Transition Metals and Coordination Chemistry

• Coordination compounds contain complex ions where molecules or ions surround a central metal atom or ion.
• Common geometries include linear, tetrahedral, square planar, and octahedral.
• Crystal field theory explains bonding in complex ions in terms of electrostatic forces.
• Ligands approaching the metal cause energy splitting in the five d orbitals.
• Crystal-field splitting is impacted by the type of ligands.
• Coordination compounds have important roles in animals and plants and therapeutic drug use.

Properties of the Transition Metals

• Transition metals have incomplete d subshells or form ions with incomplete d subshells.
• The group 2B metals (Zn, Cd, and Hg) do not fully belong in this category.
• Transition metals properties include distinct coloring, formation of paramagnetic compounds, catalytic activity, and complex ion formation.
• The first-row elements (Sc to Cu) are the most common transition metals.
• Atomic radii decrease and electronegativity increases across the period, but less rapidly than in main group elements.
• Most react with acids to produce hydrogen gas, but most are inert due to a protective layer of oxide.

Electron Configurations

• From Sc to Cu, electrons fill the 3d orbitals after the 4s orbitals.
• Cr and Cu are exceptions with outer electron configurations of 4s¹3d and 4s¹3d¹⁰.
• Half-filled and completely filled 3d subshells provide extra stability
• Electrons are removed from the 4s orbitals before the 3d when transition metals form cations.
• The outer electron configuration of Fe²⁺ is 3d⁶.

Oxidation States

• Transition metals exhibit variable oxidation states.
• Common oxidation states for each element include +2, +3, or both.
• The +3 oxidation states are more stable earlier in the series, and +2 oxidation states are more stable at the end.
• Transition metals have the highest oxidation states in compounds with electronegative elements such as O and F.
• Oxides with high oxidation numbers are covalent and acidic.
• Oxides with the low oxidation numbers are ionic and basic.

Coordination Compounds

• Coordination compound typically consists of a complex ion and counter ion.
• Werner's coordination theory explains much of coordination chemistry.
• Primary valence corresponds to oxidation number, and secondary valence corresponds to the coordination number of the element.
• Molecules or ions surrounding the metal in a complex ion are ligands.
• Interactions between a metal atom and the ligands are Lewis acid-base reactions.
• Ligands are Lewis bases that donates electrons.
• Metals are Lewis acids, accepting electrons.

Common Ligands

• An atom in a ligand bound directly to the metal atom is the donor atom.
• The coordination number is the number of donor atoms surrounding the central metal atom.
• Coordination numbers such as 4 and 6 are most common.
• Coordination numbers such as 2 and 5 are also known.
• Ligands are classified as monodentate, bidentate, or polydentate.
• Bidentate and polydentate ligands are chelating agents.
• EDTA, a polydentate ligand, is used to treat metal poisoning.

Oxidation Number of Metals in Coordination Compounds

• The net charge of a complex ion is the sum of charges on the central metal atom and the surrounding ligands.
• In [PtCl₆]²⁻, each chloride ion has an oxidation number of -1, so the oxidation number of Pt must be +4.
• If ligands do not bear net charges, the oxidation number of the metal is equal to the charge of the complex ion
• In [Cu(NH₃)₄]²⁺ each NH₃ is neutral, so the oxidation number of Cu is +2.

Naming Coordination Compounds

• The cation is named before the anion.
• Within a complex ion, the ligands are named first, in alphabetical order, and the metal ion is named last.
• Anionic ligand names end with the letter o, whereas neutral ligands are called by the molecule's name.
• Exceptions for neutral ligands, include H₂O (aqua), CO (carbonyl), and NH₃ (ammine)
• Greek prefixes, such as di-, tri-, tetra-, penta-, and hexa-, indicate multiple ligands of a particular kind.
• Roman numerals indicate the oxidation number of the metal.
• If the complex is an anion, its name ends in -ate.

