chem 241 final - 4
What does bonding in metals consist of?
Bonding in metals consists of billions of overlapping valence orbitals.
In metallic Ni, what is the highest energy band of overlapping orbitals?
The highest energy band is the 3d8 band.
What is the next lower band in metallic Ni?
The next lower band arises from billions of 4s2 orbitals overlapping.
How can these bands in metals be probed experimentally?
Using XPS, X-ray photoelectron spectroscopy, or other wavelengths of light to eject electrons.
What is the main idea about reactivity of transition metals as metals or in complexes?
It is all about σ interactions at the same time as π interactions.
What orbitals are involved in the σ interactions of transition metals?
s, p, or d-σ orbitals on the metal.
What orbitals are involved in the π interactions of transition metals?
dπ orbitals on the metal.
What broad statement is made about d-block elements?
d-block elements show rich and diverse bonding and reactivity.
What is the general trend for metals to the left of the transition series?
They are generally of lower Zeff and generally give up valence electrons and form cations.
What example is given for a left-side transition metal forming a cation?
Sc(III).
What valence electrons are donated by Sc to form Sc(III)?
4s2 and 3d1 valence electrons.
What oxidation state does Ti often form according to the notes?
Ti often forms Ti(IV), for example in TiO2.
What is the general trend for metals to the right of the transition series?
They tend to hold onto d electrons.
What example is given for a right-side transition-metal electron configuration?
Ni(II) is d8.
What example of a very hard transition-metal material is given?
WC, tungsten carbide.
What properties are emphasized for WC?
Super hard, resistant, high melting point material.
Why do transition metals show such a huge variety of compounds and properties?
Because use of 5 nd valence orbitals plus (n+1)s and (n+1)p in bonding allows for essentially infinite variety in compounds and properties.
Why is the 18-electron rule considered a stable configuration?
Because 18 valence electrons is analogous to the stable 8-valence-electron configuration of main-group elements.
What does the 18-electron rule correspond to in orbital terms?
Filled valence orbitals for the transition-metal bonding set.
What do d-block metal ions readily form?
Complexes.
What is a complex in this context?
A molecule or ion containing a transition metal bonded to ligands.
How is σ bonding between a transition metal and a main-group atom in a ligand modeled?
As a ligand lone pair donating into an empty σ orbital on the metal.
What kinds of empty metal orbitals can accept ligand σ donation?
s, p, or dσ orbitals.
What is a ligand?
A molecule or atom bonded to a metal.
What symbol is usually used for ligands?
L.
What kind of bond is the metal-ligand σ bond often called?
A dative bond.
How else can the metal-ligand bond be viewed?
As a polar covalent bond.
Why is the metal-ligand bond polar covalent?
Because there is a difference in electronegativity giving δ+ and δ− character across the M–L bond.
What common ligand example is specifically mentioned?
NH3.
What does NH3 donate to the metal?
A σ lone pair.
Why do transition-metal compounds have many possible geometries and ligand counts?
Because metal compounds can accommodate different numbers of ligands and different spatial arrangements.
What geometry is mentioned for ML6?
Octahedral.
What geometries are mentioned for ML5?
Trigonal bipyramidal or square pyramidal.
What happens when ligands dissociate or substitute in transition-metal complexes?
New compounds are produced.
What often determines the number of ligands around a transition metal?
The 18-electron rule.
How many electrons does each ligand usually contribute in electron counting?
Each ligand donates one lone pair, so 2 electrons.
What two formal descriptors does every transition metal in a complex have?
Oxidation state and coordination number.
What is the oxidation state of a metal in a complex obtained from?
From the overall charge on the compound minus the charges on the ligands in their p-electron configurations.
How are ligands regarded for oxidation-state and electron-counting purposes?
As σ electron-pair donors with the ligand in a p-electron noble-gas configuration.
What example complex is used to illustrate these ideas?
[Co(NH3)6]3+.
What is the coordination number of Co in [Co(NH3)6]3+?
6.
Why is NH3 treated as neutral in oxidation-state calculations?
Because NH3 is a neutral ligand.
What oxidation state does Co have in [Co(NH3)6]3+?
Co(III).
What d-electron count does Co(III) have in [Co(NH3)6]3+?
d6.
How many valence electrons are counted for [Co(NH3)6]3+?
d6 plus 6 × 2 from the 6 NH3 ligands = 18 valence electrons.
What is the relationship between overall charge, ligand charge, and metal oxidation state?
Oxidation state of metal = total charge − total charge on ligands.
What second cobalt example is written in the notes?
[Co(NH3)4Cl2]+.
How do you determine the oxidation state of Co in [Co(NH3)4Cl2]+?
1+ − 2(−1) = 3+, so Co is Co(III).
What is the d-electron count for Co(III) in [Co(NH3)4Cl2]+?
d6.
How many valence electrons are counted for [Co(NH3)4Cl2]+?
Co(III) d6 + 4 NH3 + 2 Cl− gives 18 valence electrons.
What is coordination number?
The number of ligands bound to the metal, or number of donor atoms attached to the metal.
What does the MO treatment of ML6 ask first?
Where are the d electrons?
