Chemical bonding in complexes of transition metals Chem 202, Sept. 28, 2010 What are transition elements? Electronic structure of atoms Naming delocalized molecular orbitals: tetrahedral and octahedral molecules Frontier MOs of many transitionmetal compounds are dbased : splitting of d AOs Rationalizing i properties of transitionmetal t compounds from dorbital splitting
Valence shells of transition metals (TMs) are made up of partially filled d orbitals 1 st row TMs: Sc Cu 2 nd row TMs: Y Ag 3 rd row TMs: La Au Lanthanides/actinides are related to TMs A lot of valence electrons results in unique chemical and physical properties: Wide range of formal oxidation states: from 4 to +7 Extensive redox chemistry: life; batteries Gases (e.g., Ni(CO) 4 ); very strong acids (e.g., HCo(CO) 4 is stronger than H 2 SO 4 ); quadruple and quintuple bonds; huge 2 4 molecules (clusters: e.g., Pt 212 L x ) Colors; magnetism catalysis Need to understand bonding to rationalize/predict chemistry
The Lewis vs. MO models of bonding in O 2 Both are approximations: Which one is appropriate depends on problem O 2: Lewis model: diamagnetic; MOs: paramagnetic Frontier (highestenergy occupied) molecular orbitals: cartoons Quantummechanical calculations (electron density) * bonding antibonding
Lewis model: dl 4 equivalent electron pairs CH 4 Photoelectron spectrum: 3 equivalent electron pairs one unique electron pair Quantumchemical calculations: 4 bonding MOs, but shapes are very different from O 2
Moreneral names of MOs: depend on molecular shape and contributing AOs! CH 4 : tetrahedral molecule; MOs derive from s,p AOs C AOs CH 4 H AOs Triply degenerate MOs Not the same as triple bond! 2p 2s t 2 a 1
Chemical bonding in sp vs. delements: CH 4 vs. C AOs Zr(CH 3 ) 4 Zr AOs Zr(CH CH 4 4 H AOs 3 ) 4 4 CH MOs 2p antibonding Metalbased 5s 4d t 2 antibonding 2s t 2 a 1 bonding bonding e a 1 t 2 Ligandbased Bonding, nonbonding ( lone pairs ) or antibonding Metalbased or ligandbased Frontier MOs: highest occupied, lowest unoccupied (most chemically important)
Bonding in octahedral complexes: very common for transition metals 4 CH MOs Zr(CH 3 ) 4 Zr or W AOs WH 6 6 H AOs a 1 (n+1)p t 1u (n+1)s a 1g antibonding Frontier MOs are based on d AOs t 2 e nd Ligand based; rarely participate in chemistry a 1 t 2 bonding t 1u a 1g
In discussing transitionmetal chemistry it often suffices to consider only dbased MOs: simpler! Crystal field theory: analog of Lewis model for transition metal complexes Simplest case (1 orbital per ligand): t 2 t ~4/9 o e d AOs o Crystalfield splitting parameter: 1. Depends on the metal, charge, ligand andgeometry 2. Smaller than splitting of AOs: Aufbau principle may not hold: highspin (as many unpaired electrons as consistent with Pauli exclusion) lowspin (as few unpaired electrons as consistent with Pauli exclusion) electronic configurations Electronic excitation requires relatively little energy: visible light or heat
Counting electrons, predicting properties: tetrahedral complexes, MCl x 4 t 2 t < electronpair repulsion; fill up MOs as if of same energy e Oxid. state d count Electronic config. Unpaired electrons Para/diamagnetic? dd transitions? Possible to oxidize? Possible to reduce? TiCl 4 VCl 4 CrCl 4 MnCl 4 2 FeCl 4 CoCl 4 NiCl 4 CuCl 4 1. Paramagnetic: at least one unpaired electron 2. dd d transitions result in color: excited state musthave the same number of unpaired electrons 3. Molecules without d electrons cannot be oxidized; molecules with 10 d electrons cannot be reduced
Chemical bonding in complexes of transition metals Chem 202, Oct. 1, 2010 Splitting of d AOs in metal complexes: 1. Population of dbased MOs Highspin vs. lowspin complexes 2. Qualitative trends in splitting parameter Spectrochemical series
Concepts from last lecture Alternative names of MOs for highsymmetry molecules Model of chemicalbonding of d elements: Splitting of dorbitals; MO origin; crystal field theory and splitting parameter, t or o Homework: Fill up the table for MCl x 4 complexes. Review text for trends in crystal field splitting
