Complexes of the d-Block Elements

1
Complexes of the d-Block Elements

• Transition Metal Chemistry

2
Complexes of the d-Block Elements

• Ions of d-block elements are excellent Lewis
  acids (electron pair acceptors).
• They form coordinate covalent bonds with
  molecules or ions that can act as Lewis bases
  (electron pair donors).
• Complexes formed in this way participate in many
  biological reactions (e.g., hemoglobin, vitamin
  B12) and are important in other ways as well
  (e.g., catalysis, dyes, solar energy conversion).

3
Terminology

• Ligand - a molecule or ion attached to a central
  metal atom or ion. Typically, there are four or
  six ligands in a complex. coordinate (verb) - to
  attach. (Ligands coordinate to the metal when
  they form the complex.)
• Coordination compound - a compound with a
  coordinate covalent bond.
• Coordination sphere - the sphere around the
  central ion made up of the ligands directly
  attached to it. Primary and secondary
  coordination sphere.
• Coordination number - the number of points where
  ligands are attached to the central metal atom.

4
(No Transcript)
5
Preparation of Complexes

• The figure at left shows cyanide ions (in the
  form of KCN), being added to an aq. solution of
  FeSO4.
• Since water is a Lewis base, the Fe2 ions were
  originally in the complex Fe(H2O)62
• The CN- ions are driving out the H2O molecules
  in this substitution reaction that form the
  hexacyanoferrate(II) ion, Fe(CN)64- .

Fe(H2O)62 6 CN-
Fe(CN)64 6 H2O
6
Various Colors of d-Metal Complexes
The color of the complex depends on the identity
of the ligands as well as of the metal..
Impressive changes of color often accompany
substitution reactions.
From left, Fe(SCN)(H2O)52, Co(SCN)4(H2O)22-,
Cu(NH3)4(H2O)22, and CuBr42-.
7
Structures and symmetries

• Six-coordinate complexes are almost all
  octahedral (a).
• Four-coordinate complexes can be tetrahedral (b)
  or square planar (c).
• (Square planar usually occurs with d8 electron
  configurations, such as in Pt2 and Au3.)

8
Representing Octahedral Shapes

• Instead of a perspective drawing (a), we can
  represent octahedral complexes by a simplified
  drawing that emphasizes the geometry of the
  bonds (b).

9
Representative Ligands and NomenclatureBidentate
Ligands

• Polydentate Ligands
• Some ligands can simultaneously occupy more than
  one binding site.
• Ethylenediamine (above) has a nitrogen lone pair
  at each end, making it bidentate. It is widely
  used and abbreviated en, as in Co(en)33.

10
(No Transcript)
11
Ethylenediaminetetraacetate Ion (EDTA)

• EDTA4- is another example of a chelating agent.
  It is hexadentate.
• This ligand forms complexes with many metal ions,
  including Pb2, and is used to treat lead
  poisoning.
• Unfortunately, it also removes Ca2 and Fe2
  along with the lead.
• Chelating agents are common in nature.

12
Porphyrins and phthalocyanins
13
Chelates

• The metal ion in Co(en)33 lies at the center
  of the three ligands as though pinched by three
  molecular claws. It is an example of a chelate,
• A complex containing one or more ligands that
  form a ring of atoms that includes the central
  metal atom.

14
Naming Transition Metal Complexes

• Cation name first then anion name.
• List first the ligands, then the central atom
• The ligand names are made to end in -O if
  negative
• Anion part of the complex ends in -ate
• Eg. Cu(CN)64- is called the hexacyanocuprate(II)
  ion
• The ligands are named in alphabetic order
• Number of each kind of ligand by Greek prefix
• The oxidation state of the central metal atom
  shown in parenthesis after metal name
• Briding is shown with ? (? -oxo)

15
Some Common Ligand Names
16
Names of Ligands (continued)
17
Examples

• Co(NH3)4Cl2Cl
• dichlorotetramminecobalt(III) chloride
• Pt(NH3)3Cl2PtCl4 di(monochlorotriammineplatin
  um(II)) tetrachloroplatinate(II).
• K3Fe(ox)(ONO)4
• potassium tetranitritooxalatoferrate(III)

