Chemistry 1000 Lecture 25: Introduction to transition metal - - PowerPoint PPT Presentation

Chemistry 1000 Lecture 25: Introduction to transition metal chemistry

Marc R. Roussel November 5, 2018

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The transition metals

The transition metals

d block of periodic table

Sc Ti V Cr Mn Fe Co Ni Cu Zn [Ar]3d14s2 [Ar]3d24s2 [Ar]3d34s2 [Ar]3d54s1 [Ar]3d54s2 [Ar]3d64s2 [Ar]3d74s2 [Ar]3d84s2 [Ar]3d104s1 [Ar]3d104s2 Y Zr Nb Mo Tc Ru Rh Pd Ag Cd [Kr]4d15s2 [Kr]4d25s2 [Kr]4d45s1 [Kr]4d55s1 [Kr]4d55s2 [Kr]4d75s1 [Kr]4d85s1 [Kr]4d10 [Kr]4d105s1 [Kr]4d105s2 La Hf Ta W Re Os Ir Pt Au Hg [Xe]5d16s2 [Xe]4f145d26s2 [Xe]4f145d36s2 [Xe]4f145d46s2 [Xe]4f145d56s2 [Xe]4f145d66s2 [Xe]4f145d76s2 [Xe]4f145d96s1 [Xe]4f145d106s1 [Xe]4f145d106s2

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The transition metals

Chemistry of the transition metals

Electronegativities range from 1.1 (La) to 2.4 (W, Au) Many transition metal (TM) compounds are essentially covalent TM atoms/ions typically act as Lewis acids Rough rule: TMs form ionic compounds in lower oxidation states, covalent compounds in higher oxidation states

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The transition metals

Common and less common oxidation states:

10

Ti V Cr Mn Fe Co Ni Cu Zn +7 +6 +5 +4 +3 +2 +1

1 4s2 3 4s2 2 4s2 5 4s1 5 4s2 6 4s2 7 4s2 8 4s2

4s1 [Ar]3d [Ar]3d [Ar]3d [Ar]3d [Ar]3d [Ar]3d [Ar]3d [Ar]3d [Ar]3d10 4s2 [Ar]3d Sc

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Complexes and ligands

Complexes and ligands

Coordinate bond: bond made between a metal ion and a Lewis base This can almost always be considered to be an ordinary polar covalent bond. The special name only serves to emphasize that coordinate bonds are typically easier to rearrange than

• ther covalent bonds.

Coordination complex: complex molecule or ion consisting of a central metal atom or ion acting as a Lewis acid with coordinate bonds to one or (usually) several Lewis bases Ligand: one of the Lewis bases in a coordination complex

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Complexes and ligands

Complexes and ligands (continued)

Coordination number: number of coordinate bonds formed by a metal centre A given metal ion in a given oxidation state typically has a preferred coordination number found in most of its compounds. Coordination complexes do not obey the VSEPR rules. Rather, the shape is connected to the coordination number.

Coordination number 6 gives octahedral complexes. Coordination number 4 gives either tetrahedral or square planar complexes.

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Complexes and ligands

Example: Chloro-anions

Many metal ions form complex anions with chloride ions. Some examples: Complex Metal oxidation state [CdCl4]2− 2 [HgCl4]2− 2 [PtCl4]2− 2 [PbCl3]− 2 [AgCl2]− 1 [CuCl3]2− 1

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Complexes and ligands

Example: Ammine-cations

Many metal ions form complex cations with ammonia. Some examples: Complex Metal oxidation state [Co(NH3)6]3+ 3 [Co(NH3)6]2+ 2 [Ni(NH3)6]2+ 2 [Cd(NH3)4]2+ 2 [Cu(NH3)4]2+ 2

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Complexes and ligands

Square bracket notation

In the preceding examples, you may have noticed that coordination complexes are shown in square brackets. This is important since it makes the connectivity clear: The metal atom/ion and its ligands are shown in square brackets together.

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Complexes and ligands

Complex salts

A complex ion is a coordination complex with a net charge. A complex salt is a compound in which at least one of the ions is a complex ion. Examples with simple counterions: Na2[CdCl4] [Co(NH3)6]Cl3 Note that we list the cation first, then the anion. The following pairs of compounds are different: [Co(NH3)5Br]SO4 and [Co(NH3)5SO4]Br [Co(NH3)6][Cr(CN)6] and [Cr(NH3)6][Co(CN)6].

