Transition Metals: Periodic Trends, Electron Configurations, and Coordination Chemistry

Atomic Radius of Transition Metals

The atomic radius is a measure of the size of an atom, typically the distance from the nucleus to the outermost electron shell. Transition metals follow general periodic trends, but with some unique features due to their electron configurations.

• Across a Period: Atomic radius generally decreases from left to right due to increasing nuclear charge, which pulls electrons closer to the nucleus. For transition metals, the decrease is less pronounced because electrons are added to inner d orbitals, which shield the outer electrons.

• Down a Group: Atomic radius increases as additional electron shells are added. However, for 5d transition metals, the increase is less than expected due to the lanthanide contraction (the poor shielding effect of the 4f electrons).

Example: Among Ni & Ti, Tc & Ru, Rh & Nb, Y & Ag, the element further down and to the left in the periodic table will generally have the largest atomic size.

Additional info: The lanthanide contraction causes 5d elements to have similar radii to 4d elements.

Density of Transition Metals

Density is the mass per unit volume of a substance. For transition metals, density increases as atomic mass increases, but the trend is influenced by atomic radius and packing structure.

• Density increases as the mass of the metal increases.

• The increase in density down a group is more significant than across a period.

Example: Os (Osmium) is a transition metal with one of the highest densities.

Transition Metals in the Periodic Table

Transition metals occupy the d-block (Groups 3-12) of the periodic table. Their unique properties arise from the filling of d orbitals.

• For cations, electrons are lost first from the highest principal quantum number (n) shell, usually the s orbital before the d orbital.

Example: The electron configuration for Ti (Z = 22) is [Ar] 4s2 3d2. For Ti3+, it is [Ar] 3d1.

Electron Configuration Exceptions

Some transition metals have exceptions to the expected electron configurations due to the stability of half-filled and fully-filled d subshells.

• Chromium (Cr, Z = 24): [Ar] 4s1 3d5 instead of [Ar] 4s2 3d4

• Copper (Cu, Z = 29): [Ar] 4s1 3d10 instead of [Ar] 4s2 3d9

These exceptions occur because half-filled (d5) and fully-filled (d10) subshells are particularly stable.

Example: The condensed electron configuration for Ag+ (Z = 47) is [Kr] 4d10.

Paramagnetic vs. Diamagnetic

Atoms and ions can be classified based on their magnetic properties:

• Paramagnetic: Contains one or more unpaired electrons; attracted to a magnetic field.

• Diamagnetic: All electrons are paired; weakly repelled by a magnetic field.

Example: Vanadium atom (V) is paramagnetic because it has unpaired electrons in its d orbitals.

Ligand Types and Classification

Ligands are molecules or ions that donate a pair of electrons to a metal cation to form a coordination complex. They can be classified as:

• Neutral Ligands: e.g., H2O, NH3, CO

• Anionic Ligands: e.g., Cl-, CN-, OH-

Ligands are also classified by the number of donor atoms:

• Monodentate: One donor atom (e.g., NH3, Cl-)

• Bidentate: Two donor atoms (e.g., ethylenediamine)

• Polydentate: More than two donor atoms (e.g., EDTA4-)

Example: Classify H2O as a monodentate ligand; ethylenediamine as bidentate.

Ligand Reactions and Adduct Formation

Ligands react with metal cations to form adducts (complexes) via Lewis acid-base reactions.

• The overall charge of the adduct is the sum of the metal cation and ligand charges.

Example: Cd2+ + 2 NH3 → [Cd(NH3)2]2+

Complex Ion Formation

A complex ion consists of a central metal cation bonded to one or more ligands. The formula is written with the metal first, followed by the ligands in brackets.

• Example: [Cu(NH3)4]2+

Example: Ti4+ + 6 CO → [Ti(CO)6]4+

Coordination Complexes and Nomenclature

Coordination complexes are ionic compounds containing complex ions and counterions to maintain charge neutrality.

• The complex ion is written in brackets, with the metal first, then the ligands.

• The counterion is written outside the brackets.

Example: [Ni(NH3)4]Cl2 contains the complex ion [Ni(NH3)4]2+ and two Cl- counterions.

Practice and Application

• Write electron configurations for transition metal ions (e.g., W4+, Cd2+).

• Determine if a species is paramagnetic or diamagnetic based on unpaired electrons.

• Identify ligand types and classify as monodentate, bidentate, or polydentate.

• Write formulas for coordination complexes and determine their overall charge.

Key Tables

Periodic Trends Table (Atomic Radius and Density):

Ligand Classification Table:

Important Formulas and Equations

• General electron configuration for transition metals: $[\text{Noble gas}]\ ns^2\ (n-1)d^{1-10}$

• For cations: Remove electrons from the highest n (s orbital) before d orbitals.

• Example: $\text{Fe}^{3+}: [\text{Ar}]\ 3d^5$

Summary Table: Paramagnetic vs. Diamagnetic

Additional info: For more details on naming coordination compounds, see IUPAC nomenclature rules for coordination chemistry.