Coordination-Bonding PDF

Angular Overlap Model (by Larsen and Lamar, 1974) LFT includes formation of bonding orbitals, but it does not use the resulting energy change; difficult to use for geometries other than Oh, SQP, and Td AOM offers flexibility to deal with a variety of geometries and ligands AOM estimates the streng...

Angular Overlap Model (by Larsen and Lamar, 1974) LFT includes formation of bonding orbitals, but it does not use the resulting energy change; difficult to use for geometries other than Oh, SQP, and Td AOM offers flexibility to deal with a variety of geometries and ligands AOM estimates the strength of the interaction between M and L orbitals based on the overlap between them and combines the values for all L and M d orbitals for the complete picture. - both σ and π interxns are considered Sigma Interaction for Angular Overlap - different coord nos. and geometries are treated Sigma Donor Interxns - strongest σ interaction is between dz2 and p; - used as reference Billones PowerNotes Angular Overlap Parameters: Sigma Interactions Billones PowerNotes Example :[M(NH ) ]n + 3 6 • NH3 has no π orbitals for bonding (donor or acceptor) with M; • The lone pair is mostly pz in composition. To calculate the energy for a d orbital, get the sum of the numbers for the appropriate ligand positions in the vertical column. dz2 orbital: Ligands at 1 and 6 positions interact strongly: 1 + 1 = 2eσ Ligands at 2, 3, 4 and 5 positions interact weakly: 1/4 + 1/4 + 1/4 + 1/4 = 1eσ Overall increase in energy of dz2 orbital = 3eσ Billones PowerNotes dx2-y2 orbital: • Ligands at 1 and 6 positions do not interact: 0 + 0 = 0eσ • Ligands at 2,3,4 and 5 positions interact strongly: 3/4 + 3/4 + 3/4 + 3/4 = 3eσ • Overall increase in energy of dx2-y2 orbital = 3eσ dxy, dxz, dyz orbitals: • These orbitals do not interact with L orbitals in σ fashion. ⮚ unchanged Ligand orbitals: • Ligands at 1 and 6 positions interact: lowered by 1eσ • Ligands at 2,3,4 and 5 positions are lowered by 1/4 eσ upon interaction with dz2 and by 3/4 eσ upon interaction with dx2-y2, for a total of 1eσ. Billones PowerNotes Energies of d Orbitals in Octahedral Complexes Resulting E pattern: ⮚ destabilized by 3eσ ⮚ unchanged ⮚ stabilized by eσ Billones PowerNotes π Acceptor Interactions • Ligands such as CO, CN-, and PR3 are π acceptors. ⮚ interacts with the metal d orbital in π fashion Billones PowerNotes Angular Overlap Parameters: π Interactions Billones PowerNotes Energies of d Orbitals in Octahedral Complexes: Sigma Donor and Pi acceptor ligands • The dxy, dxz, and dyz orbitals all have total interactions of 4eπ • In the formation of molecular orbitals, these three d orbitals undergo stabilization by -4eπ, and the ligand orbitals involved in π interactions are raised in energy. Billones PowerNotes π Donor Interactions The angular overlap model treats π-donor ligands similarly to π-acceptor ligands except that for π-donor ligands, the signs of the changes in energy are reversed, Billones PowerNotes Energies of d Orbitals in Octahedral Complexes: Sigma Donor and Pi donor ligands • The molecular orbitals with high d orbital contribution are raised in energy, whereas the molecular orbitals with high ligand π-donor orbital character are lowered in energy. Billones PowerNotes The Spectrochemical Series Ligands are classified by their donor and acceptor capabilities. The spectrochemical series is a ranking of ligands on the basis of how they result in d orbital splitting. • Ligands high in the spectrochemical series tend to cause large splitting of d-orbital energies (large values of Δ) and to favor low-spin complexes • Ligands low in the series are not as effective at causing d-orbital splitting and yield lower values of Δ. Billones PowerNotes Charge on Metal • changing the ligand or the metal affects the magnitudes of eσ and eπ. • consequence may be a change in the number of unpaired electrons Billones PowerNotes Application of Angular Overlap Model (AOM) Billones PowerNotes Billones PowerNotes Billones PowerNotes The Jahn-Teller Effect Jahn–Teller theorem states that degenerate orbitals cannot be unequally occupied. To avoid these unfavorable electronic configurations, molecules distort to render these orbitals nondegenerate. Billones PowerNotes Four- and Six-Coordinate Preferences Angular Overlap Energies of Four- and Six-Coordinate Complexes across a Transition Series. Only sigma bonding is considered. Oh and SQP both strong and weak field cases Billones PowerNotes Four- and Six-Coordinate Preferences Angular Overlap Energies of Four- and Six-Coordinate Complexes across a Transition Series. Only sigma bonding is considered. Td and SQP both strong and weak field cases Billones PowerNotes Seatwork: 1. Using the AOM, construct the energy level diagram for ML 4 having a) Td and b) SQP geometry. L is a sigma and pi donor ligand. 2. Using the AOM, construct the energy level diagram for a) trigonal planar and b) trigonal bipyramidal geometry. L is a sigma donor and pi acceptor ligand. Hint: 1)Calculate the in(de)crease in energy of each metal orbital. 2)Calculate the in(de)crease in energy of each ligand orbital. 3)Construct the diagram based on the data in 1) and 2). Billones PowerNotes