💍Inorganic Chemistry II Unit 4 – Coordination Compound Reaction Mechanisms

Key Concepts

• Coordination compounds consist of a central metal atom or ion surrounded by ligands, which are ions or molecules that donate electron pairs to the metal
• The coordination number refers to the number of ligands directly bonded to the central metal atom or ion and typically ranges from 2 to 9, with 4 and 6 being the most common
• Ligands can be classified as monodentate (one donor atom), bidentate (two donor atoms), or polydentate (multiple donor atoms) based on the number of atoms that coordinate to the metal
• The geometry of coordination compounds depends on the coordination number and can include linear, trigonal planar, tetrahedral, square planar, trigonal bipyramidal, and octahedral arrangements
• Crystal field theory explains the splitting of d-orbital energies in coordination compounds due to the electrostatic interaction between the metal and the ligands
  • The magnitude of the splitting depends on the strength of the ligand field, which is influenced by the nature of the ligands and the metal
• The spectrochemical series ranks ligands based on their ability to split d-orbital energies, with strong-field ligands causing a larger splitting than weak-field ligands
• Coordination compounds can exhibit isomerism, including structural isomers (different bonding arrangements) and stereoisomers (same bonding, different spatial arrangements)

Types of Coordination Compounds

• Werner complexes are classic coordination compounds with a metal center and surrounding ligands, such as $[Co(NH_3)_6]Cl_3$ and $[Pt(NH_3)_4]Cl_2$
• Chelate complexes contain polydentate ligands that form stable ring structures with the metal, such as the hexadentate ligand EDTA (ethylenediaminetetraacetic acid) in $[Fe(EDTA)]^-$
• Macrocyclic complexes involve cyclic ligands that encapsulate the metal ion, such as porphyrins in hemoglobin and chlorophyll
• Organometallic compounds contain metal-carbon bonds, such as ferrocene $Fe(C_5H_5)_2$ and Zeise's salt $K[PtCl_3(C_2H_4)]$
  • These compounds have important applications in catalysis and organic synthesis
• Cluster compounds contain multiple metal atoms bonded together, often with bridging ligands, such as the Fe-S clusters in ferredoxins and the Ru-Ru bonded complex $Ru_3(CO)_{12}$
• Mixed-valence compounds contain metal ions in different oxidation states, such as Prussian blue $Fe_4[Fe(CN)_6]_3$ with Fe(II) and Fe(III) centers
• Coordination polymers are extended structures with repeating units of metal ions and bridging ligands, such as the metal-organic framework (MOF) $Zn_4O(BDC)_3$ (BDC = 1,4-benzenedicarboxylate)

Ligand Substitution Reactions

• Ligand substitution reactions involve the replacement of one or more ligands in a coordination compound by other ligands or molecules
• The rate of ligand substitution depends on the lability of the metal-ligand bonds, with more labile bonds undergoing faster substitution
• Associative mechanism (A) involves the formation of an intermediate with increased coordination number, followed by the loss of a ligand
  • The rate is first-order with respect to the entering ligand and the complex: Rate = $k[ML_n][Y]$
• Dissociative mechanism (D) involves the loss of a ligand to form an intermediate with reduced coordination number, followed by the addition of the entering ligand
  • The rate is first-order with respect to the complex: Rate = $k[ML_n]$
• Interchange mechanism (I) occurs when the entering ligand interacts with the complex during the transition state, without forming a distinct intermediate
  • Associative interchange (I$_a$) has a transition state similar to the associative mechanism, with a higher-order dependence on the entering ligand
  • Dissociative interchange (I$_d$) has a transition state similar to the dissociative mechanism, with a lower-order dependence on the entering ligand
• The activation volume ($\Delta V^‡$) can help distinguish between the mechanisms, with associative processes having negative values and dissociative processes having positive values
• Trans effect refers to the ability of a ligand to labilize the bond trans (opposite) to itself, facilitating substitution at that position

