if the
  complex could be isolated as a chloride salt or a
  potassium salt. Write the formula of the
  appropriate salt.
• CN is negative one charge
• So overall charge of ion is 1 2(1) 1
• Ag(CN)2
• Need 1 to make neutral complex
• So add one K ion
• KAg(CN)2

14
Your Turn!

• What is the formula for the complex made by Cr2
  and six ammonia molecules? Add chloride or
  potassium ions to form the appropriate salt.
• K2Cr(NH3)6
• Cr(NH3)6
• Cr(NH3)6Cl2
• Cr(NH3)6Cl
• KCr(NH3)6

15
Chelate Effect

• Extra stability that results when chelate ligands
  bind to metal ion
• Ni2(aq) 6NH3(aq) Ni(NH3)62(aq)
  Kform 2.0 108
• Ni2(aq) 3en(aq) Ni(en)32(aq)
  Kform 1.4 1017
• en is bidentate ligand (NH2CH2CH2NH2)
• NH3 is monodentate ligand
• Ni(en)32 is 2 109 times more stable than
  Ni(NH3)62

16
Why this Extra Stability?

• Entropy effect
• Look at the reverse process where metal loses the
  ligands.
• Ni(NH3)62(aq) 6H2O Ni2(aq) 6NH3(aq)
  Kinst 5.0 109
• Ni(en)32(aq) 6H2O Ni2(aq)
  3en(aq) Kinst 2.4 1018
• With NH3 ligands, same number of particles on
  each side
• With chelate ligand, more molecules as reactants

17
Metal Complex Nomenclature

• IUPAC Rules for naming Coordination Compounds
• Cation named first, then anion
• Names of anionic ligands always end in suffix o
  
• Ligands whose names end in ide have suffix
  changed to o

18
Metal Complex Nomenclature

• Rules for naming Coordination Compounds
• Ligands whose names end in ite or ate become
  ito and ato respectively

19
Some Common Ligands
20
Metal Complex Nomenclature

• Neutral ligands given same name as used for
  molecule except
• H2O aqua NH3 ammine
• When there is more than one of a given ligand,
  specify number of ligands by prefixes.

Number of same ligand Simple ligand prefix Complicated ligand prefix
2 di- bis-
3 tri- tris-
4 tetra- tetrakis-
5 penta-
6 hexa-
21
Metal Complex Nomenclature

• Ordering of ligands
• In formula, symbol of metal first followed by
  ligands
• Ligands order
• Anionic ligands first in alphabetical order
• Neutral ligands next also in alphabetical order
• In the name of the complex,
• Ligands named first in alphabetical order without
  regard to charge
• Metal named last

22
Nomenclature

• If the complex ion has a negative charge, the
  suffix ate is added to the name of the metal.

Metal As Named in Anionic Complex
Aluminum Aluminate
Chromium Chromate
Manganese Manganate
Nickel Nickelate
Cobalt Cobaltate
Zinc Zincate
Platinum Platinate
Vanadium Vanadate
23
Latin Names of Metals

1. Sometimes the Latin name of the metal is used.
2. Oxidation state of the metal is designated by
   Roman numeral in parentheses

24
Learning Check

• Name the Following
• Ag(CN)2
• dicyanoargentate(I) ion
• Zn(OH)42
• tetrahydroxozincate(II) ion
• Co(NH3)63
• hexamminecobalt(III) ion
• Mn(en)3Cl2
• tris(ethylenediamine)manganese(II) chloride

25
Your Turn!

• What is the correct name for Ni(Br)(CN)(NH3)2?
• nickel(II)cyanobromodiammine
• diamminebromocyanonickel(II)
• amminebromocyanonickel
• bromocyanodiamminenickel(II)
• bromocyanoammine nickel(II)

26
Learning Check

• Predict the formula from the following names
• tetracyanocuprate(I) ion
• Cu(CN)43
• triamminethiocyanoplatinum(III) ion
• PtSCN(NH3)32
• diamminetetraaquacopper(II) ion
• Cu(NH3)2(H2O)42
• potassium hexacyanoferrate(III)
• K3Fe(CN)6

27
Your Turn!

• Predict the formula of the coordination complex
  triamminedichlorocyanocobalt(III)
• CoCl2(CN)(NH3)3
• Co(NH3)3Cl2(CN)
• Cl2(CN)(NH3)3Co
• (NH3)3Cl2(CN)Co
• CoCl(CN)(NH3)

