π-bonding & electron count rule

Question 1

What is the 18-electron rule in metal complexes?

Answer 1

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.

Question 2

How does π-bonding act as a perturbation on σ-bonding in coordination complexes?

Answer 2

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.

Question 3

What is the effect of π-acceptor ligands on the d-orbital energies of a metal in a complex?

Answer 3

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.

Question 4

Contrast the effects of π-acceptor and π-donor ligands on the HOMO and LUMO in a complex.

Answer 4

π-acceptor ligands increase Δ by lowering t2g orbital energies, whereas π-donor ligands decrease Δ by raising t2g orbital energies.

Question 5

Describe the role of CO’s π* orbitals in bonding with a metal.

Answer 5

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.

Question 6

What is the consequence of forming dxy, dxz, dyz into bonding molecular orbitals?

Answer 6

These orbitals form π-bonds with ligands, which significantly affects the electronic structure and stability of the complex.

Question 7

How does the 18-electron rule apply to different geometries like octahedral, square planar, and tetrahedral?

Answer 7

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.

Question 8

What is the significance of σ-only complexes in the context of the 18-electron rule?

Answer 8

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.

Question 9

Explain the impact of ligand field theory on understanding square planar complexes.

Answer 9

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.

Question 10

What does the term “18 electrons without an 18-electron rule” imply for tetrahedral complexes?

Answer 10

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.

Question 11

How do d-electron count and the 18-electron rule relate in complex chemistry?

Answer 11

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.

Question 12

Compare and contrast Crystal Field Theory (CFT) and Ligand Field Theory (LFT).

Answer 12

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.

Question 13

What is the effect of π-donor ligands on the spectrochemical series?

Answer 13

π-donor ligands reduce the size of Δ, affecting the electronic transitions and properties of coordination complexes.

Question 14

Why are π-acceptor ligands considered strong field?

Answer 14

They significantly increase Δ by stabilizing lower energy orbitals, which enhances the overall stability and alters the electronic structure of the complex.

Question 15

What role does symmetry play in ligand and metal orbital interactions in complexes?

Answer 15

Symmetry determines which orbitals can overlap and form molecular orbitals based on their spatial and symmetry compatibility.

Question 16

How does π-back bonding influence the bonding in complexes with CO ligands?

Answer 16

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.

Question 17

Why might a complex with π-acceptor ligands not follow the 18-electron rule?

Answer 17

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.

Question 18

How does ligand π-bonding affect the molecular orbital diagram in ML6 complexes?

Answer 18

It introduces additional π-bonding interactions that can alter the positions of the frontier orbitals and the overall electronic configuration.

Question 19

What is the relationship between ligand field stabilization and the electronic structure of transition metal complexes?

Answer 19

Ligand field stabilization directly influences the distribution and energy levels of d orbitals, affecting properties like colour, magnetism, and reactivity.

Question 20

Explain the significance of ligand-to-metal σ-bonds in octahedral complexes.

Answer 20

They are crucial for the structural integrity and electronic configuration of the complex, influencing both bonding strength and orbital hybridization.

Question 21

How do interactions between ligand π-orbitals and metal d orbitals affect molecular stability?

Answer 21

These interactions can either stabilize or destabilize the complex depending on whether they involve π-donor or π-acceptor mechanisms.

Question 22

What is the role of non-bonding orbitals in complexes without π-bonding?

Answer 22

Non-bonding orbitals in such complexes typically do not impact the electronic structure significantly, allowing flexibility in electron accommodation.

Question 23

Describe how ligand π-bonding influences the reactivity of metal complexes.

Answer 23

π-bonding can alter the electron density at the metal centre, influencing its ability to engage in further chemical reactions.

Question 24

What determines whether π-bonding will raise or lower orbital energies in a complex?

Answer 24

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.

Question 25

How does π-bonding contribute to the theoretical understanding of complex geometries?

Answer 25

It provides insights into how electron donation and back-donation occur, explaining variations in geometry and stability among complexes with different ligand types.

Question 26

What is the role of metal d orbitals in π-back bonding with CO ligands?

Answer 26

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.

Question 27

How does π-back bonding affect the bond order and strength of the CO ligand in metal complexes?

Answer 27

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.

Question 28

Why does the 18-electron rule not strictly apply in square planar complexes?

Answer 28

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.

Question 29

Explain how ligand π-donors affect the stability of metal complexes compared to π-acceptors.

Answer 29

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.

Question 30

Describe the interaction between ligand π-orbitals and the σ-bonding MOs in an octahedral ML6 complex.

Answer 30

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.

Question 31

What experimental techniques are typically used to study π-bonding interactions in coordination chemistry?

Answer 31

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.

Question 32

How does the crystal field splitting in tetrahedral geometries differ from octahedral, and what implications does this have for the 18-electron rule?

Answer 32

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.

Question 33

Discuss the importance of symmetry in determining the interactions between metal and ligand orbitals.

Answer 33

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.

Question 34

How do ligand field theory and molecular orbital theory complement each other in the study of coordination complexes?

Answer 34

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.

Question 35

What theoretical considerations are taken into account when applying the 18-electron rule to complex stability predictions?

Answer 35

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.