For a first-row transition metal in an octahedral ML6 complex, what atomic orbital sets are considered?
4p, 4s, and 3d.
What is identified as the LUMO region in the ML6 MO sketch?
The high-energy σ* antibonding orbitals derived largely from dx2−y2 and dz2 interactions.
What is identified as the HOMO region in the ML6 MO sketch?
The non-σ-bonding d orbitals dxy, dxz, and dyz.
What is assumed about the energies of the 6 ligand lone pairs in ML6?
They are the same because the ligands are identical.
Why are ligand lone-pair orbitals usually lower in energy than metal valence orbitals?
Because transition metals are to the left of most ligand atoms in the periodic table, and ligand atoms generally have higher Zeff.
What is the origin of the donor-pair σ-transition-metal bonding model?
Ligand lone pairs are lower in energy and donate into empty metal valence orbitals.
What geometry is assumed for ML6 in the closer MO treatment?
Octahedral.
In an octahedral σ-only complex, which metal orbitals are the σ-bonding metal orbitals?
(n+1)s, (n+1)px, (n+1)py, (n+1)pz, ndz2, and ndx2−y2.
For first-row transition metals in octahedral complexes, what is n?
n = 3.
Which metal d orbitals point in the regions between the metal-ligand bonds?
dxy, dxz, and dyz.
What is the bonding role of dxy, dxz, and dyz in a σ-only octahedral complex?
They are non-bonding.
Why are dxy, dxz, and dyz non-bonding in a σ-only octahedral complex?
Because they point in region of space between metal and ligand bond in a sigma only compound they are non bonding emphasis on the sigma only octahedral complex
Which metal orbitals generate σ-bonding MOs in an octahedral complex?
M(n+1)s, (n+1)px, (n+1)py, (n+1)pz, ndx2−y2, and ndz2.
How are ligand lone-pair σ-donor orbitals represented in the ML6 sketches?
As spheres with 2 electrons.
What is the lowest-energy σ-bonding MO in ML6?
The one formed from the metal (n+1)s orbital in phase with the 6 ligand σ-donor atomic orbitals.
Why is the metal (n+1)s combination the lowest-energy σ-bonding MO?
Because it overlaps in phase with all 6 ligand σ-donor orbitals in the totally symmetric combination.
How many σ-bonding MOs come from the metal p orbitals in octahedral ML6?
Three.
Which metal p orbitals form these three degenerate σ-bonding MOs?
(n+1)px, (n+1)py, and (n+1)pz.
What is said about the energy of the σ-bonding MOs from the metal p orbitals?
They are higher in energy than the (n+1)s σ-bonding MO and they are degenerate.
Which metal d orbitals generate σ-bonding MOs directed at ligands in octahedral ML6?
ndx2−y2 and ndz2.
What is special about ndx2−y2 and ndz2 in octahedral bonding?
They point directly at ligands and form σ-bonding interactions.
What is said about the energies of the σ-bonding MOs from ndx2−y2 and ndz2?
They are the same, degenerate, and higher in energy than the σ-bonding MOs from the metal s and p orbitals.
Why are the σ-bonding MOs from ndx2−y2 and ndz2 degenerate?
Because of the symmetry of the octahedral arrangement.
What does coordination number 2 usually correspond to?
ML2.
What geometry is shown for ML2?
Linear, L–M–L.
What does coordination number 3 usually correspond to?
ML3.
What geometry is shown for ML3?
Trigonal planar.
What does coordination number 4 usually correspond to?
ML4.
What geometries are shown for ML4?
Tetrahedral or square planar.
What determines whether ML4 is tetrahedral or square planar according to the notes?
It depends on the number of d electrons.
What does coordination number 5 usually correspond to?
ML5.
What geometries are shown for ML5?
Trigonal bipyramidal or square pyramidal.
What note is made about the two ML5 geometries?
They can interconvert rapidly.
What does coordination number 6 usually correspond to?
ML6.
What geometry is shown for ML6?
Octahedral.
What largely determines common transition-metal geometries according to the notes?
Largely the 18-electron rule and d-electron count.
How can you quickly remember the left-versus-right trend in the transition series?
Left-side metals more readily give up electrons and form cations; right-side metals tend to hold onto d electrons more.
How can you quickly remember the bonding picture in transition metals?
Think σ donation to metal plus possible π interactions at the same time.
How can you quickly remember what a ligand contributes in basic electron counting?
One lone pair, so 2 electrons.
How can you quickly remember oxidation state in complexes?
Metal oxidation state = overall charge minus the total ligand charge.
How can you quickly remember which d orbitals are non-bonding in a σ-only octahedral complex?
dxy, dxz, and dyz point between ligands, so they are non-bonding.
How can you quickly remember which d orbitals point at ligands in octahedral complexes?
dx2−y2 and dz2 point directly at ligands and are involved in σ bonding.
How can you quickly remember the octahedral coordination-number pattern?
Six ligands gives octahedral ML6.
How can you quickly remember the most common geometries by coordination number?
2 linear, 3 trigonal planar, 4 tetrahedral or square planar, 5 trigonal bipyramidal or square pyramidal, 6 octahedral.