Counting electrons, predicting properties: tetrahedral complexes, MCl x 4 t 2 t < electronpair repulsion; fill up MOs as if of same energy TiCl 4 VCl 4 CrCl 4 MnCl 4 2 FeCl 4 CoCl 4 NiCl 4 CuCl 4 Oxid. state +4 +3 +3 +2 +3 +3 +3 +2 d count 0 2 3 5 5 6 7 9 Electronic config. e 0 e 2 e 2 t 2 1 e 2 t 2 3 e 2 t 2 3 e 3 t 2 3 e 4 t 2 3 e 4 t 2 5 Unpaired electrons 0 2 4 5 5 4 3 2 Para/diamagnetic? Dia Para Para Para Para Para Para Para dd transitions? No Yes yes No No Yes Yes Yes Possible to oxidize? No Yes Yes Yes Yes Yes Yes Yes Possible to reduce? Yes Yes Yes Yes Yes Yes Yes Yes 1. Paramagnetic: at least one unpaired electron 2. dd d transitions result in color: excited state musthave the same number of unpaired electrons 3. Molecules without d electrons cannot be oxidized; molecules with 10 d electrons cannot be reduced e
Highspin vs. lowspin complexes Tetrahedral complexes are always high spin two possibilities for octahedral d 4 d 7 complexes o o < pairing energy Configuration with maximum spin is lowest in energy (Hund s rules) High spin complex o > pairing energy: Configuration with minimum spin is lowest in energy Low spin complex
Trends in dorbital splitting across throups Element: Heavier element, larger splitting and periods CoF 6 is high spin; RhF 6 is low spin d AOs are closer in energy to ligand MOs: less energy gap, better overlap: more antibonding character Formal charge on the metal (d count): Greater positive charge, larger splitting: CoF 6 is high spin; CoF 6 2 is low spin Same as above: larger charge, lower energy of d AOs, smaller metalligand energy gap, better orbital overlap: more antibonding character Ligand (applies (pp to octahedral complexes): Spectrochemical series: order of increasing o I < Br < S 2 < SCN < Cl < NO 3 < N 3 < OH < H 2 O < NCS < NH 3 < CN CO How to rationalize the trends using MOs? Some ligands use more than 1 orbital to bind to the metal: acids or bases How to modify the MO diagram?
MO basis of trends in crystal field splitting 6 F CoF 2 6 Co 4+ Co 2+ CoF 4 6 6 F 4p 4p 4s 4s 3d o o 3d Small energy mismatch of AOs: Metal/ligand interaction strong strongly antibonding large / gap Largeenergy energy mismatchof AOs: Metal/ligand interaction fairly weak weakly antibonding modest / gap
Counting electrons, predicting properties: octahedral complexes, MF x 6 o < electronpair repulsion energy Oxid. state d count Electronic config. Unpaired electrons Para/diamagnetic? dd transitions? Possible to oxidize? Possible to reduce? TiF 6 2 VF 6 CrF 6 MnF 6 2 FeF 6 CoF 6 NiF 6 CuF 6 Note that some metals have different oxidation states than the Cl complexes!
Counting electrons, predicting properties: octahedral complexes, MF x 6 o can be more, less or ~ electronpair repulsion energy TiF 6 2 VF 6 CrF 6 MnF 6 2 FeF 6 CoF 6 NiF 6 CuF 6 Oxid. state +4 +3 +3 +4 +3 +3 +3 +3 d count 0 2 3 3 5 6 7 8 Electronic config. 0 2 3 3 3 2 4 2 5 2 6 2 Unpaired electrons 0 2 3 3 5 4 3 2 Para/diamagnetic? Dia Para Para Para Para Para Para Para dd transitions? No Yes Yes Yes No Yes Yes Yes Possible to oxidize? No Yes Yes Yes Yes Yes Yes Yes Possible to reduce? Yes Yes Yes Yes Yes Yes Yes Yes Note that some metals have different oxidation states than the Cl complexes! The only highspin complex of Co 3+ with simple ligands.
Cyano complexes of metals o > electronpair repulsion energy t < electronpair repulsion energy t 2 e nd Ti(CN) 6 V(CN) 6 4 Cr(CN) 6 4 Mn(CN) 6 Fe(CN) 6 4 Co(CN) 6 Ni(CN) 6 4 Cu(CN) 4 Oxid. state d count Electronic config. Unpaired electrons Para/diamagnetic? dd transitions? Possible to oxidize? Possible to reduce?
Cyano complexes of metals o > electronpair repulsion energy t < electronpair repulsion energy t 2 e Ti(CN) 6 V(CN) 6 4 Cr(CN) 6 4 Mn(CN) 6 Fe(CN) 6 4 Co(CN) 6 Ni(CN) 6 4 Cu(CN) 4 Oxid. state +1 +2 +2 +3 +2 +3 +2 +1 d count 1 3 4 4 6 6 8 10 Electronic config. 1 3 4 4 6 6 6 2 e 4 t 2 6 Unpaired electrons 1 3 2 2 0 0 2 0 Para/diamagnetic? Para Para Para Para Dia Dia Para Dia dd transitions? Yes Yes Yes Yes No No Yes No Possible to oxidize? Yes Yes Yes Yes Yes Yes Yes Yes Possible to reduce? Yes Yes Yes Yes Yes Yes Yes No
Strongfield ligands (CO, CN ) interact with metal through multiple orbitals (backbonding) * 4 CO Ni Ni(CO) 4 8 * * MOs 4 MOs
Strongfield ligands (CO, CN ) interact with metal through multiple orbitals (backbonding) 6 F MnF 2 Mn Mn(CN) 6 6 6 CN * o : nonbondingantibonding gap o o o : bondingantibonding gap 2p bonding 18 electron rule: complete population of all bonding MOs requires 9 electron pairs