Transcript Slide 1
Coordination Complexes - Summary Examples of ligand substitution reactions [Ni(H2O)6]2+(aq) + 6 NH3(aq) [Ni(NH3)6]2+(aq) + 6 H2O(l) Need to know: Crystal field theory - Inert, labile, thermodynamic stability - Structural aspects of complexes (a) cis-trans in octahedral/square planar complexes (b) Chiral forms (enantiomers) in octahedral complexes Spectrochemcial series: Cl- < H2O < NH3 < CN- Crystal Field Theory z y - Mn+ - x - - Which d-orbitals are effected the most? z z x- M x dz2 along z-axis - - M x- dx2- y2 along x-y axis These two d-orbitals constitute the higher energy eg set. dzx orbital has lobes between z-x axis z Less repulsion - M - x - Likewise interactions for dxy and dyz These three d-orbitals constitute the lower energy t2g set. Octahedral Tetrahedral t2 eg Δt Δo t2g e Use diagram/spectrochemical series to explain: Absorption wavelengths due to electronic excitation Paramagnetic properties (unpaired electrons) Octahedral MO Diagram – s-bonded complex L Can be pz, s, or a hybrid L L z y Mn+ L L L x L Which d-orbitals form s-bonds with ligands at the corners of the octahedron? Octahedral MO Diagram – s-bonded complex L Can be pz, s, or a hybrid L z z y M x L M x dz2 along z-axis L dx2- y2 along x-y axis L This results in the formation of 4 MO’s (eg and eg*). L Octahedral MO Diagram – s-bonded complex L dzx orbital has lobes between z-x axis z NO OVERALP L M x L Likewise interactions for dxy and dyz L These three d-orbitals are non-bonding The Molecular Orbital Diagram t1u* 4p a1g* 4s eg* Δo t2g 3d eg t1u a1g L - AO’s Color in coordination complexes The colors are determined by Δ. Different ligands generate crystal fields of different strength. When the molecules absorb visible light, excited electrons jump from lower energy t2g to the higher energy eg orbital. The Δ (difference between energies of the two orbitals) is equal to the energy of the absorbed photon, and related inversely to the wavelength of the light. Weaker field ligands with smaller Δ emit light of longer λ and thus lower v. Similarly, stronger field ligands with larger Δ emit light of shorter λ and thus higher v. λ Absorbed 400nm Violet absorbed 450nm Blue absorbed 490nm Blue-green absorbed 570nm Yellow-green absorbed 580nm Yellow absorbed 600nm Orange absorbed 650nm Red absorbed Color observed Green-yellow observed (λ 560nm) Yellow observed (λ 600nm) Red observed (λ 620nm) Violet observed (λ 410nm) Dark blue observed (λ 430nm) Blue observed (λ 450nm) Green observed (λ 520nm) Problems 1. (a) When water ligands in [Ti(H2O)6]3+ are replaced by CN- ligands to give [Ti(CN)6]3-, the maximum absorption shifts from 500 nm to 450 nm. Is this shift in the expected direction? Explain. What color do you expect to observe for this ion? CN- is a stronger field ligand than H2O. Therefore the energy separation between the t2g and eg levels is greater. Ti3+ is a d1 metal ion. Do = hc/l. If D is larger, l is smaller. [Ti(CN)6]3eg [Ti(H2O)6]3+ eg A(lmax) = 500 nm Solution appears red A(lmax) = 450 nm Do Do Solution appears yellow t2g t2g (b) The [Fe(H2O)6]3+ ion has a pale purple color, and the [Fe(CN)6]3- ion has a ruby red color. What are the approximate wavelengths of the maximum absorption for each ion? Is the shift of wavelength in the expected direction? Explain. CN- is a stronger field ligand than H2O. Therefore the energy separation between the t2g and eg levels is greater. Fe3+ is a d5 metal ion. Do = hc/l. If D is larger, l is smaller.