Electron Transfer Mechanisms

• Electron transfer reactions involve the transfer of one or more electrons between a coordination compound and another species, resulting in a change in the oxidation state of the metal
• Inner-sphere mechanism involves the formation of a bridged intermediate between the two reactants, allowing for direct electron transfer
  • The bridging ligand facilitates the electron transfer by providing a pathway for the electrons
  • Example: $[Co(NH_3)_5Cl]^{2+} + [Cr(H_2O)_6]^{2+} \rightarrow [Co(NH_3)_5(H_2O)]^{3+} + [Cr(H_2O)_5Cl]^{2+}$
• Outer-sphere mechanism occurs when the two reactants do not form a bridged intermediate, and the electron transfer takes place through space or solvent
  • The electron transfer is influenced by the distance between the reactants and the reorganization of the solvent molecules
  • Example: $[Fe(CN)_6]^{3-} + [Ru(NH_3)_6]^{2+} \rightarrow [Fe(CN)_6]^{4-} + [Ru(NH_3)_6]^{3+}$
• Marcus theory provides a framework for understanding the rates of electron transfer reactions, considering factors such as the driving force, reorganization energy, and electronic coupling
• The rate of electron transfer depends on the self-exchange rate constants of the reactants, which measure the ability of a species to undergo electron transfer with itself
• Electron transfer can be coupled with other processes, such as proton transfer or bond cleavage, leading to more complex reaction mechanisms

Photochemical Reactions

• Photochemical reactions involve the absorption of light by a coordination compound, leading to excited states and subsequent chemical transformations
• Light absorption promotes an electron from a lower-energy orbital to a higher-energy orbital, creating an excited state with different properties than the ground state
  • The excited state can undergo various processes, such as luminescence (emission of light), non-radiative decay, or chemical reactions
• Ligand field photochemistry arises from the excitation of electrons within the d-orbitals of the metal, leading to changes in the electronic configuration and reactivity
  • Example: $[Ru(bpy)_3]^{2+}$ (bpy = 2,2'-bipyridine) undergoes metal-to-ligand charge transfer (MLCT) upon light absorption, forming a long-lived excited state that can participate in electron transfer reactions
• Charge transfer photochemistry involves the excitation of electrons from the ligand to the metal (ligand-to-metal charge transfer, LMCT) or from the metal to the ligand (metal-to-ligand charge transfer, MLCT)
  • These excited states have different redox properties and can engage in electron transfer or bond cleavage reactions
• Photoisomerization occurs when light absorption induces a change in the spatial arrangement of the ligands around the metal center, such as cis-trans isomerization or linkage isomerization
  • Example: $[Ru(NH_3)_5(NO)]^{3+}$ undergoes linkage isomerization from the N-bound nitrosyl to the O-bound isonitrosyl upon light absorption
• Photosubstitution reactions involve the replacement of a ligand in the excited state by another ligand or solvent molecule, often with different stereochemistry than the ground state substitution
• Photocatalysis harnesses the excited states of coordination compounds to drive chemical reactions, such as hydrogen production, CO2 reduction, or organic transformations

Catalysis and Applications

• Coordination compounds play a crucial role as catalysts in various industrial and biological processes, enabling efficient and selective chemical transformations
• Homogeneous catalysis involves the use of coordination compounds dissolved in the same phase as the reactants, allowing for intimate contact and fast reaction rates
  • Example: Wilkinson's catalyst, $RhCl(PPh_3)_3$, is used for the hydrogenation of alkenes and alkynes under mild conditions
• Heterogeneous catalysis employs coordination compounds immobilized on a solid support, facilitating the separation and recycling of the catalyst
  • Example: Supported metal nanoparticles, such as Pt or Pd, are used in automotive catalytic converters to reduce pollutant emissions
• Enzyme catalysis relies on the coordination of substrates to metal centers in metalloenzymes, which activate the substrates and lower the activation energy of the reaction
  • Example: Cytochrome P450 enzymes contain an Fe-heme cofactor that catalyzes the oxidation of various organic compounds, including drugs and steroids
• Asymmetric catalysis uses chiral coordination compounds to promote the selective formation of one enantiomer over the other in organic reactions
  • Example: Jacobsen's catalyst, a Mn-salen complex, catalyzes the enantioselective epoxidation of unfunctionalized alkenes
• Coordination compounds find applications in various fields, such as medicine (anticancer drugs, MRI contrast agents), energy (solar cells, fuel cells), and materials science (luminescent devices, sensors)
  • Example: Cisplatin, $cis-[PtCl_2(NH_3)_2]$, is a widely used chemotherapeutic agent that binds to DNA and induces apoptosis in cancer cells

