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Transition Metal Coordination Chemistry What Is a Transition Metal?

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Transition Metal Coordination Chemistry What Is a Transition Metal? Transition Metal Coordination Chemistry What is a transition metal? Prof S.M.Draper 2.05 SNIAMS Building [email protected] Recommended books M.J. Winter, d-block Chemistry, Oxford Chemistry Primers, OUP, 2001 M.S. Silberberg, Chemistry, 3rd Ed, McGrawHill, 2003 (chapter 23) C.E. Housecroft, A.G. Sharpe, Inorganic Chemistry, 1st Ed, PrenticeHall, 2001 J.E. Huheey, E.A. Keiter, R.L. Keiter, Inorganic Chemistry, 4th Ed., HarperCollins, 1993 1 Electrons in atoms z Shapes of d-orbitals 4s 3d 4p y x Sc Ti z z V z y y Cr y Mn x x x Fe Co yzxz xy x2-y2 z2 Ni Cu Zn 2 Working out numbers of d-electrons from oxidation states: How many d-electrons has the metal? 1 2 3 4 5 6 7 8 9 10 11 12 1st: how many electrons are there in the shell? - count along the periodic table O O en = ox = H2N NH2 e.g. Mn = 7 electrons Cu = 11 electrons -OO- 2nd: how many electrons are lost? - oxidation state complex O.S. of L O.S. of M no. d electrons 2- 0 e.g. Mn(VII) = 7 electrons lost Cu(II) = 2 electrons lost [Cr2O7] -2 +6 d - 0 [MnO4] -2 +7 d + 3rd: how many electrons left over? [Ag(NH3)2] - subtract 3+ 1 [Ti(H2O)6] 0+3d e.g. Mn(VII) = Cu(II) = 3+ [Co(en)3] 8 [PtCl2(NH3)2]-1, 0 +2 d 4- [V(CN)6] 3- 5 Hence the only valance electrons available in a transition metal ion are d-electrons [Fe(ox)3] -2 +3 d 3 Colour of transition metal complexes biological magnetic behaviour activity colour Ruby Corundum Al2O3 with impurities geometry What’s interesting about Sapphire octahedral metal centre Transition Metal Complexes?? Corundum coordination number 6 Al2O3 with and impurities coordination Emerald number medical applications Beryl oxidation states AlSiO3 containing Be with impurities 4 Haemoglobin Cisplatin Oxygen carrier in blood O 2 Porphyrin-Fe transition metal complex [PtCl2(NH3)2] Fe(II) ion is octahedrally coordinated Coordination number 6 NN NN NN FeFe NN HO C HO 2C2 NRNR HO2C HO 2C square planar Pt(II) coordination number 4 cis-isomer the first of a series of platinum coordination complex-based anti-cancer drugs (Platinol-AQ) 5 Alfred Werner - Nobel Prizewinner 1913 [Co(NH3)6]Cl3 [Co(NH3)5Cl]Cl2 [Co(NH3)4Cl2]Cl 3+ 2+ + + CoCl3 . 6NH3 yellow xs Ag 3 moles AgCl + CoCl3 . 5NH3 purple xs Ag 2 moles AgCl Werner's conclusions + CoCl3 . 4NH3 green xs Ag 1 mole AgCl 1. The metal is in a particular (primary valancy) CoCl . 3NH xs Ag+ 0 moles AgCl 3 3 2. The complex has a fixed (secondary valancy) 3. The ligands are bound to the metal via a bond which resembles a covalent bond 6 Lewis Acid-Base Concept: parallel with main group What is a coordination complex? chemistry • Adducts formed by the reaction of metal ions (Lewis acids) with several ligands (Lewis bases). n+/- H 2+ X+/- Ni2+ + H O O : Ni n 2 H . NH3 2+ NH3 2+ Ni + 6 :NH3 H3N: Ni :NH3 Central metal ion or atom surrounded by a set of ligands NH3 NH3 The ligand donates two electrons to the d-orbitals around the metal forming a • Properties of products differ from those of separate reactants (i.e., colour, composition, magnetic properties, etc.). 1 3 7 F F zz zz z z z z FFB z z z 3+ z z FFB z e.g. [Co(NH ) ] HHN z FFB z 3 6 zz zz + zz z z HHz N z H F zz HHN H H NH3 3+ _ zz H3N > BF3 NH3 BF3 zz z z z z z 3+ H3N z z NH 6 HHN z Co 3 L zz + H z z H N z z L L 3 NH3 zz NH3 L L L = coordinate or dative bond 1 CO CO Group 14 carbon monoxide - 2- e.g. NH3, H2O, OH , CO3 - - CO, PPh3, C2H4, SRH, CN , SCN Small donor atoms Larger donor atoms Electronegative H Less electronegative H Not very polarisable NH P PPh Group 15 N 3 3 Easily polarisable ammonia phosphine H R H H O S SR Group 16 O 2 2 e.g. Fe(III), Mn(II), Cr(III) e.g. Ag(I), Cu(I) water thioether R Small metals (1st row) Larger metals (2nd + 3rd row) H High oxidation