chem 241 final - 5
What is the general energy-level picture for σ bonding MOs in octahedral ML6 complexes?
The 6 ligand σ-donor atomic orbitals are lower in energy than the metal valence atomic orbitals, and donation into the metal generates 6 σ-bonding molecular orbitals.
Which orbitals make up the 6 σ-bonding MOs in octahedral ML6?
One comes mainly from metal (n+1)s, three from metal (n+1)px, py, pz, and two from metal ndz2 and ndx2−y2.
Where do the 6 donated ligand electron pairs go in octahedral ML6?
They occupy the 6 σ-bonding MOs.
What is the HOMO/LUMO picture for many octahedral transition-metal compounds ML6?
The d electrons occupy the nonbonding dxy, dxz, dyz orbitals and, if necessary, the higher dx2−y2* and dz2* orbitals.
What is the octahedral d-orbital splitting picture emphasized in the notes?
Lower set: dxy, dxz, dyz. Higher set: dx2−y2* and dz2*.
What determines the number of d electrons on the metal in a compound?
The number of d electrons in M(compound) is found from the metal oxidation state, which comes from the formal charges on the ligands and the overall compound charge.
What electron configuration idea is used for ligands in this counting method?
Ligands are assigned stable p-electron or noble-gas-like configurations.
For a d6 octahedral complex, what two electron-placement choices are shown?
Either pair electrons in the lower dxy, dxz, dyz orbitals or promote electrons into the higher dx2−y2* and dz2* orbitals.
When do electrons pair in the lower octahedral orbitals?
When the electron pairing energy is less than the energy gap between the lower dxy, dxz, dyz set and the higher dx2−y2* and dz2* set.
What kind of metal compounds tend to give this lower-orbital pairing?
Metal compounds with strong M–L bonds.
Why do strong M–L bonds favor pairing in the lower orbitals?
Because strong bonding lowers the σ-bonding orbitals and raises the σ* antibonding orbitals, making the splitting large.
What is the effect of a strong metal-ligand bond on the octahedral splitting?
σ bonding becomes lower in energy and σ* antibonding becomes higher in energy, so the gap becomes large.
Which ligands are listed as giving strong M–L σ bonds?
PR3, SR2, R3N, certain nitrogen compounds, and also CO and CN− because of π back-bonding.
Why are CO and CN− included with strong-bonding ligands?
Because π back-bonding strengthens the interaction and contributes to large splitting.
What happens to the σ* antibonding orbitals in strong-field complexes?
They are high in energy.
What is the term for complexes where spins are paired in the lower set?
Low spin.
What examples are given for weak M–L bonds?
M–OH2, M–OCH3, and etc.
What happens if the M–L bond is weak?
The splitting is smaller and electrons can occupy the higher orbitals before pairing.
What is shown for a d6 high-spin configuration?
Four electrons in the lower dxy, dxz, dyz set are not all paired first; instead electrons occupy the higher orbitals to reduce pairing.
What general statement is made about d6 octahedral compounds?
They can be either low spin or high spin.
In the strong-ligand case, what is the HOMO for the octahedral complex?
The HOMO is the t2g set.
In the strong-ligand case, what is the LUMO for the octahedral complex?
The LUMO is the eg* set.
In the weak-ligand case, what are the HOMO and SOMO ideas shown?
The occupied t2g and eg* levels can both be relevant, with singly occupied orbitals appearing in the higher set.
What symmetry labels are highlighted as good to know for octahedral complexes?
eg and t2g.
What does t2g refer to in the octahedral splitting picture?
The lower set of three d orbitals: dxy, dxz, dyz.
What does eg* refer to in the octahedral splitting picture?
The higher antibonding set: dx2−y2* and dz2*.
How is the total valence electron count of a transition-metal compound defined in the notes?
Number of d electrons on the metal plus number of electrons donated by ligands.
What is the general stability rule involving total valence electron count?
If the total is 18, you generally have a stable transition-metal compound.