Experimental Techniques

• Various experimental techniques are employed to study the structure, properties, and reactivity of coordination compounds
• X-ray crystallography provides detailed information about the solid-state structure of coordination compounds, including bond lengths, angles, and coordination geometry
  • Single-crystal X-ray diffraction is the most common method, requiring the growth of high-quality single crystals
• Spectroscopic techniques probe the electronic and vibrational transitions in coordination compounds, offering insights into the metal-ligand interactions and the coordination environment
  • UV-Vis spectroscopy measures the absorption of light in the ultraviolet and visible regions, revealing the electronic transitions between d-orbitals and charge transfer bands
  • Infrared (IR) spectroscopy detects the vibrational modes of the ligands and the metal-ligand bonds, providing information about the coordination mode and the strength of the interactions
  • Nuclear magnetic resonance (NMR) spectroscopy investigates the local environment of the ligand nuclei and the metal center, allowing for the determination of the solution structure and dynamics
• Electrochemical methods, such as cyclic voltammetry and polarography, study the redox properties of coordination compounds, measuring the oxidation and reduction potentials and the kinetics of electron transfer
• Magnetochemistry techniques, such as SQUID (superconducting quantum interference device) magnetometry, determine the magnetic susceptibility and the spin state of the metal center
• Kinetic methods, including stopped-flow and temperature-jump techniques, monitor the rates and mechanisms of ligand substitution and electron transfer reactions
  • The activation parameters, such as the activation energy ($E_a$) and the activation entropy ($\Delta S^‡$), can be derived from the temperature dependence of the rate constants
• Computational methods, such as density functional theory (DFT) and molecular dynamics (MD) simulations, provide theoretical insights into the electronic structure, reactivity, and dynamics of coordination compounds

Practice Problems and Review

1. Draw the isomers of $[Co(en)_2Cl_2]^+$ (en = ethylenediamine) and assign their configuration as cis or trans.
2. Predict the crystal field splitting and the electronic configuration of the metal in $[Fe(CN)_6]^{4-}$, given that CN- is a strong-field ligand.
3. Propose a plausible mechanism (A, D, or I) for the substitution reaction between $[Pt(NH_3)_4]^{2+}$ and pyridine (py), considering the lability of Pt(II) complexes.
4. Explain the difference between the inner-sphere and outer-sphere electron transfer mechanisms, using the reactions between $[Co(NH_3)_5Cl]^{2+}$ and $[Cr(H_2O)_6]^{2+}$ (inner-sphere) and $[Fe(CN)_6]^{3-}$ and $[Ru(NH_3)_6]^{2+}$ (outer-sphere) as examples.
5. Describe the photochemical process responsible for the light-induced isomerization of $[Ru(NH_3)_5(NO)]^{3+}$ from the N-bound nitrosyl to the O-bound isonitrosyl.
6. Discuss the role of Wilkinson's catalyst, $RhCl(PPh_3)_3$, in the homogeneous hydrogenation of alkenes, including the proposed mechanism and the factors influencing its selectivity.
7. Interpret the UV-Vis spectrum of $[Ti(H_2O)_6]^{3+}$, which shows a single broad absorption band around 500 nm, in terms of the d-orbital splitting and the electronic transition.
8. Design an experiment to determine the rate law and the activation parameters for the substitution reaction between $[Co(NH_3)_5(H_2O)]^{3+}$ and $SCN^-$ using spectrophotometric methods.