Transition Metal Chemistry CHEM261HC/SS1/01 Periodic Table 1. - - PDF document

6/4/2011 1

APPLI ED I NORGANI C CHEMI STRY FOR CHEMI CAL ENGI NEERS

Transition Metal Chemistry

CHEM261HC/SS1/01

 Elements are divided into four categories

Periodic Table

Main-group elements (S-Block) Transition metals Main-group elements (P-Block)

• 1. Main-group elements
• 2. Transition metals
• 3. Lanthanides
• 4. Actinides

( )

CHEM261HC/SS1/02

Lanthanides Actinides

6/4/2011 2

Transition metals vs. Main-group elements

Main‐group elements Transition metals

 Main‐group metals

• malleable and ductile
• conduct heat and electricity
• form positive ions

 Transition metals

• more electronegative than the

main group metals

• more likely to form covalent

compounds

• easily form complexes

Cisplatin

CHEM261HC/SS1/03

 There is some controversy about the classification of the elements i.e. Zinc (Zn), Cadmium (Cd) and Mercury (Hg) e‐ configuration [ ]ns2 n‐1d10

• form stable compounds with

neutral molecules IUPAC ‐ A transition metal is "an element whose atom has an incomplete d sub‐ shell, or which can give rise to cations with an incomplete d sub‐shell.”

Electron configuration of Transition-metal ions

• The relationship between the electron configurations of transition‐metal

elements and their ions is complex.

Example

• Consider the chemistry of cobalt which forms complexes that contain

either Co2+or Co3+ ions. Co: Co2+: Co has 27 electrons [Ar] has 18 electrons [Ar] [Ar] 4s2 3d7 3d7

CHEM261HC/SS1/04

Co3+:  In general, electrons are removed from the valence shell s orbitals before they are removed from valence d orbitals when transition metals are ionized. [Ar] 3d6

6/4/2011 3

 How do we determine the electronic configuration of the central metal ion in any complex?

• Try to recognise all the entities making up the complex
• Need knowing whether the ligands are neutral or anionic
• Then you can determine the oxidation state of the metal ion.

A simple procedure exists for the M(II) case …same as M(+2) or M2+

22 23 24 25 26 27 28 29 Ti V Cr Mn Fe Co Ni Cu

CHEM261HC/SS1/05

Cross off the first 2 gives you total No. of valence electrons left

2 3 4 5 6 7 8 9 EXAMPLES

Elements Configuration Oxidized elements Configuration Sc [Ar]4s23d1 Sc(III) [Ar] Sc [Ar]4s 3d Sc(III) [Ar] V [Ar]4s23d3 V(II) [Ar]3d3 Cr [Ar]4s13d5 Cr(III) [Ar]3d3 Fe [Ar]4s23d6 Fe(II) [Ar]3d6 Ni [Ar]4s23d8 Ni(II) [Ar]3d8 Cu [Ar]4s13d10 Cu(I) [Ar]3d10 Zn [Ar]4s23d10 Zn(II) [Ar}3d10

…variety of oxidation states !!

6/4/2011 4 Evaluating the oxidation state [CoCl(NO2)(NH3)4]+

Neutral zero charge

Net charge on complex ion (+1)

CHEM261HC/SS1/06

x = +3 X + (- 2) + 0 = +1

Co3+

zero charge

X - 2 = +1  Why do these elements exhibit a variety of oxidation states?

 Because of the closeness of the 3d and 4s energy states Sc +3

Oxidation states and their relative stabilities

Sc +3 Ti +1 +2 +3 +4 V +1 +2 +3 +4 +5 Cr +1 +2 +3 +4 +5 +6 Mn +1 +2 +3 +4 +5 +6 +7 Fe +1 +2 +3 +4 +5 +6 C +1 +2 +3 +4 +5

CHEM261HC/SS1/07

• The most prevalent oxidation numbers are shown in green.

Co +1 +2 +3 +4 +5 Ni +1 +2 +3 +4 Cu +1 +2 +3 Zn +2

6/4/2011 5

• An increase in the No. of oxidation states from Sc to Mn.
• All seven oxidation states are exhibited by Mn.
• There is a decrease in the No. of oxidation states from Mn to Zn.

WHY?

 Because the pairing of d-electrons occurs after Mn (Hund's rule) which in turn decreases the number of available unpaired electrons and hence, the number of oxidation states. Sc +3 Ti +1 +2 +3 +4 V +1 +2 +3 +4 +5 Cr +1 +2 +3 +4 +5 +6 Mn +1 +2 +3 +4 +5 +6 +7 Fe +1 +2 +3 +4 +5 +6

CHEM261HC/SS1/09

 The stability of higher oxidation states decreases in moving from Sc to Zn.

• Mn(VII) and Fe(VI) are powerful oxidizing agents and the higher
• xidation states of Co, Ni and Zn are unknown.

Co +1 +2 +3 +4 +5 Ni +1 +2 +3 +4 Cu +1 +2 +3 Zn +2

• The relative stability of +2 state with respect to higher oxidation

states increases in moving from left to right. On the other hand +3 state becomes less stable from left to right. Why? Sc +3

• This is justifiable since it will be increasingly difficult to remove the

third electron from the d-orbital.

