Transition Metal Chemistry Lecture 1-3 PDF

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Part D: Transition Metal Chemistry Prof. Chris Sumby [email protected]

Chem IA: Transition Metal Chemistry

What is life? How do we unravel (and treat) the causes of disease?

How can we achieve clean energy future? Chem IA: Transition Metal Chemistry

Aims of Transition Metal (TM) Chemistry •

• •

•

To look at coordination complexes of the d-block metals, the influence of d-orbitals and how filling of these leads to colour and magnetism. To introduce bonding theories that explain metal-ligand complex formation. To discuss the role of metallic elements in biology and medicine. Note: References to additional reading from the prescribed text (Blackman et al.) are given on the relevant slides (3rd edition but are broadly similar for 2nd – 4 th editions). Chem IA: Transition Metal Chemistry

Specifics of the Course (I) • Transition Metals – Background, electronic configurations (B13.1/B13.2), d-orbitals (B4.5) and properties – Transition Metal Complexes, see B13.4 (see also Lewis Acid and Base concepts in TMs, B11.8)

• Ligands, Ligand exchange and the Chelate Effect (B13.3/B13.4) • Isomerism in TM complexes (B13.4 p.559) • Bonding in Transition Metal Complexes (B13.4) – Crystal Field Theory (B13.4 p.570): Chem IA: Transition Metal Chemistry

Specifics of the Course (II) • Bonding in Transition Metal Complexes (cont’d) • • • •

octahedral complexes (B13.4 p.570) the Spectrochemical Series (B13.4 p.574) Magnetic properties (B13.4 p.578) tetrahedral and square planar complexes (B13.4 p.573)

– Metal Carbonyl Compounds (poorly covered in text)

• Reactivity of Metal Complexes • Metals in Biology - Bioinorganic Systems (B13.5) – Carbonic Anhydrase, Haemoglobin (B13.5 p.582), Cytochromes (B13.5 p.584) – cis-Platin (B13 p.560) Chem IA: Transition Metal Chemistry

Lecture 1: Learning Objectives •

Describe the properties of transition metals.

•

Derive the electronic configurations of the neutral elements.

•

Sketch and understand the shape and orientation of the dorbitals and how these affect the element properties.

•

Recognise and describe features of transition metal chemistry that are consistent with general trends in the periodic table.

•

Recognise and describe features of transition metal chemistry that distinguish the transition metals from other elements.

Chem IA: Transition Metal Chemistry

Transition Metals • Mark the transition between the ultra electropositive metallic elements of the s-block and the semi-metals, liquids and gases of the pblock. [Ref: extra reading only file on MyUni for TMs]

Blackman C13.1 Chem IA: Transition Metal Chemistry

Transition Metals • General properties: i. _________ ii. _________ iii. _________________ iv. _________________ Most are used prodigiously in the world around us.

Chem IA: Transition Metal Chemistry

• The source of colour in the following solutions……

Image credit (left): https://www.compoundchem.com/2014/03/05/ colours-of-transition-metal-ions-in-aqueoussolution/ Colour of glasses: https://www.compoundchem.com/2015/03/03/ coloured-glass/

Chem IA: Transition Metal Chemistry

Transition Metals – Metallurgy • Most TMs are easily oxidised and often found as oxides, sulfides and carbonates. • After being mined and separated from contaminating minerals, TMs are obtained by chemical or electrochemical reduction. • The early metals (LHS) are more difficult to extract from their ores than their late cousins and the development of civilisation reflects this. Blackman 13.6 &13.7 Chem IA: Transition Metal Chemistry

The Elements Themselves • When neutral, the ndorbitals of (n+1) period elements are higher in energy than the (n+1)s orbital. B4.6

• Thus, for example – Potassium has the electron configuration [Ar] 4s1 – Strontium has the electron configuration [Kr] 5s2 Chem IA: Transition Metal Chemistry

The Elements Themselves • Once the (n+1)s orbital is filled, the five nd orbitals are filled to give ten d-block elements. • For the 1st row of the d-block, this represents the filling of the 3d orbitals to give the elements scandium (3d1) to zinc (3d10). Blackman 13.1 & 13.2 Since the chemistry of group 12 elements does not typically use dorbital valence electrons (just selectrons), we will not discuss in detail the chemistries of zinc (3d10), cadmium (4d10) or mercury (5d10) that much. Chem IA: Transition Metal Chemistry

Recap – Orbital Filling 1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s < 4d < 5p < 6s < 5d < 6p < 7s < 6d

• Note: This energy ordering only applies when we are considering the placing of electrons into the orbitals of neutral elements. • For the first row of the TMs, when electrons are lost they come out of the 4s orbitals first! Chem IA: Transition Metal Chemistry