18
Use bis and tris for di and trifor chelating
ligands

• Co(en)3(NO3)2
• tris(ethylenediamine)cobalt(II) nitrate
• Ir(H2O)2(en)2Cl3
• bis(ethylenediamine)diaquairidium(III) chloride
• Ni(en)33MnO4
• Tris(ethylenediamine)nickel(II)
  tetraoxomanganate(II)

19
The Eta(h) System of Nomenclature

• For for p bonded ligands number of atoms attached
  to the metal atom is shown by hn

(h5 -cyclopentadienyl) tricarbonyl
manganese tetracarbonyl (h3-allyl) manganese,
Mn(C3H5)(CO)4
20
Isomers

• Both structural and stereoisomers are found.
• The two ions shown below differ only in the
  positions of the Cl- ligand, but they are
  distinct species, with different physical and
  chemical properties.

21
(No Transcript)
22
Ionization Isomers

• These differ by the exchange of a ligand with an
  anion (or neutral molecule) outside the
  coordination sphere. CoSO4(NH3)5Br has the Br-
  as an accompanying anion (not a ligand) and
  CoBr(NH3)5SO4 has Br - as a ligand and
  SO42-as accompanying anion.

23
Ionization Isomers
The red-violet solution of Co(NH3)5BrSO4 (left)
has no rxn w/ Ag ions, but forms a ppt. when
Ba2 ions are added. The dark red solution of
CoSO4(NH3)5Br (right) forms a ppt. w/ Ag ions,
but does not react w/ Ba2 ions.
24
Hydrate Isomers

• These differ by an ex-change between an H2O
  molecule and another ligand in the coordination
  sphere.
• The solid, CrCl3. 6H2O, may be any of three
  compounds.
• Cr(H2O)6Cl3 (violet)
• CrCl(H2O)5Cl2.H2O (blue-green)
• CrCl2 (H2O)4Cl.2H2O (green)
• Primary and secondary coordination spheres

25
Linkage Isomers

• The triatomic ligand is the isothiocyanato,
  NCS-. In (b) it is the thiocyanato, SCN-.
• Other ligands capable or forming linkage isomers
  are
• NO2- vs. ONO -
• CN - vs. NC - .

(a) NSC- ligand (the N is closest to the center)
(b) SCN- ligand (S is closest the center)
26
Coordination Isomers

• These occur when one or more ligands are
  exchanged between a cationic complex and an
  anionic complex.
• An example is the pair Cr(NH3)6Fe(CN)6
  andFe(NH3)6Cr(CN)6.

27
Stereoisomers

• Ionization, hydrate, linkage, and coordination
  isomers are all structural isomers.
• In stereoisomers, the formulas are the same. The
  atoms have the same partners in the coordination
  sphere, but the arrangement of the ligands in
  space differs.
• The cis- and trans- geometric isomers shown in
  next slide differ only in the way the ligands are
  arranged in space.
• There can be geometric isomers for octahedral and
  square planar complexes, but not for tetrahedral
  complexes.

28
Square Planar ComplexesGeometric Isomers

• Properties of geometric isomers can vary greatly.
  
• The cis- isomer below is pale orange-yellow, has
  a solubility of 0.252 g/100 g water, and is
  used for chemotherapy treatment.
• The trans- isomer is dark yellow, has a
  solu-bility of 0.037 g/100 g water, and shows no
  hemotherapeutic effect.

29
cis and trans-PtCl2(NH3)2
30
Trans Effect Influence
31
Preparation Geometrical Isomers
32
Optical Isomerism
The two complexes at left are mirror images. (The
gray rectangle represents a mirror, through which
we see somewhat darkly.) No matter how the
com-plexes are rotated, neither can be
superimposed on the other. Note only four of
the six ligands are different.
33
Combined Stereoisomerisms

• Both geometrical and optical isomerism can occur
  in the same complex, as below. The trans-
  isomer is green.
• The two cis- isomers, which are optical isomers
  of each other, are violet.

34
(No Transcript)
35
Identifying Optical Isomerism
If a molecule or ion belong to a point group with
a Sn axis is is not optically active
36
(No Transcript)
37
Molecular Polarity and Chirality Polarity

• PolarityOnly molecules belonging to the point
  groups Cn, Cnv and Cs are polar. The dipole
  moment lies along the symmetry axis formolecules
  belonging to the point groups Cn and Cnv.
• Any of D groups, T, O and I groups will not be
  polar

38
Chirality

• Only molecules lacking a Sn axis can be chiral.
• This includes mirror planes
• and a center of inversion as
• S2s , S1I and Dn groups.
• Not Chiral Dnh, Dnd,Td and Oh.

