Chapter 21 Transition Metals and Coordination Chemistry

Transcription

1 Chapter 21 Transition Metals and Coordination Chemistry

2 Some History In the 19 th century, chemists started to prepare colored compounds containing transition metals and other substances like ammonia, chloride, water and cyanide They were very interesting because their formulae gave little clue as to how they were bonded together Example: Co(NH 3 ) 6 Cl 3

3 Sophus Mads Jørgensen ( )

4 Chain Theory Metal ammonia complexes contain chains of ammonia molecules connected to metal with chlorides at the end of each chain

5 Alfred Werner ( )

6 Coordination Theory Octahedral arrangement of ammonia molecules around metal forming the complex ion Co(NH 3 ) 6 3+ along with three Cl - ions 3Cl-

7 Complex Ions A complex ion is a charged species consisting of a metal ion (most commonly a transition metal) surrounded by a number of molecules or ions called ligands A ligand is a Lewis base having a lone pair of electrons that can be donated to an empty orbital on the metal ion (acting as a Lewis acid) forming a coordinate covalent bond The most common ligands are H 2 O, NH 3, Cl - and CN - The charge on a complex ion is the sum of the charges of the metal ion and the ligands Examples: CoCl 4 2- = Co Cl - Ni(NH 3 ) 6 2+ = Ni NH 3 The ligands do not all have to be the same Example: Cu(NH 3 ) 4 (H 2 O) 2 2+

8 Coordination Number The number of ligands attached to the metal ion is called the coordination number The most common coordination numbers are 6, 4 and 2 Examples: Co(H 2 O) 6 2+, CoCl 4 2-, Ag(NH 3 ) 2 +

9 Complex Ion Shapes Coordination number 6 complexes will always be octahedral Coordination number 2 complexes will always be linear Coordination number 4 complex can be tetrahedral or square planar

10 Octahedral Complexes Tetrahedral Complexes

11 Square Planar Complexes Linear Complexes [H 3 N: Ag :NH 3 ] + [Ag(NH 3 ) 2 ] +

12 VSEPR Theory and Complex Ions Valence Shell Electron Pair Repulsion (VSEPR) theory works for six and two coordinate complex ions but cannot be used to predict whether a four coordinate complex will be tetrahedral or square planar!

13 Coordination Compounds A coordination compound consists of a complex ion along with one or more counterions which are anions or cations required to produce a compound with no net charge Square brackets are placed around the complex ion to indicate the composition of the complex ion Counterions are shown outside the brackets Example: [Co(NH 3 ) 5 Cl]Cl 2 = Co(NH 3 ) 5 Cl Cl - A coordination compound behaves like any other ionic compound when dissolved in water forming independent cations and anions: [Co(NH 3 ) 5 Cl]Cl 2 (s) HO Co(NH 3 ) 5 Cl 2+ (aq) + 2Cl - (aq) 2

14 Colors of Coordination Compounds Many coordination compounds are very colorful since the transition metal ions absorb light in the visible spectrum

15 Ligand Types Ligands that can form one bond to a metal ion are called monodenate or unidentate ( single toothed ) Ligands with more than one atom with a lone pair that can bond to a metal are said to chelating ligands or chelates ( clawed ) A ligand that can form two bonds to a metal ion is called a bidentate ligand Ligands that can form more than two bonds are called polydentate

16 Etheylenediaminetetraacetate (EDTA) A hexadenatate ligand!

17 Common Ligands

18 Naming Coordination Compounds

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22 Isomerism in Coordination Compounds Occurs when two or more complex ions contain exactly the same types and numbers of atoms (hence same formula) but the arrangement of ligands differ leading to different physical and/or chemical properties

23 Linkage Isomerism The composition of the complex is the same but one or more of the ligands attaches with a different atom Example: [Co(NH 3 ) 4 (NO 2 )Cl]Cl [Co(NH 3 ) 4 (ONO)Cl]Cl

24 Geometrical Isomerism Occurs when ligands occupy different spatial positions around the central atom cis trans trans cis

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26 Review of Electronic Configurations of Transition Metal Atoms

27 Iron? 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 6 or [Ar] 4s 2 3d 6

28 Chromium? Expect? 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 4 or [Ar]4s 2 3d 4 Find: 1s 2 2s 2 2p 6 3s 2 3p 6 4s 1 3d 5 or [Ar]4s 1 3d 5 Half-filled 3d level is very stable!

29 Gadolinium? 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 6 5s 2 4d 10 5p 6 6s 2 5d 1 4f 7 or [Xe]6s 2 5d 1 4f 7

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31 Electronic Configurations of Transition Metal Ions

32 In neutral transition elements, the s- and d-orbitals have very similar energies However, in transition metal ions the energy of the d-orbitals is significantly lower than that of the s-orbitals so that the electrons remaining after the ion forms occupy the lower energy d-orbitals. Example: Fe 3+ is [Ar]3d 5 not [Ar]4s 2 3d 3

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36 Bonding in Coordination Compounds Can be considered a Lewis acid-base reaction where lone pairs on the ligands are shared with orbitals on the metal ion