Background: Crystal Field Theory
The Crystal Field Theory is a bonding theory used in describing the characteristic colors and magnetic properties of complex ions due to the repulsions between the negatively charged d electrons of the central metal atom and the negatively charged electrons in the ligands. Depending on the magnitude of the repulsion, the degenerate (same energy) d orbitals can either be raised to a higher energy level or dropped to a lower energy level both with respect to the average energy of the d orbitals in a ligand field. Read More at Crystal Field Theory
The difference in energy between the higher d orbitals and the lower d orbitals is termed delta (∆) and signifies the crystal field splitting. The subscript letter following the delta specifying what type of structure the crystal field splitting is for. I.e. ∆o = octahedral, ∆t = tetrahedral, ∆sp = square planar
A prediction can be made concerning the magnitude of d-level splitting by a ligand by using the spectrochemical series – a ranking of the strengths of various ligands.
Strong Field Ligands Weak Field Ligands
CN->NO2->en>py ~ NH3>EDTA4->SCN->H20>ONO->ox2->OH->F->SCN->Cl->Br->I-
- Note 1: Water is approximately the midpoint of the specrochemical series and is typically considered a weak field ligand.
- Note 2: The spectrochemical series will be given on the exam. You will NOT have to memorize it but you will have to UNDERSTAND it.
Strong Field Ligand
Weak Field Ligand
Low Spin Complex
High Spin Complex
Note: PE = Pairing Energy
An Analogy (For an octahedral complex):
- d orbitals (dxy, dyz, dxz, dx2y2, dz2)= double decker bus (like the ones from London at the bus station near the MU)
- high energy d orbitals (dx2y2, dz2) = seats on the top deck of the bus
- low energy d orbitals (dxy, dyz, dxz) = seats on the bottom deck of the bus
- pairing energy (PE) = the energy it takes to sit next to a person
- Hund’s Rule = people typically fill up the empty seats first before they pair up with other people
- splitting energy (∆) = the energy it takes to walk up the stairs to the next level
One of the double deckers in the Davis's student-run (and student-driven) bus system. By P. C. Loadletter
Small Bus = High Spin
Bus Analogy: The seats are all full on the lower level of the bus. Since the stairs are small, it takes less energy to walk up the stairs to the next level to get your own seat.
Actual Chemistry: PE>∆; ∆ is small and PE is big. Since ∆ is small, it takes less energy to jump up to the next energy level than it does to pair with another electron.
The Result: High Spin
Large Bus = Low Spin
Bus Analogy: The seats are all full on the lower level of the bus. Since the stairs are large, it takes a lot more energy to walk up the stairs then it does to pair with another electron.
Actual Chemistry: PE<∆; ∆ is large and PE is small. Since ∆ is large, it takes less energy to pair up with another electron than it does to jump to the next energy level.
The Result: Low Spin
- Octahedral – either high spin or low spin
- Tetrahedral – typically high spin
- Square Planar – typically low spin
- Octahedral – paramagnetic (high spin) or diamagnetism (low spin)
- Tetrahedral – typically paramagnetic
- Square Planar – typically diamagnetic
The color reflects the energy that is required to move electrons from one energy level to the next. The color that is absorbed is the complementary color of the color that is reflected (the color that we see).
Note: Splitting high/low matters for d4-d7 elements.
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