Transition Metal Properties

• Transition metals possess incompletely filled d subshells or readily produce ions with the same
• The group 2B metals (Zn, Cd, and Hg) do not have the same electron configuration and are not considered transition metals
• Having incompletely filled d subshells leads to distinctive coloring, formation of paramagnetic compounds, catalytic activity and a tendency to form complex ions
• Focus on the first-row elements from scandium to copper as the most common transition metals

Trends Across the Period

• As atomic numbers increase from left to right across a period, electrons are added to the outer shell, and nuclear charge increases with added protons
• For third-period elements (sodium to argon), outer electrons weakly shield each other which results in atomic radii decreasing rapidly from sodium to argon
• Electronegativities and ionization energies steadily increase
• For transition metals, the 3d electrons shield the 4s electrons from the increasing nuclear charge, and atomic radii decreases less rapidly
• Electronegativities and ionization energies increase only slightly from scandium to copper
• Transition metals are less electropositive than alkali and alkaline earth metals
• All transition metals except copper react with strong acids to produce hydrogen gas, but a protective oxide layer often makes them inert or slow to react
• Chromium is protected by a layer of chromium (III) oxide

Electron Configurations

• Calcium has the electron configuration [Ar]4s²
• Scandium to copper add electrons to the 3d orbitals
• Scandium is 4s²3d¹
• Titanium is 4s²3d²
• Chromium is 4s¹3d⁵
• Copper is 4s¹3d¹⁰
• Chromium and copper have irregularities due to the stability associated with half-filled and completely filled 3d subshells
• When first-row transition metals form cations, 4s electrons are removed first, then 3d electrons
• Iron (II) has an outer electron configuration of 3d⁶

Oxidation States

• Transition metals exhibit variable oxidation states in their compounds
• Common oxidation states include +2, +3, or both but +3 oxidation states are more stable at the beginning of the series
• At the end, +2 oxidation states are more stable
• Third ionization energy (removing a 3d orbital electron) increases more sharply than the first and second
• Metals at the end of the row form M²⁺ ions rather than M³⁺ ions
• Manganese has a +7 oxidation state (4s²3d⁵)
• Transition metals exhibit their highest oxidation states in compounds with very electronegative elements like oxygen and fluorine for example VF₅, CrO₃, and Mn₂O₇.
• Oxides with metals that have a high oxidation number are covalent and acidic
• Oxides with metals that have a low oxidation number are ionic and basic

Coordination Compounds

• Coordination compounds form complex ions
• It typically consists of a complex ion and counter ion
• Alfred Werner prepared and characterized many coordination compounds
• Werner's coordination theory explains the two types of valence: primary (oxidation number) and secondary (coordination number)
• The formula [Co(NH₃)₆]Cl₃ means, ammonia molecules and the cobalt atom form a complex ion, while chloride ions are separate and held by ionic forces
• Molecules or ions surrounding the metal in a complex ion are ligands
• Ligands act as Lewis bases by donating electrons to metals, which act as Lewis acids

Ligands

• Ligands need to donate one or more electron pairs
• Ligands play the role of Lewis bases, metal atoms are Lewis acids
• Metal-ligand bonds are usually coordinate covalent bonds
• The donor atom in a ligand is the atom bound directly to the metal atom
• In the [Cu(NH₃)₄]²⁺ complex ion, nitrogen is the donor atom
• The coordination number is the number of donor atoms surrounding the central metal atom

Ligand Classification

• Monodentate ligands present one donor atom each in H₂O and NH₃
• Bidentate ligands have two donor atoms each in ethylenediamine
• Polydentate ligands present multiple donor atoms

Oxidation Number

• Net charge of a complex ion is the sum of the charges on the central metal atom and its surrounding ligands
• In the [PtCl₆]²⁻ ion, each chloride ion has an oxidation number of -1, the oxidation number of Pt is +4
• If the 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, oxidation number of Cu is +2.

