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TAMU CHEM 362 - Lecture_23-24

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298Chapter 23 Introduction to the Transition Elements: Ligand Field Theory Bonding in Transition Metals • Crystal Field Theory (CFT) • Ligand Field Theory (LFT) • Molecular Orbital Theory (MO) The power behind any theory is how well it explains properties and the spectroscopic behavior of compounds and, in the case of transition metals complexes, magnetic behavior. Ligand Field Theory (LFT) is much simpler than MO theory (a little more sophisticated than CFT), but it is a very useful theory.299 Transition Elements / Compounds - “d block” elements/compounds - Primarily strong, hard metals in their elemental forms that conduct electricity and heat very well. - They form colored compounds (varies with ox. state) due to electronic transitions in the visible region from one d orbital to another (small energy gap) - They are often paramagnetic (i.e. they contain unpaired electron(s)) Various bonding theories can explain the properties of T.M. (transition metal) compounds. First, show (without derivation) the M.O. approach300Bonding in Transition Metal Comlexes: Two Considerations A. Geometry ML6 Oh (octahedral) ML4 Td vs D4h (tetrahedral vs. square planar) B. Ligand Type π – acceptors π – donors σ – donors many ligands are a combination of donor types, but the “pure” donor diagrams can be considered π – acceptors CO, NO+, CNR, CN- filled empty d-orbitals π*301π – donors halides (X- = Cl, Br, I) NH2- (amide) NR2- O2- OR-, SR- σ – donors H-, NH3 Molecular Orbital Treatment Without going into the group theory considerations of how to set up symmetry adapted atomic orbitals on the metals and the ligands. First, recall MO diagram for CO. M-CO σ bond M-CO π bond s s p p σ σ σ σ* π* π C 4e- O 6e- 10e- empty C lone pair σ bond atomic orbitals lower in energy ( )302MLn How would one go about trying to build a molecular orbital diagram for a coordination complex? - Assume central atom has s,p, d orbitals in valence shell = 9 orbitals - Assume each ligand atom, L, has s and p orbitals 4 x n ligands = 4n orbitals Octahedral ML6 metal 9 orbitals ligands 4x6 = 24 orbitals Thirty – three orbitals sounds like a lot! Actually, it is not as bad as it sounds, because the orbitals can be grouped according to special rules dictated by the shape of the molecules → symmetry adapted linear combinations (SALC’S) } Total number of orbitals in the “basis set” is 33.303Electronic Structure of Transition Metal Complexes Q. What are we trying to accomplish? A. An understanding of how d orbitals are affected by bringing “n” ligands around the metal center. MLn n = 6 Octahedral basic n = 4 Tetrahedral geometry The d orbitals on M change energy according to the types of orbitals on L (σ, π, π*) }304σ – Donor Only Case Metal d s p Energy-wise d < s < p (5) (1) (3) highest occupied are d as s and p are empty for Mn+ Ligands s, pz, px, py along M-L these form axis so used in π - bonds σ - bonding on the ligands, if only σ – bonding is possible for an ML6 compound: Metal Ligand d, s, p = 9 orbitals s + pz → sp 6 x orbitals we use these to make SALC’s symmetry adapted linear combinations∴305Six SALC The Ligand Group Orbitals for :L donating a lone pair to a M-L sigma bond look like this: Now, we need to match these symmetries with the same symmetries from the metal valence orbitals. These will be the only combinations to produce overlap!306The metal orbitals are grouped by symmetry labels just like the ligand SALC’S s → A1g (one orbital) p → T1u (three orbitals so triply degenerate) In an octahedral environment, the five d orbitals split: Eg (two orbitals so doubly degenerate) dx2-y2, dz2 d T2g (three orbitals so triply degenerate) dxy, dxz, dyz since d < s < p in energy, the M.O. diagram arranges them T2g T1u A1g Eg307308Oh M.O. Diagram σ-Donor [Co(NH3)6]3+ note* order of coulomb energies for metal & ligand orbitals σ(L) < nd < (n+1)s < (n+1)p T2g Eg A1g T1u Eg T1u A1g a1g a1g (σ*) eg eg (σ*) t1u t1u (σ*) t2g non-bonding Δo important part of the MO diagram Δo changes with ligand 6 NH3 sigma bonds 12e- in 6 NH3 ligands { 3d 4s 4p metal orbitals molecular orbitals ligand orbitals309π – Donor Case you have σ and π bonding there are lone pairs that can make both types of bonds as opposed to :NH3 which only has a lone pair for σ – bonding (sp hybrid) L s, pz, px, py σ - bonds π - bonds for ML6: M: (same as before) 9 orbitals A1g, T1u, T2g Eg s p d L: 6 σ orbitals (A1g, T1u, Eg) 6 x 2 (px,py) = 12 π orbitals (T1g, T2g, T1u, T2u)( )310What are ligands that use π – bonds? (π – donors like halides for example) Group orbital made up of combinations of px and pz orbitals on four of the atoms Note: There are two more sets based on M dyz and M dxy.311Oh M.O. Diagram π – Donor [CoCl6]5- (Don’t need to sketch the whole diagram) T1g, T2g T1u, T2u Eg T2g A1g T1u A1g T1u Eg eg (σ) eg (σ*) t2g (π) t2g (π*) 3d 4s 4p π-orbitals px, py σ-orbital pz Metal Orbitals Molecular Orbitals Ligand Orbitals focus on this part only both sets of d orbitals are driven ↑ in energy due to lower lying ligand orbitals Δo312Oh M.O. Diagram π – acceptor CO, NO+, CNR, CN- T2g Eg T1g, T2g T1u, T2u A1g T1u Eg eg (σ M-L) eg (σ* M-L) t2g (π) t2g (π*) 4d Δo π* orbitals on CO (6 x 2 each) σ orbitals on CO (6 x 1 each) take a look at the CO MO diagram Mo(CO)6 Metal Orbitals (only consider the d orbitals) Molecular Orbitals Ligand Orbitals313Bottom Line and this ALL I WANT YOU TO HAVE TO BE RESPONSIBLE FOR KNOWING (you don’t have to know how to derive the previous results): M.O. Theory predicts different energy separations for the d orbitals (which are where the outer electrons reside on the metal) depending on the type of ligand To summarize Oh M.O. Splittings Octahedral, ML6, symmetry (geometry) dictates the two sets of orbitals { 1 Δo eg t2g 2 extent and type of M-L bonding dictates the separation! Δo Δo Δo eg (σ*) eg (σ*) eg (σ*) t2g (π) t2g (n.b.) t2g (π*) M-L bonding


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