Lecture 22 Electronic structure of Coordination Compounds 1) Crystal Field Theory Considers only electrostatic interactions between the ligands and the.

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Lecture 22 Electronic structure of Coordination Compounds 1) Crystal Field Theory Considers only electrostatic interactions between the ligands and the metal ion. Ligands are considered as point charges creating an electrostatic field of a particular symmetry. Main steps to estimate the energy of d-orbitals in a field of a particular symmetry: 1) An isolated metal ion. Five d-orbitals are degenerate 2) A metal ion in an averaged ligand field. The orbital energy increases due to electron (metal) – electron (ligands) repulsions. 3) A metal ion in a ligand field of certain symmetry. d-Energy levels may become split into several sublevels. (This can be learned from the appropriate character table). Some of d-orbitals become stabilized, some become less stable. The total orbital energy gain due to the stabilization is equal to the total orbital energy loss.

2) Octahedral field. ML 6 complexes In the field of O h symmetry five degenerate d-orbitals will be split into two sets, t 2g and e g orbitals (check the O h point group character table) Three t 2g orbitals be stabilized by 0.4  o and two e g orbitals will be destabilized by 0.6  o OhOh … EgEg (2z 2 -x 2 -y 2, x 2 -y 2 ) … T 2g (xz, yz, xy) …

3) Cubic and tetrahedral shapes. ML 8 and ML 4 complexes In the cases of cubic (O h ) and tetrahedral (T d ) environments d-orbitals are split into two levels, t-and e-. t-Orbitals (d xy, d xz, d yz ) are destabilized, while two e- orbitals (d z2, d x2-y2 ) are stabilized TdTd E(2z 2 -x 2 -y 2, x 2 -y 2 ) T2T2 (xy, xz, yz)

4) d-Orbital splitting in the fields of various symmetries The d-orbital splittings presented on diagram correspond to the cases of cubic shape MX 8 (O h ), tetrahedral shape MX 4 (T d ), icosahedral shape MX 12 (I h ), octahedral shape MX 6 (O h ) and square planar shape MX 4 (D 4h ). D 4h A 1g x 2 +y 2, z 2 B 1g x 2 -y 2 B 2g xy EgEg (xz, yz) … IhIh HgHg (2z 2 -x 2 -y 2, x 2 -y 2, xy, xz, yz)

5) High and low spin octahedral complexes Some consequences of d-orbital splitting: Magnetism. In the case of large  we observe low-spin, while for small  high- spin complexes (d 4 -d 7 configurations). Energy. If the occupancy of the orbitals (x) stabilized by a ligand field is more than that of the destabilized orbitals (y), the complex becomes more stable by CFSE which is (0.4x-0.6y)   for octahedral species. For d 0, d 5 (high-spin) and d 10 complexes CFSE is always zero. Redox potentials. Some oxidation states may become more stable when stabilized orbitals are fully occupied. So, d 6 configuration becomes more stable than d 7 as  o increases. M-L bond length. Ionic radii of [ML 6 ] n+ are smaller and M-L are shorter for low-spin complexes and have a minimum for d 6 configuration.