Jahn-Teller Distortions

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Alison Welch
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1 Selections from Chapters 9 & 16 The transition metals (IV) CHEM 62 Monday, November 22 T. Hughbanks Jahn-Teller Distortions Jahn-Teller Theorem: Nonlinear Molecules in orbitally degenerate states are inherently unstable with respect to distortion. Explanation: If a molecule has sufficiently high symmetry that it is possible to have two or more molecular orbitals with the same energy (by symmetry). If a degenerate set of orbitals is occupied unequally with electrons, the molecule will distort. T.M.s: Some Oxidation State Trends Maximum can never exceed the group number! Highest Ox. State: Ti IV, V V, Cr VI, Mn VII - after Mn (for 1st row), ox. states higher than +3 hard to achieve. Early metals tend to be most stable in highest oxidation state: Sc III, Y III, Ln III, Ti IV, Zr IV, Hf IV Oxides are more acidic in higher oxidation state Some broad chem. similarities are found for ox. states II & III (6 or coordinate for complexes in solution or in crystals, aqueous chem., etc.) Except for Cu I, -acid ligands and organometallics dominate for ox. states lower than. 1

2 Acidic High Oxidation State Oxides Cr VI is acidic: CrO 3 + H 2 O H 2 CrO [HCrO ] + H 3 O + Green Cr III is amphoteric: Cr 2 O 3 + H 3 O + (aq) [Cr(H 2 O) 6 ] acid -base properties similar to [Al(H 2 O) 6 ] -Cr 2 O 3 + OH (aq) soluble chromites HMnO, H 2 CrO, H 3 VO acidity trend similar to HClO, H 2 SO, H 3 PO. Oxidizing Agents (acidic soln) [MnO ] + 8 H e Mn + 2 H 2 O E = V [Cr 2 ] + 1 H e 2 Cr + 7 H 2 O E = V [VO 2 ] + + H + + e 2 V + 2 H 2 O E = V II state: Compounds of all elements, Ti-Cu, are found. in water, all M(H 2 O) 6 are known except Sc. in aerated acid solution V, Cr, and Fe are oxidized to. solid state oxides and halides known for all M II metals (with varying degrees of ionicity) M II X 2 all have CdX 2 -type structures M II O compounds surprisingly complex 2

3 CdX 2 layer III state: All T. M. elements, for Ni & Cu, ox. state III is fairly rare FeCl 3 : some similarity with AlCl 3 Both form M 2 Cl 6 molecules in liquid, gas phases Both can act as Lewis acids, e.g., as Friedel crafts catalysts but AlCl 3 not much of an oxidant! Ti Co form [M III (H 2 O) 6 ] ions Mn III Mn II & Co III Co II - quite readily [Fe III (H 2 O) 6 ] only forms in strong, noncomplexing acids III state is less stable for later TM s, e.g., Co III (H 2 O) 6 + e Co II (H 2 O) V, but Co III can be stabilized by complexation by strong field ligands, e.g., Co III (NH 3 ) 6 + e Co II (NH 3 ) V. [synthesis: CoCl 2 (soln) + NH NH 3 + O 2 [Co III (NH 3 ) 6 ]Cl H 2 O + en H en + O 2 [Co III (en) 3 ]Cl H 2 O] 3

4 actual charge oxidation state IV state: important for Ti, V (TiO 2, TiCl,, extensive chemistry of VO ), but for later first-row TM s this tate is found mainly in fluorides, oxides (oxo complexes) Higher ox. states: only oxides, fluorides, oxyfluorides For a given ligand type, higher ox. states are characterized by increasing covalence (e.g., including extensive M O bonding) First Row T.M.s vs. 2nd and 3rd T.M. Series Radii: 2nd and 3rd row ~ Å larger But 2nd and 3rd row radii close to each other (lanthanide contraction) Some 2nd and 3rd row elements have very similar chemistries (e.g., Zr and Hf). Overall, 2nd and 3rd row elements are more similar to each other than to 1st row. Metal-metal bonds stronger and more numerous examples are known for 2nd and 3rd row elements. This leads to substantial differences in lower oxidation state chemistries. Higher oxidation states generally more stable for 2nd & 3rd row. Examples of oxidation state differences CrO 3 - very powerful oxidant (oxidizes most organics), forms H 2 CrO in acidic solution; MoO 3, WO 3 - milder oxidants (will oxidize H 2 & NH 3, but not most organics) RuO, OsO - are strong oxidants, but no Fe analog exists at all MnO disproportionates to form MnO 2 and MnO (strong oxidant), while ReO 3 forms a stable metallic solid Tc 2, Re 2 - isolable solids with reasonable stability, Mn 2 - explosively oxidizing Ni IV : some fluorides (e.g., M I NiF 2 6 ) and oxides (BaNiO 3 ). Pd IV : fairly uncommon, but range of compounds larger than for Ni Pd + (HCl + HNO 3 - aqua regia) PdCl 6 or PdCl + Cl 2 PdCl 6 Pt IV : fairly common PtX have all been made, M I PtX 2 6 easily prepared (X = Cl, Br, I) amine complexes as salts: [Pd(am) 2 X 2 ]X 2 ; M I [Pd(am)X 5 ]

5 2nd & 3rd row - aqueous chem. Genuine aquo ions (containing M n+ and H 2 O only) are relatively rare. Exceptions: Mo(H 2 O) 6 can be isolated, but even for Mo III, binuclear ions (?) also occur. In ClO solutions, Pd(H 2 O) & Pt(H 2 O) can be made (e.g., from PdO or K 2 PtCl ) Rh(H 2 O) 6 & Ir(H 2 O) 6 reasonably stable, but subject to air oxidation. Why not more? For d-counts less than d 6 (maybe d 5 ), bridging and terminal oxo complexes are common Late T.M.s in lower oxidation states are oxophobic. 5

Coordination Chemistry: Bonding Theories. Crystal Field Theory. Chapter 20

Coordination Chemistry: Bonding Theories Crystal Field Theory Chapter 0 Review of the Previous Lecture 1. We discussed different types of isomerism in coordination chemistry Structural or constitutional