Dr R J Errington
CHY230 Comparative transition metal chemistry. 11 lectures
Synopsis
Trends in the Periodic Table
Sizes of atoms and ions; effect of increasing effective nuclear charge due to inefficient shielding within an icreasingly filled set of orbitals. General decrease left to right and increase down any group with major difference between first and second row elements due to filled set of f orbitals (the 'lanthanide contraction').
Orbital extension greater for 2nd and 3rd row elements; implications for M-M bonding.
Distinction between chemistry of first row element and that of heavier elements in any group.
Group 4 Ti Zr Hf
Titanium.
Binary compounds: TiO2 (solid state structures- rutile, anatase, brookite); TiCl4 covalent, molecular, Lewis acid, hydrolyses readily, forms wide range of complexes.
Aqueous chemistry: no [Ti(H2O)6]4+ due to hydrolysis to give Ti-OH species, evidence for Ti=O species. [Ti(H2O)6]3+ by reduction of Ti(IV), violet d1. Ti(II) reduces water, so no hexaaqua ion.
Complexes: Halide derivatives; anionic ligands with O-donors (e.g. OR-, acac-); octahedral geometry, but some higher coordination with chelating ligands (e.g. Ph2As C6H4AsPh2); complexes of Ti(III) e.g. [TiCl3L3] often used as 1 e- reducing agents.
Organometallic compounds: dominated by Cp2TiCl2 and related complexes, importance in alkene polymerisation catalysis.
Zirconium and Hafnium.
Similar sizes, very similar chemistry (Hf less extensively investigated).
Binary compounds: ZrO2 important refractory oxide, NOT rutile structure (Zr 7-coordinate); ZrCl4 crystalline solid, octahedral Zr (tetrahedral in gas phase).
Aqueous chemistry: although larger than Ti, Zr(IV) still sufficiently Lewis acidic to hydrolyse extensively, some evidence for Zr4+(aq) at low concn. in highly acidic solutions, [Zr4(m-OH)8(H2O)16]8+ at higher concentrations and higher pH. No evidence for Zr=O.
Complexes: Zr(IV) high coordn. numbers (e.g. 6-, 7- and 8-coordinate acac derivatives). No simple complexes of Zr(III), metal-metal bonding in [Zr2Cl6(PR3)4], edge-shared bioctahedral structure.
Organometallic compounds: less extensively investigated than Ti, but growing importance due to alkene polymerisation catalysis, Cp2ZrCl2 etc.
Metal-metal bonding
Orbital interactions in M2X10 edge-shared bioctahedral structure e.g. [Zr2Cl6(PR3)4]. Define axes, see which d orbitals interact with ligand s-orbitals, look at d-d interactions. M-M s, p and d MOs, occupancy can give M-M, M=M and M3M bonds.
Group 5 V Nb Ta
Vanadium
Binary compounds: V2O5 oxidising agent, dissolves in strong base to give VO43-, at lower pH polyvanadates e.g. [V10O28]6- produced. VCl4 readily coverted to VCl3.
Oxohalides: VOCl3, yellow liquid, covalent Lewis acid, forms complexes, hydrolyses readily.
Nature of M=O bond: s- and p-components by interaction between Op and Md orbitals, possibility of M3O. Reactivity towards isocyanates (RNCO) to give organoimido (NR2-) complexes.
Aqueous chemistry: Lewis acidity of V(V) means that [VO2(H2O)4]+ and NOT [V(H2O)6]5+ formed. [VO2L4]+ complexes by ligand substitution. Nature of cis-MO2 bonding max bond order 2.5 per M-O, general for d0 complexes. V(IV) gives [VO(H2O)5]2+ NOT {V(H2O)6]4+, wide range of [VOL4]2+ d1 vanadyl complexes, e- in d orbital perpendicular to V=O. [V(H2O)6]3+ can be produced, although still some hydrolysis to V-OH and V=O species, also [VCl3L3] complexes. [V(H2O)6]2+ prepared by reduction of V(V), d3 strongly reducing, will reduce water, large CFSE cf Cr(III), kinetically inert, slow substitution reactions.
Organometallic compounds: range of complexes, carbonyls V(CO)6 and [V(CO)6]-.
Niobium and Tantalum
Similar chemistries, very little cationic aqueous chem, but wide range of complexes.
Binary compounds: MCl5 solids, M2X10 structure, Lewis acids, complex formation [MCl5L]. NbCl4, M-M bonding in chain structure.
Oxohalides: NbOCl3 from NbCl5 by reaction with H2O or (Me3Si)2O, bridging O (NOT terminal Nb=O), wide range of Nb(V) complexes. M(IV) and M(III) complexes contain M-M bonds e.g. reduction of TaCl5 to give [Ta2Cl6(PMe3)4] (Ta=Ta), addition of H2 across Ta=Ta to give Ta-Ta hydride, but excess PMe3 gives monomer. Few M(II) complexes.
Clusters: low oxidation state halides, [M6X12]n+, n usually 2, structure based on octahedral M6 with X bridging edges, e- counting and bond order, preparation by reduction of MCl5 with M.
Organometallics: wide range including Cp complexes, s-bonded alkyls, low oxidation-state carbonyls.
Group 6 Cr Mo W
Chromium
Binary compounds: CrO3 (covalent chain structure), CrX3 and CrX2 (lattice structures).
