from Selections Chapters 9 & 16
The transition metals (IV)
CHEM 462 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: TiIV, VV, CrVI, MnVII - after Mn (for 1st row), ox. states higher than +3 hard to achieve. Early metals tend to be most stable in highest oxidation state: ScIII, YIII, LnIII, TiIV, ZrIV, HfIV Oxides are more acidic in higher oxidation state Some broad chem. similarities are found for ox. states II & III (6 or 4 coordinate for complexes in solution or in crystals, aqueous chem., etc.) Except for CuI, -acid ligands and organometallics dominate for ox. states lower than 2+.
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Acidic High Oxidation State Oxides
CrVI is acidic: CrO3 + H2O H2CrO4 Green CrIII is amphoteric: Cr2O3 + H3O+(aq) [HCrO4] + H3O+
[Cr(H2O)6]3+ acid -base
properties similar to [Al(H2O)6]3+ -Cr2O3 + OH(aq) soluble chromites HMnO4, H2CrO4, H3VO4 acidity trend similar to HClO4, H2SO4, H3PO4.
Oxidizing Agents (acidic soln)
[MnO4] + 8 H+ + 5 e [Cr2O7]2 + 14 H+ + 6 e [VO2]+ + 4 H+ + e Mn2+ + 2 H2O E = +1.51 V 2 Cr3+ + 7 H2O E = +1.33 V
2 V3+ + 2 H2O E = +0.668 V
Common Oxidation States, "Typical compounds"
II state: Compounds of all elements, Ti-Cu, are found. in water, all M(H2O)62+ are known except Sc. in aerated acid solution V2+, Cr2+, and Fe2+ are oxidized to 3+. solid state oxides and halides known for all MII metals (with varying degrees of ionicity)
MIIX2 all have CdX2-type structures MIIO compounds surprisingly complex
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CdX2 layer
Common Oxidation States, "Typical compounds"
III state: All T. M. elements,
for Ni & Cu, ox. state III is fairly rare
FeCl3 : some similarity with AlCl3
Both form M2 Cl6 molecules in liquid, gas phases Both can act as Lewis acids, e.g., as Friedel crafts catalysts but AlCl3 not much of an oxidant!
Ti--Co form [MIII(H2O)6] 3+ ions
MnIII MnII & CoIII CoII - quite readily [FeIII(H2O)6 ]3+ only forms in strong, noncomplexing acids
Common Oxidation States, "Typical compounds"
III state is less stable for later TM's, e.g., CoII(H2O)62+ +1.84 V, CoIII(H2O)63+ + e but CoIII can be stabilized complexation by by strong field ligands, e.g., CoIII(NH3)63+ + e CoII(NH3)62+ +0.1 V. [synthesis: CoCl2(soln) + 4 NH4+ + 20 NH3 + O2 4 [CoIII(NH3)6]Cl3 + 2 H2O + 4 enH+ + 8 en + O2 4 [CoIII(en)3]Cl3 + 2 H2O]
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Common Oxidation States, "Typical compounds"
"actual" charge IV state: important for Ti, V (TiO2, TiCl4 , ..., extensive chemistry of "VO2+"), 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 MO bonding)
oxidation state
First Row T.M.s vs. 2nd and 3rd T.M. Series
Radii: 2nd and 3rd row ~ 0.1 0.2 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
CrO3 - very powerful oxidant (oxidizes most organics), forms H2 CrO4 in acidic solution; MoO3, WO3 - milder oxidants (will oxidize H2 & NH3, but not most organics) RuO4 , OsO4 - are strong oxidants, but no Fe analog exists at all MnO4 2 disproportionates to form...