Transition Metals

3.1

Electronic Configurations

• d-block elements formed as the 3d, 4d, 5d are filled with e^-
• Cr: [Ar]3d⁵4s¹ & Cu: [Ar]3d¹⁰4s¹
• 2^nd row & 3^rd row configurations aren’t obvious due to shielding effects and pairing electrons

Oxidation States

• the maximum oxidation states increase with group number, +3 for group 3, +7 for group 7
• further towards the end of a series it is difficult to use 4s & 3d e^- in bonding
• ex. Zinc is almost never used in bonding bc 3d e^- are core e^-
• when an element is in a lower state than the group #, it may possess unpaired d electrons, results in magnetic & optical properties

Ligands & Complexes

• Transition-Metal Complexes: a transition metal atom bonded to several ions or molecules
  • if it carries a charge it is called a complex ion or ionic complex
  • ligand: molecule/ion bonded directly to a transition metal (can be neutral or anions)
  • coordinated anions always end in the letter “o”
  • most metal-ligand bonds are polar covalent bonds, lone pairs of electrons are shared equally between the metal and the ligand
• Notation: square brackets ex. [Co(NH₃)₆]³⁺
  • Cation is always indicated first, with ligand in the square brackets
  • Counter-ions are not part of the complex: [Co(NH₃)₆]Cl₃
• To find the charge of the metal ion:
  • X + (charge of anion) = charge of complex, solve for x

Categories of Ligands

• Monodentate (one tooth): only one donor atom
• Bidentate ligand: 2 donor atoms
• Chelate ligand: 2/more donor atoms of the same ligand bound to the same metal centre, particularly strong bonding (chelate means claw)
• Bridging or terminal ligand

Isomerism

• Different spatial arrangement, same molecular formula, different connectivity
• Linkage Isomers: complexes differ by only the donor atom (type of constitutional isomer)
• when naming donor atom is in italics after the ligand name
• if the ligands are neutral, the charge is the same as the oxidation state of the metal, if ligands are anionic, complex may be neutral or negatively charged
• Coordination Isomers: combining complex cations and complex anions to make salts, which differ only by which metal is in the cation or anion
  • Ligand interchange between the 2 metal centres
• Ionization Isomer: type of constitutional isomer, results from an interchange of an anionic ligand with in coordination sphere with anion outside

Coordination Numbers & Stereochemistry

• Structure of the complex is defined by…
  • Coordination Number: # of atoms directly bound to the metal centre (# of teeth)
  • Stereochemistry: describes how coordinated atoms are arranged in space
• Coordination # 2 is linear
• Coordination # 3 is trigonal planar
• CN 2&3 are uncommon for transition metal ions except Cu(I), Ag(I), Au(I)
• Coordination # 4 is tetrahedral or square planar
  • to make neutral complexes from Co(II)/Pt(II), 2 neutral ligands & 2 anionic ligands are req’d
  • [CoX₂L₂] only has 1 possible structure (tetrahedral) because of bond angles
  • [PtX₂L₂] can have 2 arrangements, either X-X 90º/180º
• Cis isomer: same side (90º), Trans isomer: opposite (180º)
• Geometric isomers are not superimposable, different isomers have different functions
• Coordination # 5 is trigonal bipyramidal or square pyramidal
• Coordination # 6 is octahedral
  • If arrangement is [MA₂B₄]
    • Cis or trans for the A ligand
  • If arrangement is [MA₃B₃]
    • Facial (fae) isomer: 3 of the same ligands lie on the same face
    • Meridian (mer) isomer: 2 are trans & 1 is cis to these

Carbonyl Complexes

• Ligands are CO molecules (carbonyl group) (liquid @ room temp., volatile)

Porphyrins

• Type of ligand, the metal atom bonds to the middle (Fe²⁺), basic structure
  has a charge of -2
• N donor atoms coordinate to the metal center, 2 more ligands can bind to
  above and below to form octahedral geometry

Hemoglobin

• Transports oxygen from the lungs to the rest of the body
• Tetrahedral shape
  • Subunits composed of a protein (globin) linked to a heme (Fe²⁺ porphyrin) group in a square planar configuration
  • The 5^th coordination is a histidine side chain (amino acid globin) which produces O₂ when dissolved
  • The 6^th coordination can be O₂
• Hemoglobin is made up of 4 polypeptide chains (4 heme groups), each heme group has 1 Fe²⁺ atom
• Uses Le Châtelier’s principle to release the O₂ in deoxygenated areas
• inefficient, large molecule to carry small O₂ molecule
• CO is an competitor to O₂, the brain becomes oxygen deprived

3.2

Crystal Field Theory: electrostatic field of the ligands

Assumptions:

1. Transition metal ion is treated as a free ion (not bounded to ligands)
2. The e^- are treated as point charges
3. Ligands are negatively charged

Orbital Energies (d Orbital Splitting)

• For a free metal ion, all d orbitals are degenerate (same energies)
• If the metal is part of a octahedral complex, the energy levels are not the same as free metal ion because the environment is different
• In octahedral complexes is not spherically symmetrical, therefore the 6 point charges are in an octahedral arrangement along the metal-ligand bonding directions
• d_xy, d_xz, d_yz (point between the ligands) are degenerate, called t_2g orbitals (lower energy)
• d_z², d_x²-_y² (point directly at ligands) are degenerate, called e_g orbitals (higher energy)
• the energy separation between e_g and t_2g orbitals is Δ_o (‘o’ refers to octahedral)
• total energy is unchanged from barycenter (energy in spherical field) “if something goes up, something must go down”

Factors Affecting Δ_o

1. Oxidation State of Metal Ion: increases with increasing charge of the metal
2. Identity of the Metal: increases going down a group
3. Nature of the ligand: increases along
   spectrochemical series
4. Geometry of the complex: the d orbital
   splitting pattern of an octahedral
   complex is different from that of a
   tetrahedral/square planar complex

Colours of Transition Metal Complexes

• Our eyes see the colour complementary to the colour that is absorbed
• RGB Colour Model
• Light in the visible spectrum excites e^- from a t_2g to e_g orbital

v is frequency

Absorption Spectroscopy (UV-visible)

• Colours and colour intensities absorbed can be measured by passing light through it

Magnetism in Transition Metal Complexes

• more unpaired electrons = more paramagnetism
• High-Spin Configuration: fill all orbitals before doubling up
• Low-Spin Configuration: fill all the lower energy orbitals before the higher energy levels
• If Δ_o is small it doesn’t require that much energy to get into a higher energy level, therefore e^- would rather jump up than pair up (pairing requires more energy
• Pairing energy: required to overcome the repulsion between 2 e^- occupying the same orbital
• More stable when least repulsion