28
Coordination Number (CN)

• Number of bonds formed by metal ions to ligands
  in complex ions
• Varies from 2 to 8
• Depends on
• Size of central atom
• Steric interactions of ligands
• Electrostatic interactions
• e.g. Co(NH3)63 CN 6
• PtCl42? CN 4
• Ni(CO)4 CN 4
• CN 4 and 6 most common

29
Some Common Coordination Numbers (CN) of Metal
Ions
30
Structures

• CN 2 ML2 Linear
• CN 6 ML6 Octahedral

31
Structures

• CN 4 ML4 Tetrahedral
• CN 4 ML4 Square Planar

32
Your Turn!

• What is the coordination number of cobalt in
  CoCl2(en)2?
• 2
• 3
• 4
• 5
• 6

33
Isomers

• Existence of two or more compounds with same
  chemical formula and different physical
  properties
• Consider CrCl36H2O
• Can isolate three compounds with this formula
• Each with own characteristic color and distinct
  physical properties
• Cr(H2O)6Cl3 purple
• Cr(H2O)5ClCl2H2O blue-green
• Cr(H2O)4Cl2Cl2H2O green

34
Coordination Isomers

• Results from interchange of anionic ligand in
  first coordination sphere with anion outside
  coordination sphere
• Ex.
• Co(NH3)5Br(SO4) violet
• Co(NH3)5(SO4)Br red
• Easily distinguished by tests for counterion
• SO42(aq) Ba2(aq) ?? BaSO4(s)
• Br (aq) Ag(aq) ?? AgBr(s)

35
Stereoisomerism

• Difference among isomers that arises from various
  possible orientations of atoms in space
• Same atoms attached, but in different order in
  space
• Two major types
• Geometric isomerism
• Chirality or handedness

36
1. Geometric Isomerism

• CN 4 Square planar
• trans- and cis- isomers
• Occurs only with ML2X2
• trans- isomer
• Opposite each other
• cis- isomer
• Next to each other

trans-
cis-
37
Isomerism, Geometric, CN 6

• 1. Geometric (Structural)
• CN 6 Octahedral
• ML4X2
• trans- and cis- isomers

trans-
cis-
38
Isomerism, Geometric, CN 6

• Octahedral geometry
• Also get with two bidentate ligands
• (symbol L-L)
• M(L-L)2X2

39
Your Turn!

• Which of the following coordination complexes can
  have cis- and trans- isomers?
• Fe(H2O)63
• CuCl42
• NiBr(NH3)5
• Mn(NH3)4Cl2
• Pt(en)22

40
Chirality

• More subtle form of structural isomerism
• Differ only in handedness
• Right glove doesnt fit left hand
• Mirror-image object is different from original
  object

41
Superimposable

• If you can place mirror image on top of object
  and get same 3-D one-to-one coincidence
• For molecules
• Each atom in one molecule with equivalent atom in
  other molecule
• Chiral
• Object and its mirror image are NOT
  superimposable
• Enantiomers
• Two non-superimposable isomers
• e.g. Co(en)32

42
Isomerism, Chirality

• Chiral or Optical Isomers
• Most important occurs in octahedral geometry
• M(LL)3n

43
Geometric Isomers Not Necessarily Optical Isomers

• M(LL)2X2
• Two bidentate ligands and two monodentate ligands
  
• cis- isomer has two optical isomers
• Chiral with two enantiomers

44
Geometric Isomers Not Necessarily Optical Isomers

• M(LL)2X2
• Two bidentate ligands and two monodentate ligands
  
• Trans-isomer has no optical isomers
• Not chiral

45
Unpolarized and Polarized Light

• Light possesses electric and magnetic components
  that behave like vectors
• In unpolarized light, electromagnetic
  oscillations of photons oriented at random angles
  perpendicular to axis of light propagation

46
How to Determine Chirality

• Place solution in polarimeter and pass plane
  polarized light through it
• Enantiomers rotate plane-polarized light in
  opposite directions

47
Your Turn!

• Which two structures below represent a pair of
  optical isomers?
• A. D.
• B. E.
• C.

