Properties of Transition Elements

Characteristic Features of Transition Elements

Transition elements (the d-block elements) share a group of characteristic physical and chemical properties that arise mainly from their electron configurations and from the presence of partially filled $d$ subshells. This section focuses on what is typical for transition elements as a class, not on the detailed chemistry of each individual metal.

Electronic Configuration and Variable Oxidation States

The most fundamental property of transition metals is the presence of one or more electrons in $d$ orbitals in atoms or common ions.

• Typical valence-shell pattern (for a neutral atom):
  $$\text{(noble gas core)}\; ns^2 (n-1)d^{1\text{–}10}$$
• The $ns$ and $(n-1)d$ orbitals are close in energy.
• This small energy difference allows:
• Loss of different numbers of $ns$ and $(n-1)d$ electrons
• Formation of multiple stable oxidation states for many transition metals

Examples (in common compounds):

• Titanium: Ti(II), Ti(III), Ti(IV)
• Iron: Fe(II), Fe(III)
• Manganese: Mn(II), Mn(IV), Mn(VII)
• Copper: Cu(I), Cu(II)

Some general tendencies:

• Lower oxidation states often involve more $ns$ electrons.
• Higher oxidation states increasingly involve $d$ electrons.
• The maximum oxidation state often corresponds approximately to the total number of $ns$ + $d$ valence electrons (especially in the early transition metals, e.g. V(V), Cr(VI), Mn(VII)).

These variable oxidation states underlie much of the redox chemistry and coordination chemistry of the transition elements.

Metallic and Physical Properties

Transition elements are all metals and show a number of common physical features:

• Metallic bonding
• Involves delocalized $s$ electrons and partially delocalized $d$ electrons
• Leads to typical metallic properties: luster, malleability, ductility
• High electrical and thermal conductivity
• Due to the availability of mobile electrons
• Copper, silver, and gold are particularly good conductors
• High density and high melting/boiling points
• Strong metallic bonding (contribution of $d$ electrons)
• Many transition metals are significantly denser and have higher melting points than $s$-block metals in the same period
• Hardness and mechanical strength
• Many transition metals are hard and strong (e.g. Fe, Cr, W)
• Often used structurally (steel, alloys)

Trends (qualitative):

• Melting and boiling points often rise toward the middle of a period and then decrease slightly toward the end.
• Density is generally high throughout the d-block, especially in 4d and 5d series (e.g. Pt, Au, W, Os).

Formation of Colored Compounds and Ions

A striking property of many transition-metal compounds, especially aqueous ions and complexes, is color.

Typical examples:

• $\ce{[Ti(H2O)6]^{3+}}$: purple
• $\ce{[Cr(H2O)6]^{3+}}$: violet to green (depending on ligands and concentration)
• $\ce{[Fe(H2O)6]^{2+}}$: pale green
• $\ce{[Cu(H2O)6]^{2+}}$: blue
• $\ce{[Ni(H2O)6]^{2+}}$: green

Main reasons:

• Presence of partly filled $d$ orbitals.
• In complexes, the $d$ orbitals split into sets of slightly different energies (details in coordination chemistry).
• Absorption of light promotes an electron from a lower to a higher $d$ level ($d$–$d$ transition).
• The transmitted or reflected light appears colored (complementary to the absorbed color).

Key points:

• Ions/compounds with no $d$ electrons or with fully filled $d^{10}$ configurations are often colorless or only weakly colored (e.g. $\ce{Zn^{2+}}$, $\ce{Cu^+}$ complexes).
• Changing the oxidation state or the ligand environment often changes the color significantly.

Magnetic Properties

Transition elements show characteristic paramagnetic or diamagnetic behaviors depending on the number of unpaired $d$ electrons.