Recap orbital filling 0 4s 0

4d 3d

4p Energy (J)

3s

4f

3s

3p

3d

4f

2p

2p

2s

4p

2s the energy of an orbital increases with l for a given n

-2.2E-18

0

1

1s

2

3

This effect is big enough that the energy of the 4s orbital is lower than 3d. Chem IA: Transition Metal Chemistry

The Elements Z 21 22 23 24 25 26 27 28 29

Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper

Sc Ti V Cr Mn Fe Co Ni Cu

Chem IA: Transition Metal Chemistry

More on electron configurations

• Anomalous configurations for Cr and Cu occur because the factors that control electron configurations are finely balanced. • In the d-block elements have varying electron configurations, even in the same group. • This is due to the closeness in energy of the s and d (and f) atomic orbitals, and because electron-electron repulsions are important in determining the electron configuration. Chem IA: Transition Metal Chemistry

Oxidation leads to loss of the 4s electrons • We will mostly talk about the oxidised forms of the elements. • Oxidation of a 3d TM leads to loss of the 4s electrons. • This occurs because the 4s orbitals are more radially diffuse than 3d orbitals. Thus the energy of the 3d orbitals decrease faster as Zeff increases. Chem IA: Transition Metal Chemistry

Transition Metals (TM) • The special properties of complexes of these metals derive from the five valence d-orbitals.

Blackman 4.5 Chem IA: Transition Metal Chemistry

Drawing the d-orbitals • It is required that you can draw the d-orbitals – here is your chance to practice!

Chem IA: Transition Metal Chemistry

Transition Metals (TM) • The d-orbitals are strongly directional and poorly shield the nuclear charge. • Consequently, their properties are dramatically influenced by their coordination environment. • Many of the trends seen before (e.g. for the s- and p-block) still hold e.g. metallic radius and ionisation energy

https://chem.libretexts.org/

Property 1: Oxidation States • Some properties are really distinctive though: +8 +7 +6 +5 +4 +3 +2 +1

•

Brown = Typical Green = Known Although there are multiple oxidation states for many elements, some trends can be discerned: i. High oxidation states are often preferred for the early metals. ii. +3 is common for metals in the middle. iii. +2 is preferred for the late metals.

•

This relates to an increase in Zeff across the period. Chem IA: Transition Metal Chemistry

Property 2: Paramagnetism • Unlike the elements of the p-block, the multiple oxidation states of the transition elements regularly change by +/- 1 (cf. p-block usually +/- 2). • This often leads to unpaired electrons and paramagnetism. The more unpaired electrons the greater the paramagnetism. • For the neutral metals a maximum is reached at chromium (4s13d5):

Chem IA: Transition Metal Chemistry

https://www.youtube.com /watch?v=zQgyrBnsprU

Property 3: Coordination Complexes • Much of TM chemistry is undertaken in water. • The water and metal (neutral or cation) form Lewis acid-base complexes, e.g.: [M(OH2)6]n+ • The formation of coordination complexes are intimately related to those of acids and bases. Chem IA: Transition Metal Chemistry

Recap: Brønsted-Lowry and Lewis Acids and Bases Brønsted-Lowry Acid = Proton donor Base = Proton acceptor

Lewis Acid = electron pair acceptor Base = electron pair donor

• Generally: A Lewis acid has a vacant valence orbital. A Lewis base has an electron lone pair. • By definition: A proton, H+, is a Lewis acid. All Brønsted-Lowry bases are Lewis bases Chem IA: Transition Metal Chemistry

From Lewis to Transition Metal Complexes • Lewis acid and base principles allow us to understand a large amount of chemistry: – Organic Chemistry – Main group chemistry – Transition metal complexes

• Example: Fe3+

+

Acid/Metal

6H2O



Base/Ligands

[Fe(OH2)6]3+ Complex/Adduct The Lewis bases are electron donors which are known as ligands.

Lecture 2: Learning Objectives • Understand

and

use

the

basic

terminology

associated with coordination compounds (ligand, donor atom, coordination number, coordination geometry). • Use the Lewis approach to describe coordination chemistry. • Understand and use classifications of ligands (denticity). Chem IA: Transition Metal Chemistry

Werner's Theory of Coordination Complexes • Alfred Werner developed a model to explain the following observations. • CoCl2 is dissolved in aqueous ammonia and then oxidized by air to give three complexes. • A fourth complex can be made by slightly different techniques. • These complexes have different colours and different empirical formulas. [Co(NH3)6]Cl3 [Co(NH3)5(H2O)]Cl3 [Co(NH3)5Cl]Cl2 [Co(NH3)4Cl2]Cl

CoCl3(NH3)6 CoCl3(NH3)5(H2O) CoCl3(NH3)5 CoCl3(NH3)4

orange-yellow red purple green

Experimental Observations • The reactivity of the ammonia in these complexes has been drastically reduced. – NORMALLY Ammonia reacts with hydrochloric acid to give ammonium chloride but these complexes don't react with hydrochloric acid, even at 100C!