Why are d6 octahedral compounds often 18-electron compounds?
Because they have 6 electrons from the metal d orbitals and 12 electrons from 6 ligands donating lone pairs.
What total electron count is given for d6 octahedral ML6?
18 electrons.
What is the total electron count for d4 ML6 compounds?
16 electrons.
How are d4 ML6 compounds described in terms of electron count?
They are generally electron deficient.
What chemical behavior is suggested for electron-deficient d4 ML6 compounds?
They can be good electron acceptors or may form ML7 if the metal is large and the ligand is small.
What happens to ML6 compounds with more than 18 electrons?
They tend to be labile because antibonding orbitals such as eg* become partially occupied.
Why are complexes with more than 18 electrons often labile?
Because partially occupied antibonding orbitals weaken bonding and can make the complex a good electron donor.
What electron-loss equation is written for a 19-electron ML6 complex?
ML6(19e−) → ML6+(18e−) + e−
What is the main stability statement about ML6 d6 octahedral low-spin compounds?
They are stable.
What equilibrium idea is written comparing ML6 and ML5?
ML6 ⇌ ML5 + :L
Which side of the ML6 ⇌ ML5 + L equilibrium is expected to be favored for stable 18-electron complexes?
To the left, toward ML6.
What electron count is written for ML5 formed from d8 metals?
8 electrons from the metal plus 10 electrons from 5 ligands.
What common geometries are given for d8 ML5 compounds?
Square pyramidal and trigonal bipyramidal.
What kind of species do d10 metal compounds usually form?
ML4 species.
What geometry is usually given for d10 ML4 species?
Usually tetrahedral.
What electron count is written for d10 ML4 compounds?
10 electrons from the metal plus 8 electrons from 4 ligands gives 18 electrons.
Why are d10 ML4 tetrahedral complexes common?
Because they reach 18 electrons with four ligands.
What special case is introduced for d8 ML4 compounds?
Square planar compounds.
Why is square planar ML4 noted as interesting?
Because square planar ML4 is more crowded than tetrahedral ML4 and d8 ML4 are only 16-electron compounds, so there must be special stabilization.
What is the key stabilization idea for 16-electron d8 square planar complexes?
The orbital ordering makes the strongly antibonding dx2−y2* orbital remain empty, which stabilizes the structure.
What is the MO energy ordering shown for 16-electron d8 square planar complexes?
Highest is dx2−y2*; below that are dz2 and dxy in varying order; lowest are dxz and dyz.
What is always true about dx2−y2 in square planar complexes?
It is strongly antibonding.
Why is dz2 described as net nonbonding in square planar ML4?
Because the z axis points above and below the ML4 plane.
What mixing is specifically mentioned in square planar ML4?
s-dz2 mixing.
What note is made about the order of dxy and dz2 in square planar diagrams?
Sometimes the order is reversed.
How are the 4 ligand lone pairs treated in square planar ML4?
They occupy the 4 σ-bonding molecular orbitals.
Where do the d8 metal electrons go in square planar ML4?
They occupy the nonbonding dxz, dyz, dz2, and dxy orbitals, leaving dx2−y2* empty.
What total electron count does this give for square planar d8 ML4?
16 electrons.
Why is having more than 16 electrons in square planar d8 unfavorable?
Because it would require occupying the strongly antibonding dx2−y2* orbital.
What conclusion is stated about ML4 d8 square planar complexes?
They are a stable low point.
What platinum example is given for square planar d8 chemistry?
PtCl2(NH3)2 with Pt(II), d8.
What electron count is given for PtCl2(NH3)2?
16-electron square planar compound.
What are cis and trans in square planar complexes?
They are isomers that differ by the relative placement of ligands in the same square planar coordination geometry.
What does cis mean in the PtCl2(NH3)2 example?
The NH3 ligands and the Cl ligands are on the same side of the square plane, meaning like ligands are adjacent.
What does trans mean in the PtCl2(NH3)2 example?
The NH3 ligands and the Cl ligands are on opposite sides of the square plane.
