π-bonding & electron count rule Flashcards
What is the 18-electron rule in metal complexes?
It’s a rule suggesting that a stable complex often has 18 valence electrons comprising the metal’s d-electrons plus the electrons donated by ligands.
How does π-bonding act as a perturbation on σ-bonding in coordination complexes?
π-bonding alters the molecular orbital picture by interacting with σ-bonding MOs, which may change the overall energy levels and properties of the orbitals.
What is the effect of π-acceptor ligands on the d-orbital energies of a metal in a complex?
π-acceptor ligands lower the energy of the metal’s non-bonding d-orbitals and increase the splitting (Δ) between t2g and eg* orbitals, stabilizing the complex.
Contrast the effects of π-acceptor and π-donor ligands on the HOMO and LUMO in a complex.
π-acceptor ligands increase Δ by lowering t2g orbital energies, whereas π-donor ligands decrease Δ by raising t2g orbital energies.
Describe the role of CO’s π* orbitals in bonding with a metal.
CO’s π* orbitals can accept electrons from filled metal d orbitals (π-back donation), weakening the C-O bond and strengthening the metal-ligand bond.
What is the consequence of forming dxy, dxz, dyz into bonding molecular orbitals?
These orbitals form π-bonds with ligands, which significantly affects the electronic structure and stability of the complex.
How does the 18-electron rule apply to different geometries like octahedral, square planar, and tetrahedral?
It generally applies to octahedral and trigonal bipyramidal geometries with π-acceptor ligands, does not apply to square planar, and applies “by accident” in tetrahedral geometries.
What is the significance of σ-only complexes in the context of the 18-electron rule?
In σ-only complexes (without π-bonding), the 18-electron rule does not hold as the complex can accommodate up to 22 electrons due to available non-bonding and weakly anti-bonding orbitals.
Explain the impact of ligand field theory on understanding square planar complexes.
Ligand field theory explains the large Δ in square planar complexes, favouring the 16-electron rule due to the high stabilization of certain d orbitals.
What does the term “18 electrons without an 18-electron rule” imply for tetrahedral complexes?
It implies that while tetrahedral complexes can accommodate 18 electrons due to small crystal field splitting and accessible MOs, this does not strictly follow the 18-electron rule as π-bonding is not required.
How do d-electron count and the 18-electron rule relate in complex chemistry?
The d-electron count helps determine the number of valence electrons contributing to bonding in a complex, which is crucial for applying the 18-electron rule effectively.
Compare and contrast Crystal Field Theory (CFT) and Ligand Field Theory (LFT).
CFT provides an electrostatic model of d-orbital splitting without considering covalent interactions, while LFT offers a more comprehensive molecular orbital approach that includes covalent bonding and multiple bonds.
What is the effect of π-donor ligands on the spectrochemical series?
π-donor ligands reduce the size of Δ, affecting the electronic transitions and properties of coordination complexes.
Why are π-acceptor ligands considered strong field?
They significantly increase Δ by stabilizing lower energy orbitals, which enhances the overall stability and alters the electronic structure of the complex.
What role does symmetry play in ligand and metal orbital interactions in complexes?
Symmetry determines which orbitals can overlap and form molecular orbitals based on their spatial and symmetry compatibility.
How does π-back bonding influence the bonding in complexes with CO ligands?
π-back bonding involves electron donation from metal d orbitals to CO’s π* orbitals, strengthening the metal-CO bond and weakening the C-O bond.
Why might a complex with π-acceptor ligands not follow the 18-electron rule?
If the complex’s geometry or the nature of the ligands leads to additional stabilization without needing to fulfil 18 electrons, the rule may not apply.
How does ligand π-bonding affect the molecular orbital diagram in ML6 complexes?
It introduces additional π-bonding interactions that can alter the positions of the frontier orbitals and the overall electronic configuration.
What is the relationship between ligand field stabilization and the electronic structure of transition metal complexes?