• Solutions of the Cl- ion react with Ag+ ion to form a white precipitate of AgCl. CoCl3(NH3)6

orange-yellow

three moles of AgCl are formed

CoCl3(NH3)5(H2O) red

three moles of AgCl are formed

CoCl3(NH3)5

purple

two moles of AgCl are formed

CoCl3(NH3)4

green

one mole of AgCl formed

Experimental Observations (2) • Measurements of the conductivity tell us about the number of ions in aqueous solution. • Suggest: CoCl3(NH3)6

orange-yellow

total of four ions

CoCl3(NH3)5(H2O) red

total of four ions

CoCl3(NH3)5

purple

total of only three ions

CoCl3(NH3)4

green

total of only two ions

Hypothesis • Werner explained these observations by suggesting that transition-metal ions have: – A primary valence, i.e. the number of negative ions needed to satisfy the charge on the metal ion (oxidation state). – A secondary valence, i.e. the number of ions or molecules that are coordinated to the metal ion (coordination number). [Co(NH3)6]Cl3 [Co(NH3)5(H2O)]Cl3 [Co(NH3)5Cl]Cl2 [Co(NH3)4Cl2]Cl

CoCl3(NH3)6 CoCl3(NH3)5(H2O) CoCl3(NH3)5 CoCl3(NH3)4

orange-yellow red purple green

• The cobalt ion is coordinated to a total of six ligands in each complex (secondary valence). • Each complex also has a total of three chloride ions (primary valence).

Evidence to support hypothesis • The cobalt ion is coordinated to a total of six ligands in each complex, which satisfies the secondary valence of this ion. Each complex also has a total of three chloride ions that satisfy the primary valence. • Some of the Cl- ions are free to dissociate when the complex dissolves in water. Others are bound to the Co3+ ion and neither dissociate nor react with Ag+. [Co(NH3)6]Cl3 [Co(NH3)5(H2O)]Cl3 [Co(NH3)5Cl]Cl2 [Co(NH3)4Cl2]Cl

CoCl3(NH3)6 CoCl3(NH3)5(H2O) CoCl3(NH3)5 CoCl3(NH3)4

orange-yellow red purple green

• Werner assumed that transition-metal complexes had definite shapes. According to his theory, the ligands in six-coordinate cobalt(III) complexes are oriented toward the corners of an octahedron.

Consistent with the observations… • The ammonia is not free to react – it is coordinated. • Some of the Cl- ions are free to dissociate when the complex dissolves in water. Others are bound to the Co3+ ion and neither dissociate nor react with Ag+. • Werner assumed that transition-metal complexes had definite shapes.

[Co(NH3)6]Cl3

[Co(NH3)5Cl]Cl2

[Co(NH3)4Cl2]Cl* * Only one isomer shown

Alfred Werner The Nobel Prize in Chemistry 1913 was awarded to Alfred Werner "in recognition of his work on the linkage of atoms in molecules by which he has thrown new light on earlier investigations and opened up new fields of research especially in inorganic chemistry".

Primary and secondary valence – oxidation state and coordination number

Coordination Chemistry [Fe(OH2)6]2+ + 6CN–  [Fe(CN)6]4– + 6H2O • This reaction type dominates the chemistry of transition metals. • Complexes are often formed by simply displacing one molecule or anion by another.

Chem IA: Transition Metal Chemistry

Nomenclature and Definitions • For a typical example:

Metal

Co = Lewis acid = metal. OH2/Br = Lewis base = ligand. O/Br are lone pair donors.

Sulfate Counterion

Charge on complex Ligand A complex (ion) is a (transition) metal surrounded by coordinated ligands.

A compound containing a complex (ion) is called a coordination compound.

