Coordination Complexes

Chapter 20 What we learn from Chap 20

• We begin the chapter with what is among the most important coordination complexes of all, iron in hemoglobin (magnesium in chlorophyll might be another). In Section 20.1, we introduce the coordinate covalent bond and reintroduce Lewis acids (central atom) and bases (ligands) as the bonding species in coordination complexes. We also discuss how carbon monoxide can replace oxygen in hemoglobin.

I. Bonding in Coordination Complexes II. Ligands III. Coordination Number IV. Structure V. Isomers VI. Formulas and Names A. Formulas B. Nomenclature VII. Color and Coordination Compounds A. Transition Metals and Color B. Crystal-Field Theory C. Orbital Occupancy D. The Result of d Orbital Splitting E. Magnetism VIII. Chemical Reactions A. Ligand Exchange Reactions B. Electron Transfer Reactions

• Central metal atom – transition metal. • Coordinate covalent bond • Ligand – anion or neutral compound ·· such as H2O: or :NH3.

2+ [Cu(NH3)] 3- [Fe(CN)6]

Protein Ferrodoxin Plastocyanin Wilkin’s catalysyst

• Lewis bases having a lone pair of e- to donate. • Bidentate ligands form two bonds to the metal. • Chelates are polydentate ligands.

• Coordination number (CN) is the number of donor atoms bonded to central metal atom. • Common coordination numbers are 4 and 6. • May be as low as 2 and as high as 8. • CN determined by – The nature of metal ions : eg. Size – Charge on the ligand & metal – Electron configuration of metal

• CN = 2 – linear complex, 180o bond angles. • CN = 4 – tetrahedral complex, 109o bond angles. – square planar complex (nd8electron configuration), 90o bond angles. • CN = 6 – octahedral complex, 90o bond angles.

Tetrahedral Square planar 2+ 2+ 2- eg) Cu(NH3)4 , Ni(II), Pd(II), Pt(II) eg) Zn(NH3)4 , CoCl4 cf) VSEPR Model

Predict the geometry of the following complexes: + + [Ni(NH3)6]2 [Pt(NH3)2 Cl2] [Au(CN)]

+ [Ni(NH3)6]2 CN = 6, octahedral

[Pt(NH3)2Cl2] CN = 4, square planar [Au(CN)]+ CN = 2, linear

Cisplatin

Write the formulas for the following: • Sodium hexafluorocobaltate(III) • Bisethylenediaminecopper(II) chloride

Sodium hexafluorocobaltate(III) = Na3[CoF6]

Bisethylenediaminecopper(II) chloride=[Cu(en)2]Cl2

Name the following coordination compounds:

[Co(H2O)2Cl2]Cl K3[Fe(CN)6]

[Co(H2O)2Cl2]Cl diaquodichlorocobalt(III) chloride

K3[Fe(CN)6] potassium hexacyanoferrate(III)

Copyright © Houghton Mifflin Company. All rights reserved. 20 | 31 •K2[NiCl4] •[Co(NH3)6]Cl3 •[Co(NO2)2(NH3)4]2SO4 • Diamminebis(ethylenediamine) chronium(II) sulfate • Ammonium hexacyanoferrate(III) • Potassium tetrachloronikelate(II) • Hexaamminecobalt(III) Chloride • Diamminedinitrocobalt(IV) sulfate

• [Cr(NH3)2(en)2]SO4 •(NH4)3[Fe(CN)3]

• Coordination compounds are usually colored. • The color is due to partially filled d orbitals separated by an energy difference. • A photon causes a lower energy electron to move to a higher energy level. • The color is the results of the light, missing the absorbed photon, being reflected from the metal.

• A free gaseous metal atom or ion does not show an energy level difference among the d orbitals. • In the presence of ligands, the d orbital of the metal are split by a slight energy

difference, ∆o

Approach of six ligands to transition metal cation splits d orbitals into two sets of different• energy: explain color and magnetic properties eg

d 2 2 , d 2 E x −y z △ 3/5 o ∆ ∆o o dxy , dyz , dxz 2/5△o

t Free metal ion in Spherical field 2g in Oh field

small ∆o large ∆o

•Cristal field splitting energy ∆o , Cristal Field Stabilization Energy

2+ [CoCl ]2- [Co(H2O)6] 4

• The nature of the ligand determines the

magnitude of the crystal field splitting, ∆o

- - - - - Cl < F < OH < H232 O < NH < NO < CN < CO (small ∆∆ ) (large )

• Ferromagnetism • Paramagnetism • Diamagnetism

• Magnetic moment, µ=[n(n+2)]1/2 n: # unpaired electron

2+ 4- [Mn(H2O)6] µ=5.9 vs. [Mn(CN)6] µ= 2.2

Paramagnetism illustrated:

• Ligand Exchange

2+ 2+ [Cu(H2O)4] + 4NH3 → [Cu(NH3)4] + 4H2O K=4x108 2+ 2+ [Ni(NH3)6] + 3en → [Ni(en)3] +6NH3 Chelate effect (entropy) K=5x109

- 2+ [CoCl4] [Co(H2O)6]

•Inert complexes: 3 min. 1 day exchange ligands slowly

+ [Cr(H O) ]3+ [CrCl2(H2O)4] 2 6