B and C
48
Crystal Field Theory

• Localized electron model doesnt work
• No information about how energies of d orbitals
  are affected by ligands when they form
• Transition metal complexes are usually colored
• Different ligands often give different colors

49
Crystal Field Theory

• 2. Magnetic properties of transition metal
  complexes often affected by what ligands are
  attached to metal
• Because transition metals have incomplete d
  subshells, complexes often paramagnetic
• But for given metal, number of unpaired spins
  varies
• e.g. Fe(H2O)62 four of its six 3d electrons
  are unpaired Fe(CN)64 has no unpaired spins
• Any theory that attempts to explain bonding in
  transition metal complexes must account for color
  and magnetic properties

50
Crystal Field Theory

• Crystal field theory
• Simplest model
• Purely electrostatic (ionic) model
• Ignores covalent bonding interactions with
  transition metals
• Assumes ligand lone pair point negative charge
• Repels electrons in d orbital on transition
  metals
• Allows us to understand and correlate all those
  properties that arise from presence of partly
  filled d subshells

51
Crystal Field Theory

• What is the effect of point negative charges on
  partially filled d orbitals?
• Look at d orbitals
• Four have same shape, but point in different
  directions
• Fifth has two lobes pointing along z axis and
  donut-shaped ring around center in x-y plane

52
Crystal Field Theory

• It is important what directions orbitals point
• Three point in between the two axes (x, y, and z)
  that are their label
• dxy, dxz, and dyz
• Other two point along the axis named in their
  label
• dx2 y2 , dz2

dx2 y2
dxy
dxz
dyz
dz2
53
Crystal Field Theory

• Now construct octahedral metal complex using this
  coordinate system
• Metal at origin
• Ligands coming in along positive and negativex,
  y, and z axes
• What is effect of point negative charges on
  energies of partially filled d orbitals?

54
Why are d orbitals split?

• Electron repulsions
• Place transition metal ion in octahedral ligand
  field of six ligands each with pair of electrons
• Come in along x, y, and z axes
• Two d orbitals are repelled more and
• Incoming lone pairs on ligands are pointing
  directly toward d orbitals containing electrons
• Three d orbitals repelled less dxy, dyz, dzx
• Incoming lone pairs on ligands pointing in
  between d orbitals containing electrons

55
Octahedral Crystal Field Splitting
(dx2 y2, dz2)
(dxy, dxz, dyz)

• ? Crystal field splitting h? hc/?
• Splitting of d orbitals leads to magnetic
  properties
• Magnitude of ? depends on ligand and metal

56
Spectrochemical Series

• Ligand that produces large ? with one metal
  produces large ? with other metals
• Ligands arranged in order of their effectiveness
  in producing large crystal field splitting
• Spectrochemical Series
• Common ligands in decreasing strength
• CN gt NO2 gt en gt NH3 gt H2O gt C2O42 gt OH gt F gt
  Cl gt Br gt I
• With same metal,
• CN produces largest ?
• I produces smallest ?

57
? Depends on Three Factors

• 1. ? depends on nature of ligand
• Some ligands produce larger splitting of d
  orbitals than other
• ?o increases as ligand field strength increases
• e.g. CN always gives large splitting
• F always gives small splitting
• Consequence
• Changing ligand changes ?
• Same metal ion can form variety of complexes with
  wide range of colors

58
? Depends on Three Factors

• 2. ? depends on oxidation state of metal
• For given metal and ligand set
• ? increases as oxidation state of M increases
• e.g. Fe lt Fe2 lt Fe3
• Why?
• As electrons are removed from M
• Charge on Mn becomes more positive and
• Ion size becomes smaller
• Result
• Ligands attracted to metal more strongly
• Greater repulsion with electrons in dx2 y2 and
  dz2 orbitals
• Leads to greater splitting of d orbitals and
  larger ?

59
? Depends on Three Factors

• 3. ? depends on row in which metal occurs
• For a given ligand set and oxidation state, ?
  increases as you go down a group.
• Fe3 lt Ru3 lt Os3
• e.g. Compare Ni2 and Pt2
• Pt2 has larger ?
• Pt2 is a larger ion
• Has larger and more diffuse d orbitals that
  extend farther from nucleus in direction of
  ligands
• Produces larger repulsion between electrons in
  ligands and d orbitals that point at them

60
Learning Check

• Which of the following pairs of complexes has the
  larger ??
• Fe(H2O)62 or Fe(H2O)63
• Fe(H2O)63 as higher metal oxidation state
• Cr(NH3)62 or Mo(NH3)62
• Mo(NH3)63 as metal lower in periodic table
• Pt(CN)42 or Pt(Cl)42
• Pt(CN)42 as CN is stronger field ligand