• Paramagnetism
• Occurs in species with one or more unpaired electrons.
• Substances are weakly attracted into a magnetic field.
• Common for many first-row transition metal ions in solution (e.g. $\ce{Ti^{3+}}$, $\ce{V^{2+}}$, $\ce{Mn^{2+}}$, $\ce{Fe^{2+}}$, $\ce{Co^{2+}}$, $\ce{Ni^{2+}}$).
• Diamagnetism
• All electrons are paired.
• Substances are weakly repelled by a magnetic field.
• Examples: $\ce{Zn^{2+}}$ ($3d^{10}$), some low-spin complexes of $\ce{Fe^{2+}}$ and $\ce{Co^{3+}}$.
• Ferromagnetism
• Strong, permanent magnetism in bulk materials.
• Arises from ordered alignment of atomic magnetic moments.
• Classical examples: Fe, Co, Ni, and some of their alloys.

Magnetism can provide information about:

• The number of unpaired electrons
• The oxidation state
• The type (high-spin vs. low-spin) of certain coordination complexes

Complex Formation and Coordination Tendencies

Transition elements have a pronounced tendency to form coordination compounds (complexes) with ligands.

Characteristic aspects:

• The metal center accepts electron pairs into vacant $d$, $s$, or $p$ orbitals.
• Typical ligands: $\ce{H2O}$, $\ce{NH3}$, halide ions ($\ce{Cl^-}$, $\ce{Br^-}$), $\ce{CN^-}$, $\ce{CO}$, and many organic ligands.
• Common coordination numbers: 4, 5, 6 (but others occur).

Consequences:

• Many transition metal ions in solution exist almost exclusively as complexes (e.g. $\ce{[Cu(H2O)6]^{2+}}$, $\ce{[Fe(H2O)6]^{3+}}$).
• Complex formation strongly influences:
• Solubility
• Color
• Redox potential
• Reactivity and catalytic behavior

These complex-forming tendencies are central to:

• Coordination chemistry
• Bioinorganic chemistry (metal ions in enzymes, hemoglobin, etc.)
• Analytical and industrial applications

Catalytic Activity

Many transition metals and their compounds act as catalysts in a wide range of reactions.

General reasons:

• Availability of multiple oxidation states.
• Ability to form intermediates via coordination of reactants.
• Possibility of reversible electron transfer and bond formation/breaking at the metal center.

Examples (types, not mechanisms):

• Heterogeneous catalysts: Fe in ammonia synthesis, Ni and Pt in hydrogenations.
• Homogeneous catalysts: organometallic complexes of Rh, Pd, Pt, etc., in organic synthesis and industrial processes.

Typical characteristics:

• Catalysts often involve change of oxidation state during the catalytic cycle.
• Ligands and environment around the metal strongly influence catalytic activity and selectivity.

Trends Across a Transition Series

Within a given transition-metal period (e.g. 3d from Sc to Zn), some systematic trends can be observed:

• Atomic and ionic radii
• Decrease slightly from left to right due to increasing nuclear charge.
• Contribute to similar sizes and chemistry across the series.
• Ionization energies
• First ionization energy increases moderately left to right.
• Successive ionizations show characteristic jumps when new subshells are involved.
• Preferred oxidation states
• Early transition metals (e.g. Ti, V, Cr, Mn) often favor higher oxidation states.
• Later ones (e.g. Fe, Co, Ni, Cu) stabilize lower oxidation states.
• Very late metals (e.g. Zn) typically exhibit only one common oxidation state (Zn(II)).
• Magnetic and color trends
• Number of unpaired $d$ electrons rises to a maximum near the middle of the series and then decreases.
• Magnitude of paramagnetism and intensity of $d$–$d$ colors often follow these changes.

Comparison with Main Group Metals

Transition elements differ from many main group metals in characteristic ways:

• Frequent occurrence of several oxidation states vs. usually one or two for main group metals.
• Strong tendency to form colored, paramagnetic complexes.
• More extensive catalytic roles and redox flexibility.
• Often harder and higher melting due to stronger metallic bonding.

These differences make the transition elements particularly important in:

• Structural materials (steels, alloys)
• Catalysis and industrial chemistry
• Biological systems
• Electronic and magnetic applications