CHEM 511 Chapter 20 page 1 of 21 Chapter 20 d-metal complexes: electronic structures and properties Recall the shape of the d-orbitals... Electronic structure Crystal Field Theory: an electrostatic approach to understand d-orbital complexes. It provides an approximate description of the electronic energy levels that determine the UV and visible spectra, but does not describe the bonding. It predicts that the d-orbitals will not be degenerate. d-orbitals split. Why?
CHEM 511 Chapter 20 page 2 of 21 Electronic spectra and split orbitals Imagine a d 1 electron species: Ti(H2O)6 3+ o is different for every complex, but there are patterns. The Spectrochemical Series I - <Br - <S 2- <SCN - <Cl - <NO2 - <N3 - <F - <OH - <C2O4 2- <H2O<NCS - <CH3CN<py<NH3<en<bipy<phen <NO2 - <PPh3<CN - <CO o also depends on the metal o increases with higher oxidation number o increases going down a group Spectrochemical Series for metals (partial list) Mn 2+ <Ni 2+ <Co 2+ <Fe 2+ <V 2+ <Fe 3+ <Co 3+ <Mn 4+ <Mo 3+ <Rh 3+ <Ru 3+ <Pd 4+ <Ir 3+ <Pt 4+
CHEM 511 Chapter 20 page 3 of 21 Crystal Field Stabilization Energy (CFSE) The split d-orbitals results in a lowering of the energy for three orbitals (dxy, dxz, dyz) and an increase in energy for two orbitals (dx2-y2, dz2) relative to the orbitals in a spherical field For a d 0 metal, Ca 2+, Sc 3+, Ti 4+, CFSE = 0 For a d 1 metal (e.g., Ti 3+ ) For a d 2 metal (e.g., V 3+ ) For a d 3 metal (e.g., Cr 3+ )
CHEM 511 Chapter 20 page 4 of 21 For a d 4 metal (e.g., Cr 2+ ) Which configuration for d 4? Depends on placement in the spectrochemical series! If you have ligands low on the series and metals low on the series, you will generally get a maximum for unpaired electrons This is called a high spin complex or a weak field complex EX. [CrCl6] 4- For ligands high in the series and metals high in the series, you get a low spin complex (strong field case) EX. [Ru(NO2)6] 3- (what is NO2 - called in this case?)
CHEM 511 Chapter 20 page 5 of 21 What about the in-between cases? Best to have experimental evidence on unpaired electrons Generalizations: o Ligands high on the series and 3d metals = strong field cases o 4d and 5d metals with just about any ligand = strong field cases Measuring electron spin Use a Guoy balance to measure paramagnetism. Paramagnetism: attraction to a magnetic field due to unpaired electrons Diamagnetism: repulsion of a magnetic field by paired electrons (weaker than paramagnetism) Can measure the magnetic moment ( ) = 2 (S(S+1)) ½ B Also, = (N(N+2)) ½ B S = spin quantum number (total spin), B = Bohr magneton N = number of unpaired electrons EX. The magnetic moment of an octahedral complex for Co 2+ is 4.0 B. What is the electron configuration?
CHEM 511 Chapter 20 page 6 of 21 Tetrahedral complexes For Td complexes, d-orbitals are split, but opposite of an Oh complex. See the appropriate character table in the Resource Section of your textbook. What labels are given to the orbitals? Which are higher in energy? T is smaller than o because there are fewer ligands Td complexes are only weak field cases EX. What is the CFSE for [CoCl4] -?
CHEM 511 Chapter 20 page 7 of 21 Splitting patterns for various geometries of metal complexes. How well does this correlate with the character tables of the correct point groups?
CHEM 511 Chapter 20 page 8 of 21 The Jahn-Teller Effect If the ground electronic configuration of a non-linear complex is orbitally degenerate, the complex will distort so as to remove the degeneracy and achieve a lower energy # of e - 1 2 3 4 5 6 7 8 9 10 high spin W w s w w s low spin W w w w s s w = weak s = strong blank = no distortion Tetragonal distortion and square planar complexes Distortion of Oh symmetry causes a change in orbital energies and severe distortion could cause loss of ligands! Mainly affects d 7, d 8, d 9 complexes
CHEM 511 Chapter 20 page 9 of 21 Distortion for d 8 complexes can be enough to cause spin pairing in the dz2 and you lose the ligands along the z-axis. To accomplish this, need a strong field ligand or a strong field metal. [NiCl4] 2- [Ni(CN)4] 2- [PdCl4] 2-
CHEM 511 Chapter 20 page 10 of 21 MO theory and octahedral complexes (aka Ligand Field Theory) While crystal field theory is useful in making correlations between experimental evidence (e.g., λmax and electron configuration), it doesn t necessarily explain why using MO theory can held explain the whys. Complexes with σ-bonding As with previous MO theory, there must be matching symmetry for a bond to occur. What are the symmetries of the metal orbitals in an octahedral field? Let s assume we have a 3d metal (see character table). What symmetries will the ligands adopt? What symmetries will overlap in the MO? and as importantly, what doesn t have the right symmetry? eg electrons in complex are not strictly confined to the metal atom o is still a function of t2g and eg separation.