Naming Coordination Compounds

• Cation is named before the anion, as in other ionic compounds
• Ligands are named first alphabetically within a complex ion, and the metal ion is named last
• Anionic ligands end with the letter "o," while neutral ligands are usually called by the molecule name
• Multiple ligands of a kind use Greek prefixes such as di-, tri-, tetra-, penta-, and hexa-
• The ligands in the cation [Co(NH₃)₄Cl₂]⁺ are "tetraamminedichloro" and prefixes are ignored when alphabetizing ligands
• If the ligand contains a Greek prefix, prefixes bis (2), tris (3), and tetrakis (4) indicates the ligands present
• In bis(ethylenediamine, ethylenediamine contains "di" so use "bis"
• Oxidation number of the metal is written in Roman numerals
• [Cr(NH₃)₄Cl₂]⁺ is called tetraamminedichlorochromium(III) ion
• Anion names end in "-ate", so, the anion [Fe(CN)₆]⁴⁻ in K4[Fe(CN)₆] is called hexacyanoferrate(II) ion
• Metal atoms in anionic complexes are named using alternate names
• Tetracarbonylnickel(0) has a Ni atom with no net charge
• Sodium tetrafluoroaurate(III) is NaAuF₄ which leads to the compound requiring an Au with +3 charge
• Potassium hexacyanoferrate(III) has 3 negative charges in [Fe(CN)₆]³⁻ due to cyanide ions each with -1 charge
• In tris(ethylenediamine)chromium(III) chloride, "en" as ethylenediamine, meaning the compound is [Cr(en)₃]Cl₃

Geometry of Coordination Compounds

• A metal atom's structure and coordination number are related to each other
• A coordination number of 2 has a Linear structure
• A coordination number of 4 has a Tetrahedral or square planar structure
• A coordination number of 6 has an Octahedral structure
• Stereoisomers are compounds with the same types and numbers of atoms bonded together but differ in spatial arrangements
• Two types of stereoisomers: geometric isomers and optical isomers (enantiomers), coordination compounds may exhibit one or both types of isomerism
• [Ag(NH₃)₂]⁺ complex ion has a coordination number of 2 and a linear geometry
• Examples are [CuCl₂]⁻ and [Au(CN)₂]⁻
• [Zn(NH₃)₄]²⁺ and [CoCl₄]²⁻ ions have tetrahedral geometry, while [Pt(NH₃)₄]²⁺ ion has the square planar geometry
• Square planar complex ions with two different monodentate ligands exhibit geometric isomerism
• The cis and trans isomers of diamminedichloroplatinum(II) (Pt(NH₃)₂Cl₂) has the two Cl atoms adjacent to each other in the cis isomer, and diagonally across in the trans isomer
• Complex ions with a coordination number of 6 have octahedral geometry
• geometric isomers are possible in octahedral complexes when two or more different ligands are present
• The tetraamminedichlorocobalt(III) ion has two geometric isomers, which have different colors and properties but the same ligands and bond types
• Certain octahedral complex ions can give rise to enantiomers, such as cis isomers of dichlorobis(ethylenediamine)cobalt(III) ion and their mirror images
• The trans isomer and its mirror image are superimposable

Crystal Field Theory

• A satisfactory theory of bonding in coordination compounds must account for color, magnetism, stereochemistry, and bond strength
• Crystal field theory accounts for both the color and magnetic properties of many coordination compounds
• Crystal field theory explains the bonding in complex ions using electrostatic forces
• One kind of electrostatic interaction binds ligands to the metal (positive metal ion and negatively charged ligand)
• The other is electrostatic repulsion between ligand lone pairs and the metal's d orbital electrons
• D orbitals have different orientations, but have the same energy in the absence of external disturbance, but all five d orbitals experience electrostatic repulsion
• The magnitude of repulsion depends on the orientation of the d orbital

Crystal Field Splitting in Octahedral Complexes

• The lobes of the dx²-y² orbital point toward corners of the octahedron which results in greater repulsion from the ligands compared to electrons in the dxy orbital
• The dz² orbital's energy is also greater because its lobes point at the ligands along the z axis
• The five d orbitals are split between two energy levels: a higher level with two orbitals (dx²-y² and dz²) and a lower level with three orbitals (dxy, dyz, and dxz)
• The crystal field splitting (Δ) is the energy difference between two sets of d orbitals in a metal atom when ligands are present
• Crystal field splitting depends on the metal and ligands and affects color and magnetic properties

Color

• White light is a combination of all colors
• A substance appears black if it absorbs all visible light, and white/colorless if it absorbs none
• An object appears green if it absorbs all light but reflects the green component or reflects all colors except red
• Transmitted light passes through the medium
• Hydrated cupric ion, [Cu(H₂O)₆]²⁺, absorbs light in the orange region of the spectrum, so a solution of CuSO₄ appears blue
• When a photon energy equals the difference between the ground state and the excited state, absorption occurs
• Energy of a photon is given by E = hv
• Crystal field splitting is best measured using spectroscopy
• [Ti(H₂O)₆]³⁺ shows an example because Ti³⁺ has one 3d electron and 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