What general point is made about isomers of transition-metal complexes?
Isomers can have vastly different chemistry.
What is the important medicinal example given?
cis-PtCl2(NH3)2, cis-platin.
What property is assigned to cis-platin?
Strong anticancer properties.
What is said about trans-platin?
It is a toxin with weak anticancer properties.
Why is it important to distinguish cis-platin from trans-platin?
Because only cis-platin has the strong desired anticancer activity.
What practical note is given about cis-platin and water?
cis-platin is not readily dissolved in water.
What practical preparation note is given for cis-platin?
People used to heat water and cis-platin to speed up dissolution.
How can you quickly remember strong-field versus weak-field behavior in octahedral complexes?
Strong M–L bonds give large splitting and low spin; weak M–L bonds give small splitting and high spin.
How can you quickly remember the octahedral labels?
t2g is the lower set and eg* is the upper antibonding set.
How can you quickly remember when 18-electron complexes are favored?
When the bonding and nonbonding levels are filled without putting electrons into strongly antibonding orbitals.
How can you quickly remember the stability of d6 octahedral low-spin complexes?
d6 low-spin octahedral plus 6 ligands gives 18 electrons, so it is especially stable.
How can you quickly remember d10 ML4 geometry?
d10 ML4 is usually tetrahedral.
How can you quickly remember d8 ML4 geometry?
d8 ML4 is often square planar because keeping dx2−y2* empty is stabilizing.
How can you quickly remember cis versus trans in square planar complexes?
cis = like ligands adjacent; trans = like ligands opposite.
How can you quickly remember the platinum drug example?
cis-platin works as an anticancer drug; trans-platin does not.
What is a complex in coordination chemistry?
A complex is written as M(L)n.
What does n mean in M(L)n?
n is the coordination number, largely determined by electron counting such as the 18-electron rule.
What are ligands?
Ligands are groups bonded to the metal.
What is a σ-donor ligand?
A ligand that donates a lone pair into an empty σ-type orbital on the metal.
What examples of σ-donor ligands are given?
NH3, PR3, and related ligands.
Can σ-donor ligands give strong or weak bonding?
Yes, σ-donor ligands can be strong or weak bonding.
What is a π-accepting ligand?
A ligand with empty antibonding orbitals that can accept electron density from dπ orbitals on the metal.
What examples of π-accepting ligands are shown?
CO, N2-related multiple-bond ligands, CN−, and related ligands.
Why are π-accepting ligands special?
They have empty antibonding orbitals that accept electron density from dπ orbitals on the metal.
What metal carbonyl example is given?
Ru(CO)4.
How is Ru(CO)4 electron-counted in the notes?
Ru(0) is d8 and 4 CO ligands donate 8 electrons total, giving a 16-electron compound.
What geometry is associated with Ru(CO)4 in the notes?
Square planar d8, 16-electron, with π bonding between Ru and CO.
How are ligands classified by coordination mode?
As monodentate or poly-/multidentate.
What is a monodentate ligand?
A ligand that binds through one donor atom, written conceptually as :L.
What is a polydentate or multidentate ligand?
A ligand that binds through two or more donor atoms.
What is a bidentate ligand?
A ligand that binds through two donor atoms to the same metal.
What is the slide’s definition of a chelate?
A ring structure involving a metal and a multidentate ligand.
What is the chelate effect?
The enhanced affinity of chelating ligands for a metal ion compared to similar nonchelating monodentate ligands for the same metal.
What is the main driving force for the chelate effect in the notes?
A positive entropy change, ΔS.
Why is ΔS positive in the chelate-effect example?
Because the reaction produces more molecules in the products than in the reactants.
What reaction pattern is used to explain the chelate effect?
Two monodentate ligands on a metal are replaced by one bidentate ligand, releasing two free monodentate ligands.
Why does the chelate effect make complex formation more favorable in terms of free energy?
Because ΔG = ΔH − TΔS, and a positive ΔS makes −TΔS more negative.