High Spin Or Low Spin?

Home , Crystal Field Theory, Ligand

Central Tenants of Crystal Field Theory • The metals (Lewis acids) have d orbitals that are partially filled with electrons.

• Ligands, that are Lewis bases with lone pairs, come in and form a covalent bond.

• Electrostatic repulsion between the ligand lone pairs with the d-orbital subshell leads to higher energies

• Each metal orbital will either be stabilized or destabilized depending on the amount of orbital overlap with the ligand. CFT (Octahedron)

(dz2, dx2-y2)

Orbitals point directly at ligands eg stronger repulsion dx2y2 E dz2 higher energy

(dxy, dyz, dxz)

Orbitals point between ligands t2g dxy dyz dxz weaker repulsion lower energy Energy Levels of d-Orbitals in an Octahedron

• crystal field splitting energy = 

• The size of  is determined by;  metal (oxidation state; row of metal,  for 5d > 4d >> 3d)  ligand (spectrochemical series)

Spectrochemical Series

eg  

t2g

3+ [Cr(H2O)6]

Greater 

       I < Br < Cl < F < OH < H2O < NH3 < en < NO2 < CN < CO Small Δ Large Δ Weak Field Strong Field Weak M-L interactions Strong M-L interactions How Do the Electrons Go In? • Hund’s rule: one electron each in lowest energy orbitals first

• what happens from d4 to d7? d6 Crystal Field Spin Pairing Splitting vs. Δ P High Spin vs. Low Spin

Co3+ (d6)

High Spin Low Spin Δ < P Δ > P High Spin vs. Low Spin Configurations High Spin vs. Low Spin Configurations

high spin low spin

d4 3d metal 3d metal weak field strong field ligand d5 ligand

4d & 5d 6 d always

d7 What About Other Geometries? linear (CN = 2) z

y x tetrahedral (CN = 4) z

y x square planar (CN = 4) z how do the electrons on the d orbitals y interact with the ligands in these cases? x Tetrahedral vs. Octahedral

dxy dyz dxz dz2 dx2-y2

Δtet Δoct dxy dyz dxz dz2 dx2-y2

Energy Spherical dz2 dx2-y2 crystal field

Tetrahedral dxy dyz dxz crystal field Octahedral crystal field Δtetrahedron ≈ (4/9) Δoctahedron Consequently, tetrahedral complexes are always high spin. Square Planar d-level Splitting

z All z-levels lower in z y energy due to less x e− repulsion y x dz2 more stable so lower in energy than dxy orbital

Common for 4d & 5d metals octahedral square planar Low Spin (CN = 6) (CN = 4) High Spin or Low Spin?

1) What is the Coordination Geometry ? Tetrahedral Square Planar (CN = 4) (CN = 4)

High Spin Octahedral Low Spin (CN = 6)

2) Is it a 1st (3d), 2nd (4d) or 3rd (5d) row Transition Metal ?

If 1st Row 2nd or 3rd Row

Depends on Ligand & Metal Low Spin F, Cl, Br, I = High Spin CN, CO = Low Spin Orbital Splitting vs. Geometry Electron Configurations of Complexes

coord. # geom. d-e− config.

+ [Rh(CN)2(en)2]

4 [MnCl6] Electron Configurations of Complexes

coord. # geom. d-e− config.

2 [NiCl4]

2+ [Pt(NH3)4] 2– [FeCl4] is tetrahedral. How many unpaired 2– electrons will [FeCl4] have? Magnetism • paramagnetism • diamagnetism – compounds with one or more – compounds with no unpaired electrons are unpaired electrons repel a attracted by a magnetic field magnetic field – The more unpaired electrons – much weaker effect the greater the attractive force

Octahedral Crystal Field

t2g t2g

d6 High Spin (i.e. Co3+) d6 Low Spin (i.e. Co3+) Color of Transition Metal Complexes

MnSO4 FeSO4 CoSO4 NiSO4 CuSO4 ZnSO4 The color of these compounds comes from the absorption of light that causes an excitation of an electron from one d-orbital to a different d-orbital on the same metal cation. Color of Transition Metal Complexes • Spectrochemical series – relative strength of metal ion/ligand interactions – independent of metal octahedral cobalt(III) complex ions

– – 2– a. CN b. NO2 c. phen d. en e. NH3 f. gly g. H2O h. ox i. CO3

1,10-phenanthroline glycine Absorption Of Light • compounds absorb light if the light has the correct energy to cause an electron to move from a lower energy state to a higher energy state octahedral d3 (i.e. Cr3+)

“GROUND STATE” “EXCITED STATE”

eg eg Light (hv)

oct (photon) oct E

t2g t2g

The size of oct dictates the wavelength of light needed, and therefore, the color of the compound. Absorption Dependence on d-electrons Different numbers of transitions will occur for different d-electron configurations

g eg E t 2g t2g

d10 = colorless d0 = colorless

d1-d9 = various visible transitions