state Low oxidation state 2 π - bonded ligands - z CN - - Group 14 CNz Ph H C CH cyanide 2 2 phenyl Cl CH2 K+ Cl Pt Cl CH2 RC CR NO- - Group 15 z z NCS NOz nitrous NCS z isocyanate 2 - [PtCl3(η -C2H4)] Group 16 OH- SCN- z O H hydroxide S C N z thiocyanate Monodentate donor atom per ligand Bidentate donor atoms per ligand Group 17 X halide H hydride Tridentate three donor atoms per ligand Multidentate many donor atoms per ligand 3 Neutral bidentate ligands: 2 donor atoms Anionic bidentate ligands π-donor bidentate ligand O O H2NNH2 oxalate = ox2- zz e.g. [PtCl2(en)] zz - - OO Fe C C C O O 1,2-diaminoethane = ethylene diamine = en O O 4 [Fe(CO)3(η -C4H6)] - H3C acetate = ac O Ph PPPh 2 zz 2 OH zz NN R zz zz R' NN N O O O zz zz Pd M R' O N C N N C 1,2-diphenylphosphineethane 2,2'-bipyridine 1,10-phenanthroline HO R H H dppe bpy phen Pd(II)-oxime complex 4 Tridentate ligands: three donor atoms Tetradentate ligands Hexadentate ligand tetraanion of NH2 ethylenediaminetetraacetic acid N H NNHNH2 zz N 2 zz zz zz EDTA N N zz zz NH2 NH2 O O tris(2-aminoethyl)amine - - diethylenetriamine 2,2':6',2"-terpyridine O O tren - NN - dien tpy O O N O O HN NH NH N NH N 1,2,4-triazacyclonane zz zz N N macrocyclic ligand N HN N HN zz N N M H porphyrin phthalocyanin 5 Inner coordination sphere = Outer coordination sphere = SO 2- H2O 4 H2O H2O H2O OSO3 2+ H2O Mn OH2 Mn2+ OH2 OH H2O 2 H O 2 H2O [Mn(OH2)6][SO4]: outer sphere complex [Mn(OH2)5(SO4)5]: inner sphere complex 6 Ligand Exchange Reactions 2+ - + [Fe(OH2)6] +CN [Fe(OH2)5(CN)] + H2O 2+ + + + hexaaquo iron(II) cyanide + complex ion const = [Fe(OH2)5(CN) ][H2O] K1 = [H O] 2+ - 2 [Fe(OH2)6 ][CN] 1 The reaction continues…. The reaction continues…. - + - + [Fe(OH2)3(CN)3 ] [Fe(OH ) (CN)] +CN [Fe(OH ) (CN) ] +HO - - 2 5 2 4 2 2 β3 = [Fe(OH2)4(CN)2]+CN [Fe(OH2)3(CN)3] +H2O 2+ - 3 [Fe(OH2)6 ] [CN ] K =[Fe(OH) (CN) ] 2- 2 2 4 2 [Fe(OH2)2(CN)4 ] - - 2- [Fe(OH ) (CN) ] +CN [Fe(OH ) (CN) ] +HO β4 = 2 3 3 2 2 4 2 [Fe(OH ) 2+] [CN-]4 + - 2 6 [Fe(OH2)5(CN) ] [CN ] 3- [Fe(OH2)(CN)5 ] 2- - 3- β5 = [Fe(OH2)2(CN)4] +CN [Fe(OH2)(CN)5] +H2O 2+ - 5 [Fe(OH2)6 ] [CN ] + [Fe(OH2)5(CN) ] [Fe(OH2)4(CN)2] K K = β = x 1 2 2 [Fe(CN) 4-] + - 6 2+ - - 4- [Fe(OH ) ][CN] [Fe(OH2)5(CN) ] [CN ] β6 = 2 6 [Fe(OH2)(CN)5]+CN [Fe(CN)6] +H2O 2+ - 6 [Fe(OH2)6 ] [CN ] [ML ] Overall Stability Constant βn = n [M] [L]n Stability constants are often expressed in log form ie log βn 2 Values of βn The Chelate Effect 2+ 4- 2+ [M(NH3)6] n+ n+ [M(en) ]2+ + 6 +6 NH3 H2N 3 H NH 2 NH H N 3 N 2 3 M M NH NH H3N 3 N 2 H2 NH3 H2N 4- [Fe(CN)6 ] β = ~ 1035 6 [Fe(OH ) 2+] [CN-]6 2 6 n+ n+ [M(OH2)6] +6 NH3 [M(NH3)6] +6 H2O [M(OH ) ]n+ + 3 en [M(en) ]n+ +6 HO log β6 = 35 2 6 3 2 Kn < … < K3 < K2 < K1 3 Entropy of chelate formation ΔGo = ΔHo -TΔSo ΔGo = -RT ln K 2+ 2+ [Cu(OH2)6] +2 NH3 [Cu(OH2)4(NH3)2] +2 H2O β 107.7 log β = 7.7 ΔGo = ΔHo -TΔSo 2 2 ΔHo = - 46 kJ mol-1 ΔSo = - 8.4 J K-1mol-1 Enthalphy changes similar Entropy changes differ 2+ 2+ [Cu(OH2)6] +en [Cu(OH2)4(en)] +2 H2O n+ n+ [M(OH2)6] +6 NH3 [M(NH3)6] +6 H2O n+ n+ β 1010.6 log β = 10.6 [M(OH2)6] + 3 en [M(en)3] +6 H2O 2 1 ΔHo = - 54 kJ mol-1 ΔSo = + 23 J K-1mol-1 ΔSo is large and positive - TΔSo is large and negative 4 Thermodynamic vs Kinetic Stability The Macrocyclic Effect Complexes containing macrocyclic rings have enhanced stability compared to acyclic ligands 2+ 2+ H H H H N N N N Cu Cu N N N N H2 H2 H H 3+ 3 e.g [Cr(OH2)6] d kinetically inert slow substitution of Ls 3+ 5 cf. [Fe(OH2)6] hs d kinetically labile rapid substitution of Ls log K1 = 23.9 log K1 = 28.0 acyclic chelating ligand macrocyclic ligand (thermodynamic stability similar) ΔG always favours formation of macrocyclic complex 5 Tour of Coordination Numbers and their Geometries Most Common Geometries of Transition Metal Complexes The Kepert Model The shape of a complex is usually dictated by the number of coordinated atoms.
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