39
Optical Activity
40
Bonding and electronic structure

• Bonding Theories of Transition Metal Complexes
• Valance Bond Theory
• Crystal Field Theory
• Ligand Field Theory or Molecular Orbital Theory

41
Valance Bond Theory

• Outer orbital" (sp3d2) and Inner orbital"
  (d2sp3)
• CoF63- - Co3 d6
• Co(NH3)63 - Co3 d6

42
Crystal Field Theory

• In the electrical fields created by ligands
• The orbitals are split into two groups a set
  consisting of dxy, dxz, and dyz stabilized by
  2/5Do, known by their symmetry
• classification as the t2g set, and a set
  consisting of the dx2-y2 and dz2, known as the eg
  set, destabilized by 3/5Do where Do is the gap
  between the two sets.

43
Crystal Field Splitting of d Orbitals
44
Octahedral Crystal Field Splitting
45
Crystal Field Stabilization Energy

• Crystal Field stabilization parameter Do

46
Crystal Field Stabilization Energy

• d7 case.
• Weak field case
• The configurations would be written t2g5 eg2
• 5(-2/5Do) 2(3/5Do) -4/5Do
• Strong field case
• The configurations would be written t2g6 eg1
• 6(-2/5Do) 1(3/5Do) -9/5Do

47
CFSE Paring Energy

• Fe(H2O)62. Iron has a d6 configuration, the
  value of Do is 10,400 cm-1 and the pairing
  energy is 17600cm-1. (1 kJ mol-1 349.76 cm-1.)
  We must compare the total of the CFSE and the
  pairing energy for the two possible
  configurations.

48

• high spin (more stable)
• CFSE 4 x -2/5 x 10400 2 x 3/5 x 10400
  -4160cm-1 (-11.89 kJ mol-1)
• Pairing energy (1 pair) 1 x 17600 17600 cm-1
  (50.32 kJ mol-1
• Total 13440 cm-1 (38.43 kJ mol-1)
• low spin
• CFSE 6 x -2/5 x 10400 -24960 cm-1 (-71.36 kJ
  mol-1)
• Pairing energy (3 pairs) 3 x 17600 52800
  (151.0 kJ mol-1)
• Total 27840 cm-1 (79.60 kJ mole-1)

49
Tetrahedral complexes

• Splitting order or reversed. eg is now lower
  energy and t2g is hgher energy
• Because a tetrahedral complex has fewer ligands,
  the magnitude of the splitting is smaller. The
  difference between the energies of the t2g and eg
  orbitals in a tetrahedral complex (t) is slightly
  less than half as large as the splitting in
  analogous octahedral complexes (o)
• Dt 4/9Do

50
Tetrahedral Ligand Arrangement
Dt 4/9Do Mostly forms high spin complxes
51
Octahedral Crystal Field Splitting
52
Square-planar Complexes-D4h
53
Generalizations about Crystal Field Splittings

• The actual value of D depends on both the metal
  ion and the nature of the ligands
• The splitting increases with the metal ion
  oxidation state. For example, it roughtly doubles
  going from II to III.
• The splitting increases by 30 - 50 per period
  down a group.
• Tetrahedral splitting would be 4/9 of the
  octahedral value if the ligands and metal ion
  were the same.

54
Spectrochemical Series for Ligands

• It is possible to arrange representative ligands
  in an order of increasing field strength called
  the spectrochemical series
• I lt Br lt -SCN lt Cl lt F lt OH lt C2O42 lt H2O
  lt -NCS lt py lt NH3 lt en lt bipy lt o-phen lt NO2 lt
  CN lt CO

55
Spectrochemical Series for Metals

• It is possible to arrange the metals according to
  a spectrochemical series as well. The approximate
  order is
• Mn2 lt Ni2 lt Co2 lt Fe 2 lt V2 lt Fe3 lt Co3
  lt Mn3 lt Mo3 lt Rh3 lt Ru3 lt Pd4 lt Ir3 lt Pt
  3

56
Spectrum of Ti(H2O)63.
d1 t2g1eg0 gt t2g0eg1
57
Hydration Enthalpy.

• M2(g) 6 H2O(l) M(O2H)62(aq)

