Transition Metals and Coordination Compounds

Atomic Radius & Density of Transition Metals

Transition metals exhibit unique trends in atomic radius and density due to their electron configurations and position in the periodic table.

• Atomic Radius: The atomic radius of transition metals generally decreases across a period due to increasing nuclear charge, but the change is less pronounced than in main group elements.

• Density: Density increases as the atomic mass increases down the group. The increase in density down the group is more significant than across the period.

• Lanthanide Contraction: The effective nuclear charge (Zeff) increases, causing a slight decrease in atomic radius for elements after the lanthanides.

Example: Which element from each period would you predict to have the biggest atomic size? Answer: Y (yttrium) in period 5, La (lanthanum) in period 6.

Electron Configurations of Transition Metals

Transition metals occupy the d-block of the periodic table. Their electron configurations are determined by filling the 4s orbital before the 3d orbital, but exceptions exist.

• General Rule: For cations, electrons are lost from the highest principal quantum number (n) first.

• Condensed Electron Configuration: Uses the previous noble gas in brackets, followed by the remaining electrons.

Example: Condensed electron configuration for V3+: [Ar] 3d2

Exceptions in Electron Configurations

• Starting from chromium (Z = 24), exceptions occur due to stability associated with half-filled and fully-filled d subshells.

• Example: Cr (Z = 24): [Ar] 4s1 3d5 (not [Ar] 4s2 3d4)

• Additional exceptions are observed in period 5 (e.g., Nb, Mo, Ru).

Paramagnetism and Diamagnetism

The magnetic properties of transition metals depend on the presence of unpaired electrons.

• Paramagnetic: Atoms or ions with unpaired electrons; attracted to a magnetic field.

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

Example: V atom ([Ar] 4s2 3d3) is paramagnetic due to three unpaired electrons.

Ligands and Coordination Compounds

Ligands are molecules or ions that act as Lewis bases, donating electron pairs to a metal cation to form coordination compounds.

• Neutral Ligands: H2O, NH3, CO

• Anionic Ligands: Cl-, CN-, OH-

Ligand Classification

• Monodentate: Ligands with one donor atom (e.g., NH3, Cl-).

• Bidentate: Ligands with two donor atoms (e.g., ethylenediamine).

• Polydentate: Ligands with more than two donor atoms (e.g., EDTA).

Chelating Agents

• Polydentate ligands that form ring structures with the metal ion, increasing stability.

• Example: EDTA can bind to a metal ion through six donor atoms.

Coordination Number and Geometry

The coordination number is the number of ligand donor atoms bonded to the central metal ion.

• Common coordination numbers: 2, 4, 6

• Geometry depends on coordination number and ligand arrangement:

Naming Coordination Compounds

Coordination compounds are named using systematic rules:

• Name ligands in alphabetical order (prefixes do not count).

• Anionic ligands end in -o (e.g., chloro, cyano).

• Neutral ligands use their molecule name (exceptions: aqua for H2O, ammine for NH3).

• Metal name follows ligands; if the complex is an anion, use the Latin name of the metal with -ate ending.

• Indicate oxidation state of the metal in Roman numerals in parentheses.

Example: [Ag(NH3)2]Cl is named diamminesilver(I) chloride.

Writing Formulas of Coordination Compounds

To write formulas from names:

1. Identify cation and anion.

2. List formula of metal, ligands, and counterion.

3. Write formula of complex ion and identify its charge.

4. Balance charges outside the brackets.

Example: Triaquatriamminechromium(III) chloride: [Cr(H2O)3(NH3)3]Cl3

Isomerism in Coordination Complexes

Isomers are molecules with the same molecular formula but different connectivity or spatial orientation.

• Structural Isomers: Coordination isomers (ligands and counterions switch places), linkage isomers (ligand attaches through different atoms).

• Geometric Isomers: Ligands have different spatial orientation (cis/trans).

Example: [Co(NH3)4Cl2]+ can have cis and trans isomers.

d Orbital Orientations

d orbitals have different orientations in space, affecting their interactions with ligands.

• dz2 and dx2-y2 orbitals lie along the axes.

• dxy, dxz, dyz orbitals lie between the axes.

Crystal Field Theory

Crystal field theory explains the electronic structure and properties of coordination compounds by considering the effect of ligand electric fields on metal d orbitals.

• Octahedral Complexes: Ligands approach along the axes, causing splitting of d orbitals into t2g (lower energy) and eg (higher energy) sets.

• Tetrahedral Complexes: Ligands approach between the axes; splitting is reversed and smaller than in octahedral complexes.

• Square Planar Complexes: Strongest splitting, with dx2-y2 orbital at highest energy.

Crystal Field Splitting Energy: (octahedral) (tetrahedral)

Magnetic Properties of Complex Ions

The magnetic properties depend on the number of unpaired electrons and the strength of the ligand field.

• Low Spin Complexes: Strong-field ligands cause pairing of electrons, resulting in fewer unpaired electrons.

• High Spin Complexes: Weak-field ligands allow maximum number of unpaired electrons.

Example: [Co(NH3)6]3+ is a low-spin complex if NH3 acts as a strong-field ligand.

Strong-Field vs. Weak-Field Ligands

The magnitude of crystal field splitting () depends on the ligand:

• Strong-Field Ligands: CN-, CO, NO2-

• Weak-Field Ligands: H2O, F-, Cl-

Summary Table: Common Ligands and Their Field Strength

Practice and Application

• Determine electron configurations for transition metal ions.

• Identify ligand types and classify as mono-, bi-, or polydentate.

• Write systematic names and formulas for coordination compounds.

• Draw and identify isomers (structural, geometric, linkage).

• Predict magnetic properties and crystal field splitting diagrams.

Additional info: These notes cover the essential concepts for Chapter 24: Transition Metals and Coordination Compounds, including atomic properties, electron configurations, ligand types, coordination geometry, nomenclature, isomerism, and crystal field theory, as required for a General Chemistry college course.