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Coordination chemistry

Monodentate ligands

Monodentate: literally, having one ‘tooth’ These are ligands that coordinate only once to a metal. Examples of monodentate ligands: F−, Cl−, Br−, I−, OH−, H2O, NH3, CO Note that these are all Lewis bases, but not necessarily Brønsted bases. For example, Cl− is a Lewis base, but it is not a Brønsted base. How do we know that Cl− is not a Brønsted base?

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Coordination chemistry

Polydentate ligands

Bidentate: coordinates twice to a metal, i.e. can donate lone pairs from two different atoms Polydentate: coordinates more than once to a metal Denticity: number of donor atoms in a ligand through which it coordinates

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Coordination chemistry

Polydentate ligands (continued)

Structure Denticity Abbreviation

:O: C :O: O .. 2− .. .. ..

1 or 2 CO2−

3

C C :O: :O: :O: :O: .. 2− ..

2

• x2−

N

2

H C N H2 H2 ..

2C

.. H

2 en

N N .. ..

2 phen

2

H 2 CH 2 CH C :O: C C

2

H

2

H C O: :O: C O: :O: N CH2 N C :O :O: C :O .. 4− .. .. .. .. .. .. .. .. ..

6 EDTA4−

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Coordination chemistry

Chelates

Chelate: a complex in which a bidentate or polydentate ligand forms a closed ring with a metal atom by forming two or more coordinate bonds Chelating agent: a ligand that can form chelates Examples of chelates:

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Coordination chemistry

Chelate effect

Chelates tend to be much more stable than similar complexes containing monodentate ligands (chelate effect). Example: Cu2+

(aq) + 4NH3(aq) ⇋ [Cu(NH3)4]2+ (aq)

K = 1.1 × 1013 Cu2+

(aq) + 2en(aq) ⇋ [Cu(en)2]2+ (aq)

K = 1.0 × 1020 These equilibrium constants are such that Cu2+ will preferentially bind en at any reasonably comparable concentrations of en and ammonia.

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Coordination chemistry

Chelation makes ions unavailable in solution. Applications: Added to detergents to reduce water hardness (EDTA) Food additive to prevent catalysis of oxidation by metal ions (EDTA) Chelation therapy for metal poisoning, esp. hypercalcemia, mercury or lead poisoning (dimercaptosuccinate)

CH C :O: :O: :S H C :O: CH :S H :O .. .. .. .. .. 2−

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Isomerism

Isomerism

Isomers are compounds with the same chemical formula (same atoms), but arranged differently. Structural isomers differ in what is bonded or coordinated to what. Stereoisomers have identical chemical bonds, but are arranged differently in space.

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Isomerism

Linkage isomerism: an example of structural isomerism

Linkage isomers contain a ligand coordinated to the metal centre through different donor atoms. Example:

N 3 H N 3 H N 3 H N H3 N H3 Co N O O 2+

yellow-orange and

N 3 H N 3 H N 3 H H N

3

H N

3

Co O N O 2+

red-orange

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Isomerism

Some types of stereoisomerism

Geometrical isomers have identical bonds, but the distances between some nonbonded atoms are different due to a different arrangement of the bonds in space. In square planar complexes with chemical formula MX2Y2 or in

• ctahedral complexes with formula MX2Y4, the two X ligands can be

adjacent (cis) or opposed (trans). Example:

NH3 NH3 Pt Cl Cl

and

NH3 NH3 Pt Cl Cl

cis trans (cisplatin) (transplatin)

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Isomerism

Example:

NH 3 NH 3 H 3N Cl H 3N Co Cl +

cis and

NH 3 NH 3 H 3N Cl NH 3 Co Cl +

trans

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Isomerism

In octahedral complexes with formula MX3Y3, the three X ligands can be in the same face of the octahedron (fac) or along a meridian (mer): Example:

NH 3 NH 3 Co NO 2 O2N NO 2 H 3N

fac and

NH 3 NH 3 NH 3 Co NO 2 O2N O2N

mer

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