Aqueous chem: Cr(II) d4 [Cr(H2O)6]2+ bright blue, air sensitive, reducing agent, high/low spin complexes; [Cr2(O2CCH3)4] red, diamagnetic, quadruple Cr-Cr bond. Bonding analysis to show s2p4d2 electronic configuration. Cr(III) d3 inert complexes. Cr(VI) oxo compounds, CrO42-, HCrO4-, Cr2O72-.
Molybdenum and Tungsten
Binary compounds: MO3 not strongly oxidising, MO2 distorted rutile structure with M-M bonds; WCl6, MoCl5 also MCl4, MCl3, MCl2.
Oxohalides: MOCl4, chain structure in solid state.
Aqueous chemistry: M(VI) no cationic chemistry, anionic species WO42- and polynuclear species. M(V) no [M(H2O)6]5+, diamagnetic [M2O4(H2O)6]2+ and derivatives. M(IV) trinuclear species with m3-O [M3O4(H2O)9]4+, M-M bonding. Further reduction to give tetranuclear [M4O4L9]n+ complexes.
Non-aqueous chemistry: complexes of halides and oxyhalides, multiple M-M bonding, extensive chemistry of M2X6 complexes, quadruple M-M bonds in [M2(O2CR)4]. Clusters containing M6X84+ units, structures based on octahedral M6 with X triply bridging faces of octahedron.
Lower oxidation state complexe: p-acceptor ligands, CO, N2, PR3.
Early transition-metal polyoxometalates.
Groups 5 & 6, MxOyn-, formed by acidification of MO4n- by condensation reactions. Contrast Cr chemistry.
Group 7 Mn Tc Re
Manganese
Binary compounds: MnO2 (black, oxidising), MnO (green, reacts with O2).
Aqueous chemistry: Mn(II) most stable ox. state, most comlexes high spin d5.
Higher ox. states with oxo ligands, lower ox. states with p-acceptors.
Technetium and Rhenium
Similar, but markedly different from Mn.
Very little cationic Mn+ chemistry and few compoounds with M(II).
Extensive M(IV) and M(V) chemistry
M-M bonds in ox. states up to M(IV). Binary compounds: ReO3, ReCl5, Re3Cl9.
Multiple bonding in Re2Cl82-. Oxo complexes. Technetium coordination chemistry in medicine. Polyhydrides, Re3H92-.
Group 8 Fe Ru Os
Iron
Aqueous chemistry: Fe(II) d6 [Fe(H2O)6]2+ pale blue-green, most complexes octahedral (tetrahedral with p-donors); Fe(III) d5 [Fe(H2O)6]3+ pale purple, hydrolysis unless solution strongly acidic.
Organometallics: Wide range including Cp2Fe (ferrocene), carbonyl and phosphine complexes.
Ruthenium and Osmium
Little resemblance to Fe chem, higher ox. states more prevalent.
Oxo compounds: ox. states (IV) - (VIII); RuO4, OsO4.
Complexes: Ru(III) d5, e.g. full range of {Ru(H2O)5Cl]2+ to [RuCl6]3-; Ru(II) d6 electron-rich metal centre, [Ru(NH3)6]2+ reducing agent, complexes with p-acceptors including N2. Mixed oxidation state complexes.
Group 9 Co Rh Ir
Cobalt
No ox. states > (IV) and few compounds containing Co(IV).
Co(III) relatively unstable w.r.t. Co(II) in 'simple' compounds, but many stable complexes with low-spin d6 configuration.
Co(II) in [Co(H2O)6]2+, oxidation to Co(III) unfavourable in absence of complexing ligands, but with stronger field ligands, Co(III) relatively more stable (CFSE).
Rhodium and Iridium
Rh(I), Rh(III) and some Rh(II); Ir(I) - Ir(IV).
Many complexes of M(III), Ir(III) complexes inert; cationic and neutral Rh(III) inert, but anionic Rh(III) complexes labile.
M&endash;M bonding in M(II) complexes e.g. [Rh2(O2CCH3)4].
Large number of M(I) complexes, especially with p-acceptors. Square-planar, 16-e-, d8. [RhCl(PPh3)3] (Wilkinson's catalyst); [Ir(CO)Cl(PPh3)2] (Vaska's compound); oxidative addition.
Group 10 Ni Pd Pt
Nickel
Only Ni(II) important in coordination chemistry, tetrahedral and square-planar geometries.
Palladium and Platinum
M(II) and M(IV) ox. states; Pd(H2O)4]2+ diamagnetic in aq. solution; very large number of d8 square planar, 16-e- complexes for Pd(II) and Pt(II). Pd complexes more labile, large number of phosphine complexes (important in catalysis).
Substitution in square planar complexes used to study trans-effect. This is an effect on the rate of reaction i.e. kinetic, activation energy affected. Ligands can be arranged in order of trans-effect:
CO, CN- > C2H4 > PR3 > Me- > Ph-, I-, NO2-, Br-, Cl- > py, NH3, OH-, H2O
Theorectical explanations in terms of polarisation of ligands (ground state observation of bond weakening referred to as trans-influence) and stabilisation of transition state in substitution reaction by p-acceptor ligands.
Use of trans-effect in synthesis of specific isomers e.g. cis and trans isomers of [PtCl2(NH3)2] and of [PtCl2(NO2)(NH3)]-.