Ligand field stabilization directly influences the distribution and energy levels of d orbitals, affecting properties like colour, magnetism, and reactivity.
Explain the significance of ligand-to-metal σ-bonds in octahedral complexes.
They are crucial for the structural integrity and electronic configuration of the complex, influencing both bonding strength and orbital hybridization.
How do interactions between ligand π-orbitals and metal d orbitals affect molecular stability?
These interactions can either stabilize or destabilize the complex depending on whether they involve π-donor or π-acceptor mechanisms.
What is the role of non-bonding orbitals in complexes without π-bonding?
Non-bonding orbitals in such complexes typically do not impact the electronic structure significantly, allowing flexibility in electron accommodation.
Describe how ligand π-bonding influences the reactivity of metal complexes.
π-bonding can alter the electron density at the metal centre, influencing its ability to engage in further chemical reactions.
What determines whether π-bonding will raise or lower orbital energies in a complex?
The nature of the ligand (π-donor or π-acceptor) and the existing electron configuration of the metal’s d orbitals dictate the direction of energy adjustment.
How does π-bonding contribute to the theoretical understanding of complex geometries?
It provides insights into how electron donation and back-donation occur, explaining variations in geometry and stability among complexes with different ligand types.
What is the role of metal d orbitals in π-back bonding with CO ligands?
Metal d orbitals, especially non-bonding or filled d orbitals, can donate electron density back to the π* anti-bonding orbitals of CO, enhancing the metal-ligand bonding and altering the electronic properties of the ligand.
How does π-back bonding affect the bond order and strength of the CO ligand in metal complexes?
π-back bonding increases the electron density in CO’s π* orbitals, which decreases the C-O bond order and weakens the C-O bond, potentially increasing the reactivity of the ligand.
Why does the 18-electron rule not strictly apply in square planar complexes?
In square planar complexes, the electronic and spatial arrangement typically leads to a stable 16-electron configuration due to the strong field created by the ligands and the specific d orbital occupation that stabilizes the complex without the need for 18 electrons.
Explain how ligand π-donors affect the stability of metal complexes compared to π-acceptors.
π-donors raise the energy of the non-bonding d orbitals by donating electron density, potentially destabilizing the complex, whereas π-acceptors lower these orbital energies, providing greater stability and a larger splitting of d orbital energies.
Describe the interaction between ligand π-orbitals and the σ-bonding MOs in an octahedral ML6 complex.
Ligand π-orbitals interact minimally with the σ-bonding MOs due to significant energy differences; however, these interactions are crucial when they do occur, as they can subtly influence the electronic structure and bonding characteristics of the complex.
What experimental techniques are typically used to study π-bonding interactions in coordination chemistry?
Techniques such as X-ray crystallography, infrared / Raman spectroscopy, and electronic absorption spectroscopy are used to analyse the structure, bonding, and electronic transitions in complexes, revealing the effects of π-bonding.
How does the crystal field splitting in tetrahedral geometries differ from octahedral, and what implications does this have for the 18-electron rule?
Crystal field splitting in tetrahedral geometries is smaller and inverted compared to octahedral, with fewer low-energy orbital options for electrons to occupy, making the 18-electron rule less applicable without π-bonding considerations.
Discuss the importance of symmetry in determining the interactions between metal and ligand orbitals.
Symmetry determines which orbitals can overlap effectively; orbitals must share the same symmetry properties to interact and form molecular orbitals, crucial for predicting and understanding bonding in complex geometries.
How do ligand field theory and molecular orbital theory complement each other in the study of coordination complexes?
Ligand field theory provides insights into the effects of ligand arrangement on metal d orbital energies, while molecular orbital theory offers a detailed description of bonding interactions, covalency, and the formation of molecular orbitals across the complex.
What theoretical considerations are taken into account when applying the 18-electron rule to complex stability predictions?
Theoretical considerations include the nature of the metal and ligands, orbital symmetries and energies, electron count, and the overall molecular geometry, which collectively influence whether the rule predicts stability accurately.