CHEM 511 Chapter 20 page 11 of 21 Complexes with π-bonding -orbitals (bonding and antibonding) may form between d-orbitals and (filled) p-orbitals or d- orbitals and (empty) antibonding orbitals on a ligand d-orbital and p-orbital Consider a ligand with a filled p-orbital (called -donor ligands) d-orbital and antibonding -orbital Recall CO
CHEM 511 Chapter 20 page 12 of 21 What orbitals would overlap with the antibonding orbitals in CO? Δo increases with the following trend: -donor < weak -donor < no effects < -acceptor I - < Br - < Cl - < F - < H2O < NH3 < PR3 < CO (this is the spectrochemical series) Ligands may be neither π-donors nor π-acceptors, but can still be strong σ-donors: CH3 -, H - Skip sections 20.6-20.9
CHEM 511 Chapter 20 page 13 of 21 Thermodynamic aspects: ligand field stabilization energies (LFSE) The data in the table of high and low spin Δoct can be plotted as follows: This correlation with CFSE (LFSE) is found repeated in several manifestations: Lattice energy In a MCl2 lattice, we find the following lattice energies. Hydration energy This graph is for M 2+ (aq) ions
CHEM 511 Chapter 20 page 14 of 21 Irving-Williams Series Ranks the stability of M 2+ complexes as the following Ba 2+ < Sr 2+ < Ca 2+ < Mg 2+ < Mn 2+ < Fe 2+ < Co 2+ < Ni 2+ < Cu 2+ > Zn 2+ Compare electronic structure of Ni 2+ and Cu 2+ How many d e -? What's the expected e - configuration? Comparing multiple substitution of NH3 for M(H2O)6 2+
CHEM 511 Chapter 20 page 15 of 21 Thermochemical correlations with crystal/ligand field stabilization energies The data in the table of high and low spin Δoct can be plotted as follows: This correlation with CFSE (LFSE) is found repeated in several manifestations: Hydration energy This graph is for M 2+ (aq) ions
CHEM 511 Chapter 20 page 16 of 21 Irving-Williams Series Ranks the stability of M 2+ complexes as the following Ba 2+ < Sr 2+ < Ca 2+ < Mg 2+ < Mn 2+ < Fe 2+ < Co 2+ < Ni 2+ < Cu 2+ > Zn 2+ Compare electronic structure of Ni 2+ and Cu 2+ How many d e -? What's the expected e - configuration?
CHEM 511 Chapter 20 page 17 of 21 Comparing multiple substitution of NH3 for M(H2O)6 2+ Lattice energy In a MCl2 lattice, we find the following lattice energies. Ligand Field Theory While crystal field theory is useful in making correlations between experimental evidence (e.g., λmax and electron configuration), it doesn t necessarily explain why using MO theory can help explain the whys.
CHEM 511 Chapter 20 page 18 of 21 Complexes with σ-bonding As with previous MO theory, there must be matching symmetry for a bond to occur. What are the symmetries of the metal orbitals in an octahedral field? Let s assume we have a 3d metal (see character table). What symmetries will the ligands adopt? What symmetries will overlap in the MO? and as importantly, what doesn t have the right symmetry?
CHEM 511 Chapter 20 page 19 of 21 Build the MO diagram... eg electrons in complex are not strictly confined to the metal atom o is still a function of t2g and eg separation. Complexes with π-bonding -orbitals (bonding and antibonding) may form between d-orbitals and (filled) p-orbitals or d- orbitals and (empty) antibonding orbitals on a ligand d-orbital and p-orbital Consider a ligand with a filled p-orbital (called -donor ligands)
CHEM 511 Chapter 20 page 20 of 21 Build the MO showing π-bonding between the metal and p-orbitals. d-orbital and antibonding -orbital Recall CO (build the MO) What orbitals would overlap with the antibonding orbitals in CO?
CHEM 511 Chapter 20 page 21 of 21 Build the MO between d-orbitals and antibonding orbitals in the ligands Δo increases with the following trend: -donor < weak -donor < no effects < -acceptor I - < Br - < Cl - < F - < H2O < NH3 < PR3 < CO (this is the spectrochemical series) Ligands may be neither π-donors nor π-acceptors, but can still be strong σ-donors: CH3 -, H -