Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Color and Bonding in Transition Metal Complexes Chemistry 123 Dr. Patrick Woodward Supplemental Lecture 3 Intra-atomic (localized) excitations – – Transition metal ions, complex ions & compounds (d-orbitals) Lanthanide ions and compounds (f-orbitals) [Ni(NH3)6]2+ NiSO4 Cu3(CO3)2(OH)2 CuSO4 Malachite In these complexes the color comes from absorption of light that leads to excition of an electron from an occupied d-orbital to an empty or ½-filled d-orbital. The energy separation between d-orbitals depends upon the interaction between the dorbitals and the ligands. There are two ways to rationalize the energy separation Crystal Field Theory & Ligand Field Theory. 1 Energy Crystal Field Splitting (Octahedron) dz2 & dx2-y2 orbitals (e (eg) •Point directly at the ligands •Stronger (repulsive) interaction with the ligands dxy, dyz & dxz orbitals (t2g) dz2 dx2-y2 dyz dxz •Point between the ligands •Weaker (repulsive) interaction with the ligands dxy Electrons in d-orbitals are repelled from the electrons in the ligands, based on electrostatic interactions. This causes two of the d-orbitals (dz2 & dx2-y2) to be at higher energy than the other three (dxy, dyz, dxz) Ligand Field Splitting (Octahedron) dz2 & dx2-y2 orbitals (e (eg) •Point directly at the ligands •Sigma antibonding interaction with the ligands dz2 σ* dx2-y2 σ* dxy, dyz & dxz orbitals (t2g) •Point between the ligands •Pi antibonding interaction with the ligands dxy π* dyz π* dxz π* Ligand field theory is based on covalent interactions between the metal and the surrounding ligands, we can use MO theory to understand it. The splitting of orbitals into a lower energy t2g set of orbitals (non bonding, or piantibonding) & and a higher energy eg set of orbitals (sigma antibonding). 2 d-orbital Splitting (Octahedron) dz2 dx2-y2 dz2 & dx2-y2 orbitals (e (eg) eg •Point directly at the ligands •Sigma antibonding interaction with the ligands Δ = Crystal Field Splitting Energy dxy, dyz & dxz orbitals (t2g) •Point between the ligands •Pi antibonding interaction with the ligands t2g dxy dyz dxz Cr3+ 5 dd-orbitals on Cr (Cr3+ = d3 ion) 3 electrons in the d-orbitals [Cr(NH3)6]3+ Octahedron :NH3 : N H HH 6 Ligand Orbitals Nitrogen lone pairs (all containing 2 e-) Only sigma interactions are allowed 3 [Cr(NH3)6]3+ Antibonding (σ*) Metal-Ligand MO’s eg orbitals Δ = Crystal Field Splitting Energy t2g orbitals Energy Metal (Cr) d-orbitals Nonbonding Metal d MO’s Nonbonding Ligand MO’s Ligand (N) lone-pair orbitals Δ ~ 3.0 eV (~410 nm) Absorption = Violet Color = Yellow Bonding (σ) Metal-Ligand MO’s eg eg Δ Δ t2g t2g Cl– Small Δ Spectrochemical Series < F- < H2O < NH3 < NO2- < CN- Weak Field Ligand Weak M-L interaction Large Δ Strong Field Ligand Strong M-L interaction 4 High spin & low spin states Large Δ Small Δ Low Spin Configuration High Spin Configuration The t2g set of d-orbitals are completely filled before electrons fill the eg orbitals All five dorbitals are filled before pairing up 2 electrons in one orbital Diamagnetism – All electrons are paired up, which leads to equal numbers of spin up and spin down electrons (i.e. [Co(CN)6]3-) Paramagnetism – Unpaired electrons, which leads to unequal numbers of spin up and spin down electrons ((i.e. [CoF6]3-) Cr3+ Gemstones Corundum - Al2O3 Beryl - Be3Al2Si6O18 Ruby Al2O3:Cr3+ In both gemstones Cr3+ substitutes for Al3+, which is surrounded by 6 oxygen ions in an octahedron. The color comes from a d-to-d excitation on the Cr3+ center. Emerald Be2Al2Si6O18:Cr3+ 5 Measurements show that the crystal field splitting, Δ, of the Cr3+ ion in ruby and emerald are: 1. Ruby = 2.3 eV (540 nm) & Emerald = 1.9 (650 nm) 2. Ruby = 1.9 eV (650 nm) & Emerald = 2.3 (540 nm) 50% 10 50% Ruby = 2.3 eV (540 nm) & Emerald = 1.9 (650 nm) Ruby = 1.9 eV (650 nm) & Emerald = 2.3 (540 nm) Do you expect the Cr-O distances to be shorter in ruby (Δ = 2.3 eV) or emerald (Δ = 1.9 eV) 1. Shorter in Ruby 2. Shorter in Emerald 50% 50% 10 Shorter in Ruby Shorter in Emerald 6 Ligand Field Splitting (Tetrahedron) dxy, dyz & dxz orbitals (t2) •Stronger antibonding interaction with the ligands •Higher energy dxy dyz dxz dz2 & dx2-y2 orbitals (e) •Weaker antibonding interaction with the ligands •Lower energy dz2 dx2-y2 The crystal field splitting, Δ, for a tetrahedron is considerably smaller than for an octahedron Ligand Field Splitting (Tetrahedron) dxy, dyz & dxz orbitals (t2) dxy dyz dxz •Stronger antibonding interaction with the ligands •Higher energy t2 Δ = Crystal Field Splitting Energy dz2 & dx2-y2 orbitals (e) •Weaker antibonding interaction with the ligands •Lower energy e dz2 dx2-y2 The crystal field splitting, Δ, for a tetrahedron is considerably smaller than for an octahedron 7 5 dd-orbitals on Cr (Cr6+ = d0 ion) 0 electrons in the dd-orbitals Cr3+ O 4 Ligand Orbitals Oxygen lone pairs (all containing 2 e-) CrO42- Tetrahedron [CrO4]2- t2 orbitals (antibonding) e orbitals (antibonding) CT Energy Metal (Cr) d-orbitals Nonbonding Oxygen 2p MO’s e orbitals (bonding) t2 orbitals (bonding) PbCrO4 12 Oxygen 2p orbitals (4 oxygens x 3 p orbitals) CT ~ 3.3 eV (~375 nm) Absorption = Violet Color = Yellow 8 Charge Transfer in Sapphire • The deep blue color the gemstone sapphire is also based on impurity doping into Al2O3. The color arises from the following charge transfer excitation: Fe2+ + Ti4+ → Fe3+ + Ti3+ (λmax ~ 2.2 eV, 570 nm) • The transition is facilitated by the geometry of the corundum structure where the two ions share an octahedral face, which allows for favorable overlap of the dz2 orbitals. Fe2+ Ti4+ • Unlike the d-d transition in Ruby, the chargetransfer excitation in sapphire is fully allowed. Therefore, the color in sapphire requires only ~ 0.01% impurities, while ~ 1% impurity level is needed in ruby. 9