Transcript Slide 1
Lecture 22 Electronic structure of Coordination Compounds 1) Crystal Field Theory • Considers only electrostatic interactions between the ligands and the metal ion. 1 • Ligands are considered as point charges creating an electrostatic field of a particular symmetry. 3 2 E M 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 M o M x y 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). free ion the ion in an averaged ligand field the ion in a sertain ligand field 2x = 3y x + y = o x = (3/5)o y = (2/5)o 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. ML6 complexes • In the field of Oh symmetry five degenerate d-orbitals will be split into two sets, t2g and eg orbitals (check the Oh point group character table) • Three t2g orbitals be stabilized by 0.4o and two eg orbitals will be destabilized by 0.6o L L L eg 1 L L dz2=0.5(dy2-z2+dx2-z2) z 1 z dx2-y2 4 z 4 L 2x = 3y x + y = o eg t2g the ion in an averaged ligand field 2 3 y y x = 0.6o y = 0.4o x 2 3 dy2-z2 2 x the ion in an octahedral ligand field dyz 1 dx2-z2 x z t2g 4 3 y (2z2-x2-y2, x2-y2) … x … y 3 … T2g 1 y x Oh Eg 4 (xz, yz, xy) 2 3) Cubic and tetrahedral shapes. ML8 and ML4 complexes • • In the cases of cubic (Oh) and tetrahedral (Td) environments d-orbitals are split into two levels, t-and e-. t-Orbitals (dxy, dxz, dyz) are destabilized, while two eorbitals (dz2, dx2-y2) are stabilized dyz z dx2-y2 44 3 y eg 5 E (2z2-x2-y2, x2-y2) T2 (xy, xz, yz) 6 3 the ion in a cubic or tetrahedral ligand field 2 1 x 4 1 4 the ion in an averaged ligand field t2g z MX4 4 y x dyz z 2 x = 0.6o y = 0.4o 5 x dx2-y2 y 8 x Td 1 2 8 6 44 3 1 2 z MX8 e y y 4 3 x t2 4) d-Orbital splitting in the fields of various symmetries MX4 • The d-orbital splittings presented on diagram correspond to the cases of cubic shape MX8 (Oh), tetrahedral shape MX4 (Td), icosahedral shape MX12 (Ih), octahedral shape MX6 (Oh) and square planar shape MX4 (D4h). dx2-y2 E b1g Oh Td Ih Oh D4h MX6 dz2 dx2-y2 MX8 eg Ih Hg MX4 dyz dxz t2g dxy (2z2-x2-y2, x2-y2, xy, xz, yz) free ion x2+y2, z2 B1g x2-y2 B2g xy Eg (xz, yz) … b2g dyz dxz t2 dxy D4h A1g dxy MX12 hg averaged ligand field e dz2 dx2-y2 eg a1g dz2 dz2 dx2-y2 dyz dxz t2g dxy eg dyz dxz 5) High and low spin octahedral complexes Some consequences of d-orbital splitting: • • low spin d6 high spin d6 Magnetism. In the case of large we observe low-spin, while for small highspin complexes (d4-d7 configurations). eg Oh 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 d0, d5 (high-spin) and d10 complexes CFSE is always zero. • Redox potentials. Some oxidation states may become more stable when stabilized orbitals are fully occupied. So, d6 configuration becomes more stable than d 7 as o increases. • E M-L bond length. Ionic radii of [ML6]n+ are smaller and M-L are shorter for low-spin complexes and have a minimum for d6 configuration. MX6 dx2-y2 dz2 eg large o small o t2g t2g dxy dxz dyz