61
Your Turn!

• Which of the following complexes will have the
  largest ? splitting?
• Co(en)33
• Co(NH3)63
• Co(H2O)63
• Co(I)63
• Co(OH)63

62
Using Crystal Field Theory

• Can use d orbital splitting to explain relative
  stabilities of oxidation states of metals
• Ex. Cr2 easily oxidized to Cr3 in Cr(H2O)62
  and Cr(H2O)63
• To explain, look at electron configurations
• Cr Ar3d 5 4s1
• For Cr2 remove two electrons
• Cr2 Ar3d 4
• For Cr3 remove three electrons
• Cr3 Ar3d 3

63
Using Crystal Field Theory

• Put electrons into d orbitals using Hunds Rule
• With Cr2 we have choice
• Experimentally find fourth e goes into higher
  energy d set
• Cr3 only has three electrons all go into lower
  energy set of d orbitals
• Oxidizing Cr2 means removing an electron from
  higher energy orbital, leaving a lower energy
  complex

dx2 y2, dz2
dxy, dxz, dyz
Cr2 Ar 3d 4
dx2 y2, dz2
dxy, dxz, dyz
Cr3 Ar 3d 3
64
Why Transition Metal Complexes are Colored

• When light absorbed by molecule, atom or ion
• Energy of photon raises electrons to higher
  energy level
• Large energy difference, UV light required
• e.g. NaCl
• Appears white, as no visible light absorbed
• Small energy difference, visible light required
• e.g. Transition metal complexes d-orbital energy
  levels

65
Absorption of Light

• E h? hc /? ?
• When photon of light is same energy as spacing
  between d levels,
• Light absorbed
• Electron transfers from dxy, dyz, or dxz orbital
  to dx2 y2 or dz2 orbital

66
Color Wheel

• Green-blue is complementary color to red
• Yellow is complementary color to violet-blue
• If substance absorbs given color when bathed in
  white light
• Perceived color of reflected or transmitted light
  is complementary color

67
Absorption of Light

• Cr(H2O)63
• When an electron moves from one set of d orbitals
  to other
• Absorbs light with ? 5.22 1014 Hz
• Corresponds to yellow light
• Transmits violet, so solution appears violet

68
Effect of Ligand on ?

• Color absorbed depends on magnitude of ?
• As ? increases, energy of h? increases, and
  frequency of light increases
• For transition metals with same oxidation state,
  ? depends on ligand

Cr(H2O)63
Cr(NH3)63
NH3 induces larger ? than H2O Cr(NH3)63
absorbs higher energy light than Cr(H2O)63
Cr(NH3)63 absorbs blue light so appears
orange-yellow
69
Using Crystal Field Theory

• Cr2 has d 4 electron configuration
• First three go into lower level according to
  Hunds rule and aufbau principle
• Where does fourth electron go?
• May enter lower d level
• Must pair two electrons in one orbital, leads to
  repulsion
• Pairing Energy P
• Energy required to overcome Coulombic repulsion
  of putting two electrons in one orbital

70
Using Crystal Field Theory

• Where does fourth electron go?
• May enter higher d level
• Cost is ?
• Which Occurs?
• Depends on magnitude (size) of ?
• If ? gt P
• then most stable is pair of electrons in lower d
  level
• If ? lt P
• then most stable is to put fourth electron in
  higher d level

71
Using Crystal Field Theory

• Ex. Cr(H2O)63 vs. Cr(CN)63
• CN is strong field ligand
• ? gt P fourth electron pairs up in lower level
• H2O is relatively weak field ligand
• ? lt P Get minimum pairing of electrons

Large ?
Small ?
?
?
Cr(CN)64
Cr(H2O)62
72
Magnetic Properties by CFT

• Low spin complex
• Case with minimum number of unpaired spins
• Occurs with stronger field ligands (CN, en, bpy)
• High spin complex
• Case where you have maximum number of unpaired
  spins
• Occurs with weaker field ligands (H2O, Cl,
  Br,RS)

High spin
Low spin
?
?
Cr(H2O)62
Cr(CN)64
73
Learning Check

• Draw the CFT energy diagram for Fe(CN)64 which
  is diamagnetic
• CN is strong field ligand
• Large ?
• Low spin case

• Draw the CFT energy diagram for Fe(H2O)62
  which is paramagnetic
• H2O is weak field ligand
• Induces small ?
• High spin case

Low spin
High spin
?
?
Fe(H2O)62
Fe(CN)64
74
Your Turn!