Geometry of Coordination Compounds

• Metal atoms with monodentate ligands have four different geometric arrangements including Linear, Tetrahedral, Square planar, and Octahedral
• Stereoisomers are compounds with the same types and numbers of atoms bonded together in the same sequence but with different spatial arrangements.
• Types of stereoisomers are geometric isomers and optical isomers (enantiomers).
• [Ag(NH₃)₂]⁺ has a coordination number of 2 and a linear geometry.
• [Zn(NH₃)₄]²⁺ and [CoCl₄]²⁻ ions have tetrahedral geometry, whereas the [Pt(NH₃)₄]²⁺ ion has the square planar geometry.
• Square planar complex ions with two different monodentate ligands can exhibit geometric isomerism.
• Geometric isomers differ in properties like melting point, boiling point, color, solubility, and dipole moment.
• Complex ions with a coordination number of 6 all have octahedral geometry.
• Certain octahedral complex ions can give rise to enantiomers

Bonding in Coordination Compounds: Crystal Field Theory

• A bonding theory must account for properties like color and magnetism.
• Crystal field theory accounts for the color and magnetic properties.
• It explains that bonding is in complex ions in purely electrostatic forces.
• Electrostatic repulsion is between the lone pairs on the ligands and the electrons in the d orbitals of the metals.
• Magnitude of that repulsion depends on the orientation of the d orbital.
• Because of metal-ligand interactions, the five d orbitals in an octahedral complex are split between two sets of energy levels
• Crystal field splitting (Δ) is the energy difference between two sets of d orbitals in a metal atom when ligands are present.
• The magnitude of Δ directly affects the color and magnetic properties of complex ions.

Color

• White light is a combination of all colors.
• Substances appear black if it absorbs all visible light and white or colorless if it absorbs no visible light.
• An object appears green if it absorbs all light but reflects the green component.
• Reflected light applies to transmitted light.
• A hydrated cupric ion, [Cu(H₂O)₆]²⁺, absorbs light in the orange region and appears blue.
• Best to measure crystal field splitting is to use spectroscopy to determine the wavelength at which light is absorbed.
• [Ti(H₂O)₆]³⁺ absorbs light in the visible region of the spectrum.
• A d-to-d transition must occur for a transition metal complex to show color.
• Ions with d⁰ or d¹⁰ electron configurations are usually colorless.

Magnetic Properties

• The magnitude of the crystal field splitting also determines magnetic properties.
• [Ti(H₂O)₆]³⁺, having only one d electron, is always paramagnetic.
• [FeF₆]³⁻ and [Fe(CN)₆]³⁻ are an example of distributing the five d electrons among the d orbitals.
• All five electrons enter the dxy, dyz, and dxz orbitals with one unpaired electron.
• The electrons promoted to the higher-energy dx²-y² and dz² orbitals is the exception.
• The low- and high-spin complexes result in the distribution of electrons among d orbitals.
• Actual arrangement of the electrons is determined by stability gained by maximum parallel spins vs. the energy investment to promote electrons to higher d orbitals.
• F⁻ is a weak-field ligand, so the five d electrons enter five separate d orbitals with parallel spins to create a complex.
• The cyanide ion is a strong-field ligand, so it is energetically preferable for all five electrons to be in the lower orbitals.
• High-spin complexes are more paramagnetic than low-spin complexes.
• Aided by spectroscopic data, chemists calculated the crystal splitting for each ligand and established a spectrochemical series.
• Strong-field ligands cause a large splitting of the d orbital energy levels.
• Weak-field ligands split the d orbitals to a lesser extent.

Reactions of Coordination Compounds

• Complex ions undergo ligand exchange (or substitution) reactions in solution.
• Rates vary widely based on metal ion and ligands.
• Kinetic lability is a complex ion's tendency to react.
• Stability is a thermodynamic property measured in terms of the species' formation constant K_f.
• Tetracyanonickelate(II) is stable because it has a large formation constant, K_f ≈ 1 × 10³⁰
• Labile complexes undergo rapid ligand exchange reactions.

Coordination Compounds in Living Systems

• Coordination compounds are essential in the storage and transport of oxygen, as electron transfer agents, catalysts, and in photosynthesis.
• Coordination compounds containing the porphyrin group and cisplatin are anticancer drugs.
• Hemoglobin functions as an oxygen carrier. Hemoglobin contains oxygen carrying subunits.