Additional reading

Nomenclature: extra reading only file on MyUni for TMs See also Blackman 13.4, p.562

Chem IA: Transition Metal Chemistry

Thermodynamic Stability • The stability of the new complex to the reverse reaction, otherwise known as the stability constant, Kf, can be calculated. • For [Fe(OH2)6]2+ + 6CN– [Fe(CN)6]4– + 6H2O Kf

=

[[Fe(CN)6]4-] [[Fe(OH2)6]2+][CN-]6

• The higher the value of Kf, the greater the stability. Chem IA: Transition Metal Chemistry

Thermodynamic vs. Kinetic stability • Complexes with a high value of Kf are thermodynamically stable, i.e. the equilibrium lies toward the products and we see the compound. • This does not mean the complex is kinetically stable:

• Complexes can either be labile (undergo rapid ligand exchange) or inert (stable to ligand exchange). Blackman 13.4 p.568 Chem IA: Transition Metal Chemistry

Colour of Transition Metal Complexes • The most apparent distinction between the chemistries of the main group and transition elements is the frequent vivid colouration of TM complexes. • This occurs because the energy required for electronic transitions between d-orbitals is comparable to that of visible light.

Question: Why do these colours vary? Chem IA: Transition Metal Chemistry

Why do these colours vary? • The d-orbitals are strongly directional and poorly shield the nuclear charge. • Consequently, their properties are dramatically influenced by their coordination environment.

Chem IA: Transition Metal Chemistry

Coordination Number • To start to understand properties like colour we need to know the structures of complexes. • Compounds with coordination numbers of 1 to greater than 10 are possible. • Luckily, most compounds are four or six coordinate. • The ligands take up a well-defined geometry around the metal centre. • For example: tetrahedral, Td (4 ligands) octahedral, Oh (6 ligands) Blackman 13.4 Chem IA: Transition Metal Chemistry

Coordination Number: 6 • Octahedral (Oh) six-coordinate complexes are by far the most prevalent. • Examples: [M(OH2)6]n+ [Fe(CN)6]4[Mo(CO)6] [PtCl6]2-

M

Black arrows forward/dashed arrows backward

Chem IA: Transition Metal Chemistry

Coordination Number: 4 • Most four coordinate complexes are tetrahedral (Td), i.e. the four ligands are placed at the apices of a tetrahedron.

• Examples: [CoCl4]2[CrO4]2[Zn(NH3)4]2+

Chem IA: Transition Metal Chemistry

M

Coordination Number: 4 • There are some exceptions to this, most having square-planar geometry, with the ligands located at the corners of a square. • Examples: [RhCl(PPh3)3] [RhI2(CO)2

M

]-

[PtCl2(NH3)2] • Occurs commonly for complexes with d8 electron configurations – see CFT lectures. Chem IA: Transition Metal Chemistry

Julia Lermontova (German) whose experimental chemistry established the right place for the elements of the platinum group (Ru, Rh, Pd, Os, Ir, Pt) in the periodic table.

Drawing TM complexes • It is required that you can draw TM complexes – here is your chance to practice!

Chem IA: Transition Metal Chemistry

What kind of ligands do we encounter??? • Molecules with one or more lone pair/s. • By their nature these are normally neutral or anionic. • Examples include H2O (neutral) and Clˉ (anionic):

H

O

H

Cl

• Ligands may have one or more donor atoms. Blackman 13.3 Chem IA: Transition Metal Chemistry

Monodentate Ligands • Those with one donor are termed monodentate (meaning one ‘tooth’) and can only occupy one coordination site. • Examples:

HOˉ, NH3, OH2, Clˉ, Brˉ, Iˉ, CO, NCˉ

Chem IA: Transition Metal Chemistry

Bidentate Ligands • Ligands with two donors that can participate in coordination together are known as bidentate (two ‘teeth’) and normally occupy two adjacent sites. • Examples are: 1,2-ethylenediamine, en:

2,2’-bipyridine, bpy:

Chem IA: Transition Metal Chemistry

Bidentate Ligands [Co(OH2)6]3+ + 3 en  [Co(en)3]3+ + 6 H2O

The tris(ethylenediamine)cobalt(III) ion Chem IA: Transition Metal Chemistry

Definitions • The donor atom is the atom directly coordinated to the transition metal (these are the lone pair donors). • The coordination number is the number of donor atoms – not the number of ligands – coordinated to the transition metal in a complex.

Note the number of ligands!

Chem IA: Transition Metal Chemistry

Tridentate Ligands • Tridentate (three ‘teeth’): e.g. diethylenetriamine, dien:

H N

H2N

N

2,2’,6’,2”-terpyridine, tpy:

N

H2 O

N

n+

H2 N

[M(OH2)6]n+ + dien 

NH 2

NH M

H2 O OH2 Chem IA: Transition Metal Chemistry

+ 3 H2O N H2

Tetradentate Ligands • An example of a tetradentate ligand is porphyrin: (n - 2)+ N N

H 2O N N

N

N

+ [M(OH2) 6] n+

M

N

N

OH2

+ 4 H 2O

Chem IA: Transition Metal Chemistry

+...