Topic 1: Inorganic Chemistry I - Introduction to Transition Metals

Transition Metals (TMs)

Transition metals are elements in groups 3 to 11 of the periodic table that have partially filled d-orbitals.

Criteria for TMs

Elements must have an incompletely filled d-shell to be considered transition metals.

TM Exclusions

Zinc (Zn), Cadmium (Cd), and Mercury (Hg) in the 12th group are not counted as TMs by IUPAC because they have full d-shells (d¹⁰) and no valence d-orbitals.

Rows of TMs

There are four rows of transition metal elements. The first row includes chromium, iron, nickel, copper, and zinc. The fourth row elements are man-made and mainly studied in experiments.

Bonding with d-orbitals in TMs

d-orbitals are involved in bonding, unlike in most carbon-based chemistry where only s and p-orbitals are involved.

Capacity of d-Shell

A d-shell consists of 5 orbitals and can hold up to 10 electrons.

Nature of d-Orbitals

They are five degenerate orbitals (l = 2, ml = -2, -1, 0, 1, 2) in free atoms or ions that are highly directional, and poorly shielded.

d-Orbital Labels

d-orbitals are conventionally labeled d_xy, d_xz, d_yz, d_x²-y², and d_z².

d-Orbital Labels: d_xy, d_xz, d_yz

These orbitals have 4 lobes pointing in between the principal axes.

d-Orbital Labels: d_x²-y² and d_z²

• These orbitals have lobes pointing along the axes.

• d_z² has a dumbbell along Z axis and a torus in the XY plane.

General Electronic Configuration of TMs

[Noble gas] ns² (n-1)d^x, where:

• n ranges from 4 to 6

• x ranges from 1 to 10.

Filling Order of TMs

For neutral atoms, ns orbitals fill before (n-1)d orbitals: 4s before 3d, 5s before 4d, 6s before 5d.

• Example: 

  • Titanium (Ti) configuration is [Ar] 4s² 3d².

  • Vanadium (V) configuration is [Ar] 4s² 3d³.

Filling Order of TMs: Exceptions

Half-filled (d⁵) or fully filled (d¹⁰) subshells are extra stable and alter the filling order.

• Example:

  • Chromium (Cr) is [Ar] 4s¹ 3d⁵ instead of [Ar] 4s² 3d⁴.

  • Copper (Cu) is [Ar] 4s¹ 3d¹⁰ instead of [Ar] 4s² 3d⁹.

5th and 6th Period TMs 

Electronic configurations differ due to relativistic effects which become significant in heavier atoms → he high nuclear charge (Z) in heavy atoms causes → inner-shell electrons (and electrons with significant probability density close to the nucleus, like s electrons) to move at speeds approaching the speed of light 

*Beyond scope of the course

Variable Oxidation States in TM Ions

Transition metal cations exhibit multiple oxidation states.

• Left-side TMs commonly +3;

• right-side +2;

• middle elements (like Mn) range from +2 to +7.

TM Ionization Rule

When forming cations, ns electrons are removed before (n-1)d electrons, despite ns electrons being filled first as well

Electrons in Complexes

In complexes, as a result of the ionization rule, the 1st series TMs have remaining valence electrons in d-orbitals, not in 4s.

• Example:

  • Vanadium(II) ion: Neutral V is 4s² 3d³, V²⁺ loses 4s electrons to become [Ar] 4s⁰ 3d³.

d-Electron Count Formula

d-electron count = group number - oxidation number aka valence electrons - charge

Properties of TMs Due to d-Electrons

Transition metal complexes often show color, catalytic activity, and magnetic properties.

Properties of TMs Due to d-Electrons: Color

TM salts and complexes are often colored due to d-d transitions, while s- and p-block salts are usually colorless.

• Examples: Pink rubies from Cr³⁺, blue sapphires from Ti³⁺.

d-d Transition Mechanism

The ligands’ negative charges repel the d-orbitals differently depending on their orientation → degeneracy removed → d-orbitals in the same sub-shell split into lower and higher energy groups → when d-orbitals are partially filled; photons are absorbed by electrons matching the d-d energy gap → electron jumps

Perceived Color after d-d Transition

• While transitions with bigger gaps falls in the UV region (undetectable to the naked eye), d-d transitions have smaller gaps that fall in the visible light region.

• The color we see is complementary to the light absorbed by d-d transitions.

Enzyme and Catalyst Function of TMs

• TMs are critical in enzymes (biological catalysts) and industrial catalysts due to variable oxidation states.

• Variable oxidation states allow for flexibility → better bioavailability

Enzyme and Catalyst Function of TMs: Nitrogenase

• Enzyme that makes nitrogen more bioavailable

  • The N² is triple bonded, requiring a lot of energy to separate

  • Nitrogenase cracks that apart and converts it: N₂ + 6e^- + 8H⁺ → 2NH₄⁺

  • At the center is a cluster of metals: Fe₇MoS₉C

  • Having it in ammonium form makes it more water-soluble

Enzyme and Catalyst Function of TMs: Cytochrome-C Oxidase

• Enzyme that aids in respiration

• Made up of coordination compounds with metals like iron and sulfur surrounded by ligands

• It catalyzes: C₂ + 8H⁺ + 4e^- (in) → 2H₂O + 4H⁺ (out)

Paramagnetic

Compounds with unpaired electrons, strongly attracted to magnetic fields.

Diamagnetic

Compounds with all paired electrons, weakly repelled by magnetic fields.

Magnetic Properties of TMs: Spin-Crossover

Compounds can switch between diamagnetic and paramagnetic with temperature changes.