• Which of the following crystal field diagrams is
  correct for Mn(CN)63 where CN is cyanide, a
  strong field ligand?
• D.
• E.

75
Learning Check

• Which complex should be expected to absorb
  highest energy light Fe(CN)64 (a yellow
  solution) or Fe(H2O)62 (a purple solution)?
  What color of light is absorbed in each case?
• Fe(CN)64
• Yellow solution
• Absorbs purple light
• Higher energy
• Makes sense as CN is a strong field ligand and
  will induce a larger ?

• Fe(H2O)62
• Purple solution
• Absorbs yellow light
• Lower energy

76
Your Turn!

• A complexCoA63 is red while the complex
  CoB63 is green. Which ligand, A or B,
  produces the larger crystal field splitting, ??
• A, because red is higher energy light
• B, because green is higher energy light
• Both ligands induce the same ?
• A, because it absorbs green light which is
  higher energy
• B, because it absorbs red light which is higher
  energy

77
Crystal Field Theory for Other Geometries

• Square planar
• Formed by removing ligands along z-axis
• d-orbitals along z-axis go down in energy
• Ligands in xy plane
• More tightly held
• More repulsions
• d-orbitals pointing along x and y axes go up in
  energy

78
Learning Check

• What is the distribution of electrons in the
  Ni(CN)62 ion?
• Ni2 has 8 valence electrons in d orbitals
• CN is a strong field ligand so electrons will
  pair up
• Using the diagram for square planar gives

79
Tetrahedral Ligand Field

• Ligands are approaching metal between axes
• Order of energy levels exactly opposite
• ? smaller for tetrahedral than for octahedral
• ?tet ? 4/9 ?
• ?tet always lt pairing energy
• After the lower orbitals are half filled, the
  next electron fills thehigher energy orbitals
• Tetrahedral complexes always high spin

80
Learning Check

• How many unpaired electrons are there in the
  tetrahedral complex, CoCl42? Show the crystal
  field splitting diagram to support this.
• Co(II) 3d 7 electron configuration
• Tetrahedral complexes are always high spin

81
Your Turn!

• Ni(CN)2Br2 is a diamagnetic, red complex with the
  formula. What color light does this complex
  absorb and does it have a square planar or
  tetrahedral geometry?
• Red, tetrahedral
• Green, square planar
• Red, square planar
• Green, tetrahedral
• Not enough information

• If it appears red, it absorbs green.
• Ni2 is d 8
• Square planar, diamagnetic
• Tetrahedral, paramagnetic

82
Some Biological Functions of Metals
  Body Function     Metal  
Blood pressure and blood coagulation   Na, Ca  
Oxygen transport and storage   Fe  
Teeth and bone structure   Ca  
Urinary stone formation   Ca  
Control of pH in blood   Zn  
Muscle contraction   Ca, Mg  
Maintenance of stomach acidity   K  
Respiration   Fe, Cu  
Cell division   Ca, Fe, Co  
83
Myoglobin (Mb) Hemoglobin (Hb)

• Both contain iron protoporphyrin IX (heme b)
• heme b active site of myoglobin and hemoglobin

heme b
84
Hemoglobin

• Each heme ring has Fe in center
• octahedral coordination
• Oxy-hemoglobin
• O2 bound
• Color is bright red
• O2 is strong field ligand
• Induces large ?
• ? E h? hc/?
• So shorter ?

85
Hemoglobin

• Deoxy-hemoglobin
• O2 not bound
• Color is blue-violet
• H2O replaces O2
• H2O is weak field ligand
• ? smaller
• So longer ?

Fe(II)
86
Vitamin B12

• Co2 in octahedral environment
• Ligands
• 4 nitrogen atoms in Corrin ring
• CN and adenosine
• Co-factor for many enzymes
• Essential to diet
• Humans do not make this in their bodies
• Absence leads to pernicious anemia
• Enzymes where vitamin B12 is needed
• Ribonuclease reductase
• Glutamate mutase
• Diol dehydratase
• Methionine synthetase