58
Irving-Williams Series
59
Ligand Field Splitting and Metals

• the transition metal also impacts Do increases
  with increasing oxidation number
• Do increases as you move down a group (i.e.
  with increasing principal quantum number n)

60
Ligand Field Stabilization Energies

• LFSE is a function of Do
• weighted average of the splitting due to the
• fact that they are split into groups of 3 (t2g)
• and 2 (eg)

61
Weak Field vs. Strong Field

• now that d orbitals are not degenerate how do we
  know what an electronic ground state for a d
  metal complex is? need to determine the relative
  energies of pairing vs. Do

62
Splitting vs. Pairing

• when you have more than 3 but fewer than 8 d
• electrons you need to think about the relative
  merits
• pairing vs. Do
• high-spin complex one with maximum number of
  unpaired electrons
• low-spin complex one with fewer unpaired
  electrons

63
Rules of Thumb for Splitting vs Pairing

• depends on both the metal and the ligands
• high-spin complexes occur when o is small Do is
  small when
• n is small (3 rather than 4 or 5) high spin
  only really for 3d metals
• oxidation state is low i.e. for oxidation
  state of zero or 2
• ligands is low in spectrochemical series eg
  halogens

64
Four Coordinate Complexes Tetrahedral

• Same approach but different set of orbitals with
  different ligand field
• Arrangement of tetrahedral field of point
  charges results in splitting of energy where dxy,
  dzx, dyz are repelled more by Td field of
  negative charges
• So the still have a split of the d orbitals
  into triply degenerate (t2) and double degenerate
  (e) pair but now e is lower energy and t2 is
  higher.

65
Tetrahedral Crystal Field Splitting
66
Ligand Field Splitting Dt

• describes the separation between
  reviouslydegenerate d orbitals
• Same idea as Do but Dt lt 0.5 Do for comparable
  systems
• So Almost Exclusively Weak Field

67
Electron configurations in octahedral fields
Weak field and strong fieled cases
68
Tetragonal Complexes

• Start with octahedral geometry and follow the
• energy as you tetragonally distort the octahedron
• Tetragonal distortion extension along z and
• compression on x and y
• Orbitals with xy components increase in
• energy, z components decrease in energy
• Results in further breakdown of degeneracy
• t2g set of orbitals into dyz, dxz and dxy
• eg set of orbitals into dz2 and dx2-y2

69
Tetragonal Complexes
70
Square Planar Complexes

• extreme form of tetragonal distortion
• Ligand repulsion is completely removed from
• z axis

Common for 4d8 and 5d8 complexes Rh(I),
Ir(I) Pt(II), Pd(II)
71
Jahn Teller Distortion

• geometric distortion may occur in systems
• based on their electronic degeneracy
• This is called the Jahn Teller Effect
• If the ground electronic configuration of a
• nonlinear complex is orbitally degenerate, the
• complex will distort to remove the degeneracy
• and lower its energy.

72
Jahn Teller Distortions

• Orbital degeneracy for octahedral geometry
• these are
• t2g3eg1 eg. Cr(II), Mn(III) High spin complexes
• t2g6eg1 eg. Co(II), Ni(II)
• t2g6eg3 eg. Cu(II)
• basically, when the electron has a choice between
  one of the two degenerate eg orbitals, the
  geometry will distort to lower the energy of the
  orbital that is occupied.
• Result is some form of tetragonal distortion

73
Ligand Field Theory

• Crystal field theory simple ionic model, does
  not accurately describe why the orbitals are
  raised or lowered in energy upon covalent
  bonding.
• LFT uses Molecular Orbital Theory to derive the
  ordering of orbitals within metal complexes
• Same as previous use of MO theory, build ligand
  group orbitals, combine them with metal atomic
  orbitals of matching symmetry to form MOs

74
LFT for Octahedral Complexes

• Consider metal orbitals and ligand group orbitals
• Under Oh symmetry, metal atomic orbitals
  transform as
• Degeneracy Mulliken Label
  Atomic Orbital
• 2 eg
  dx2-y2, dz2
• 3 t2g
  dxy, dyz, dzx
• 3 t1u
  px, py, pz
• 1 a1g
  s

75
Sigma Bonding Ligand Group Orbitals
76
Combinationsof Metal andLigand SALCs
77
Molecular Orbital Energy Level Diagram Oh
78
PI Bonding