Spectrochemical Series

• Ligands are arranged in increasing order of their abilities to split the d orbital energy levels
• This order is; I⁻ < Br⁻ < Cl⁻ < OH⁻ < F⁻ < H₂O < NH₃ < en < CN⁻ < CO
• CO and CN⁻ are called strong-field ligands, which cause a large splitting of the d orbital energy levels
• Halide ions and hydroxide ion are weak-field ligands, because they split the d orbitals to a lesser extent

Magnetic Properties

• The magnitude of crystal field splitting also determines the magnetic properties of a complex ion
• [Ti(H₂O)₆]³⁺ has one d electron and is always paramagnetic
• In [FeF₆]³⁻ and [Fe(CN)₆]³⁻ for example, the electron configuration of Fe³⁺ is [Ar]3d⁵, and there are two distribution possibilities of five d electrons
• Maximum stability is reached when electrons are placed in five separate orbitals with parallel spins
• With five electrons, two electrons must be promoted to the higher-energy dx²-y² and dz² orbitals
• All five electrons enter the dxy, dyz, and d₁₂ orbitals at no energy investment, thus there will be only one unpaired electron
• The arrangement of electrons in low- and high-spin complexes is determined by the stability gained by having maximum parallel spins versus the investment required to promote electrons to higher d orbitals
• F⁻ is a weak-field ligand, the five d electrons enter five separate d orbitals with parallel spins to create a high-spin complex
• The cyanide ion is a strong-field ligand, all five electrons prefer to be in the lower orbitals, so a low-spin complex is formed
• High-spin complexes are more paramagnetic than low-spin complexes.
• The number of unpaired electrons/spins can be found through magnetic measurements, which support crystal field splitting predictions
• A distinction between low and high-spin complexes is made if the metal ion contains more than three and fewer than eight d electrons

Tetrahedral and Square-Planar Complexes

• Octahedral complexes are previously shown
• Splitting of d orbital energy levels in tetrahedral and square-planar complexes can be accounted for
• A tetrahedral ion’s splitting pattern is the reverse of that of octahedral complexes
• Ligands are more closely directed at the dxy, dyz, and dxz orbitals which therefore possess more energy
• Tetrahedral complexes are high-spin complexes due to reduced magnitude of metal-ligand interactions with a smaller value The splitting pattern is the most complicated for square-planar complexes
• The dx²-y² orbital has the highest energy (like octahedral), and the dxy orbital has the next highest
• The relative placement of the dz² and the dxz and dyz orbitals cannot be determined simply by inspection and must be calculated

Reactions of Coordination Compounds

• Complex ions undergo ligand exchange (or substitution) reactions in solution but rates vary
• Stability is often expressed using kinetic lability (tendency to react) and the species' formation constant Kf
• Tetracyanonickelate(II) is stable and has a large formation constant (Kf ≈ 1 × 10³⁰)
• This equilibrium is established when the species are mixed

[Ni(CN)₄]²⁻ + 4*CN⁻ ⇌ [Ni(*CN)₄]²⁻ + 4CN⁻

• [Ni(CN)₄]²⁻ is a labile complex due to rapid ligand exchange reactions
• A thermodynamically stable species isn't necessarily unreactive
• Smaller activation energy makes a larger rate constant and a greater rate
• [Co(NH₃)₆]³⁺ is thermodynamically unstable in acidic solution, equilibrium constant 1 × 10²⁰:

[Co(NH₃)₆]³⁺ + 6H⁺ + 6H₂O ⇌ [Co(H₂O)₆]³⁺ + 6NH₄⁺

• The reaction takes several days to complete due to the inertness of the [Co(NH₃)₆]³⁺ ion
• [Co(NH₃)₆]³⁺ ion is an inert complex, a complex ion that undergoes very slow exchange reactions
• Inertness does not mean low reactivity, it just determines if a species is chemically reactive
• Rate of reaction is determined by its activation energy, which is high
• Most complex ions and compounds containing Co³⁺, Cr³⁺, and Pt²⁺ are kinetically inert
• They are easy to study in solution because they exchange ligands slowly
• Knowledge comes from studying these compounds’ bonding, structure, and isomerism

Coordination Compounds in Living Systems

• Coordination compounds are essential in oxygen storage and transport, as electron transfer agents, catalysts, and in photosynthesis
• Coordination compounds that contain the porphyrin group and cisplatin are used as an anticancer drug
• Hemoglobin functions as an carriers of oxygen for metabolic processes
• Hemoglobin has four folded long chains called subunits and carries oxygen in the