• pi interactions alter the
• MOELD that results from
• sigma bonding
• interactions occur between
• frontier metal orbitals and the
• pi orbitals of L
• two types depends on the ligand
• pi acid - back bonding accepts e- density from M
• pi base -additional e- density donation to the M
• type of bonding depends on relative energy
  level
• of pi orbitals on the ligand and the metal
  orbitals

79
PI Bases and the MOELD Oh

• pi base ligands
• contribute more
• electron density to
• the metal
• t2g is split to form a
• bonding and
• antibonding pair of
• orbitals
• Do is decreased
• halogens are good
• pi donors

80
PI Acids and the MOELD Oh

• pi acids accept electron
• density back from the
• metal
• t2g is split to form a
• bonding and antibonding
• pair of orbitals
• the occupied bonding
• set of orbitals goes
• down in energy so ..
• Do increases
• typical for phosphine
• and carbonyl ligands

81
Magnetic Properties of Atoms

• a) Diamagnetism?
• Repelled by a magnetic field due to paired
  electrons. b)Paramagnetism?
• attracted to magnetic field due to un-paired
  electrons.
• c) Ferromagnetism?
• attracted very strongly to magnetic field due to
  un-paired electrons.
• d)Anti-ferromagnetic?
• Complete cancelling of unpaired electrons in
  magnetic domains

82
Magnetic Suceptibility Vs Temperature
83
(No Transcript)
84
Magnetic Properties

• A paramagnetic substance is characterised
  experimentally by its (molar) magnetic
  susceptibility, cm. This is measured by
• suspending a sample of the compound under a
  sensitive balance between the poles of a powerful
  electro-magnet,

85
Number of Unparied Electrons

• The magnetic moment of the substance is given by
  the Curie Law
• m 2.54(cmT)½ (in units of Bohr
  magnetons)
• The formula used to calculate the spin-only
  magnetic moment can be written in two forms
• m ?n(n2) B.M.

86
Magnetic Properties of Atoms

• Paramagnetism?
• Ferromagnetism?
• Diamagnetism?
• Gouvy Balance

87
Octahedral Complexes
88
Tetrahedral Complexes
89
The lifetimes for ligand substitutions
90
Inert Labile Complexses

• The lifetimes for ligand substitutions span the
  range 109 s (i.e. diffusion limited) to 109 s
• (for heavier d-metals in high oxidation states,
  e.g. IrIII, PtIV ).
• complexes of the s-block metals are generally
  labile
• for s- and p-block metals complex lability
  increases in the order M3ltM2ltM
• apart from inert Vaq2 and very labile Cu2, 3d
  M2 complexes are moderately labile, lability
  decreasing with increase in d-electron count
• octahedrally co-ordinate Cu2 undergoes
  Jahn-Teller distortion monodentate axial ligands
  are weakly bound and readily exchanged
• M3 complex ions of the first transition series
  metals are not necessarily less labile than their
  M2 counterparts
• d-electron configuration is influential, d3 and
  d8 confer low lability
• complexes of low oxidation state d10 ions are
  very labile

91
Stepwise Formation Constants
M 4L -gt ML4
l. M L -gt ML K1 ML / M
L 2. ML L -gt ML2 K2 ML2 /
ML L 3. ML2 L -gt ML3 K3
ML3 / ML2 L 4. ML3 L -gt ML4
K4 ML4 / ML3 L Alternatively, we can
write the "Overall Foramtion Constant" thus
M 4L -gt ML4 b4 ML4/ M
L4 b4 K1.K2.K3.K4 or more generally, bn
K1.K2.K3.K4 --------------Kn
Normally K1gtK2gtK3gtK4
92
The Chelate Effect

• The chelate effect can be seen by comparing the
  reaction of a chelating ligand and a metal ion
  with the corresponding reaction involving
  comparable monodentate ligands.

93
Mechanisms of Ligand substitution

• The dissociative reaction is the predominant
  mechanism for substitution in octahedral
  complexes.
• Ni(OH2)62 L --gt Ni(OH2)5L2 H2O

94
Mechanisms of Ligand substitution

• The associative reaction is the predominant
  mechanism for substitution in tetrahedral
  complexes.
• Pt(Cl2) (py)2 L --gt Pt(Cl) L(py)2 Cl-