22 23 24 25 26 27 28 29 Ti V Cr Mn Fe Co Ni Cu

Example

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

CHEM261HC/SS1/10

Ti V Cr Mn Fe Co Ni Cu

M = [Ar]4s23dx M+2 = [Ar]3dx loss of the two s electrons M+3 = [Ar]3dx-1 more difficult Ni +1 +2 +3 +4 Cu +1 +2 +3 Zn +2

6/4/2011 6

• Oxidized by HCl or H2SO4 to form blue Cr2+ ion
• Cr2+ oxidized by O2 in air to form green Cr3+

Chromium

A i t 1

• Cr also found in +6 state as in CrO4

2− and

Cr2O7

2− are strong oxidizer

 Write down balance equations that show the two reactions Assignment 1

Cr2O7 are strong oxidizer

 Use balanced equations to show that CrO4

2−

and Cr2O7

2− are strong oxidizing agents

Assignment 2

Assignment 1

Solution Cr + H SO Cr SO + H 2 Cr(s) + 4 HCl(aq) 2 CrCl2(aq) + 2H2(g) Cr(S) + H2SO4(aq) Cr2SO4(aq) + H2(g) 2CrCl2(aq) + O2(g) Cr2O2Cl2(aq) + Cl2(g)

6/4/2011 7

• Fe exists in solution in +2 or +3 state
• Elemental Fe reacts with non-oxidizing acids to

Iron

form Fe2+, which oxidizes in air (O2) to Fe3+

• Brown water running from a faucet is caused by

insoluble Fe2O3

• Fe3+ soluble in acidic solution, but forms a

hydrated

• xide

as red-brown gel in basic solution solution Assignment 3 Fe2O3  Complete and balance the following equation

+

HCl

Coordination Chemistry

 A coordination compound (complex), contains a central metal atom (or ion) surrounded by a number of oppositely charged ions or neutral molecules (possessing lone pairs of electrons) which are known as ligands.  If a ligand is capable of forming more than

• ne bond with the central metal atom or ion,

then ring structures are produced which are known as metal chelates

• the ring forming groups are described as

CHEM261HC/SS1/13

the ring forming groups are described as chelating agents or polydentate ligands.  The coordination number of the central metal atom or ion is the total number of sites occupied by ligands.

• Note: a bidentate ligand uses 2 sites, a tridentate 3 sites etc.

6/4/2011 8

Ligands

molecular formula Lewis base/ligand Lewis acid donor atom coordination number formula base/ligand acid atom number [Zn(CN)4]2-

CN- Zn2+ C 4

[PtCl6]2-

Cl- Pt4+ Cl 6

[Ni(NH3)6]2+

:NH3 Ni2+ N 6

CHEM261HC/SS1/14

Mono-dentate Multidentate ligands

Abbreviation Name Formula en Ethylenediamine

CHEM261HC/SS1/15

• x2-

Oxalato EDTA4- Ethylenediamine- tetraacetanato

6/4/2011 9

 Chelating ligands bond to metal

• Five or six atoms rings are common
• forms rings – chelate rings

 Coordination numbers and geometries

(i.e. including metal)

Li

CHEM261HC/SS1/16

Linear Square planar Tetrahedral Octahedral

Nomenclature of Coordination Compounds

• The basic protocol in coordination nomenclature is to name the ligands

attached to the metal as prefixes before the metal name.

• Some common ligands and their names are listed above.

6/4/2011 10

 As is the case with ionic compounds, the name of the cation appears

first; the anion is named last.

 Ligands are listed alphabetically before the metal. Prefixes denoting

the number of a particular ligand are ignored when alphabetizing.

Example [Co(NH3)5Cl]Cl2 Pentaamminechorocobalt(III) chloride

cation anion 5 NH3 ligands Cl‐ ligands cobalt in +3

• xidation states

 The names of anionic ligands end in “o”; the endings of the

names of neutral ligands are not changed.

 Prefixes tell the number of a type of ligand in the complex.

If th f th li d it lf h h fi If the name of the ligand itself has such a prefix, alternatives like bis-, tris-, etc., are used.

[Co(NH2CH2CH2NH2)2Cl2]+ dichlorobis(ethylenediammine)cobalt(III) cation

Example

2 Cl‐ ligands 2 enligands with 2 NH2 groups cobalt in +3

• xidation states

en = ethylenediammine

6/4/2011 11

 If the complex is an anion, its ending is changed to -ate.  The oxidation number of the metal is listed as a roman numeral

in parentheses immediately after the name of the metal.

Example Na2[MoOCl4]

Exercise 1

Name the following coordination complexes: (i) Cr(NH3)Cl3 (ii) Pt(en)Cl2 (ii) Pt(en)Cl2 (iii) [Pt(ox)2]2-

Exercise 2

Give the chemical formular for the following coordination complexes: (i) Tris(acetylacetanato)iron(III) (ii) Hexabromoplatinate(2-) (iii) Potassium diamminetetrabromocobaltate(III)

6/4/2011 12

(i) Cr(NH3)Cl3 Ammine

Solutions

chromium ammine chloro (III) tri trichlorochromium(III) Ammine (ii) Pt(en)Cl2 Dichloro Platinum ethylenediammine chloro (II) di trichlorochromium(III) ethylenediammineplatinum(II) (iii) [Pt(ox)2]2- Dioxalato Platinate

• xalato

(II) di y p ( ) platinate(II) (i) Tris(acetylacetanato)iron(III) Fe(acac)3

Solutions

Fe acac

3+

( )3 Fe(acac)3 (ii) Hexabromoplatinate(2-) [PtBr6]2- (ii) P t i di i t t b b lt t (III) Pt Br [ ]2-

6

(ii) Potassium diamminetetrabromocobaltate(III) K[Co(NH3)2Br4] K NH3 Br Co ( )2

4 3+

6/4/2011 13

Isomers

 Primarily in coordination numbers 4 and 6.  Arrangement of ligands in space and also the ligands themselves. Types Ionization isomers

• Isomers can produce different ions in solution

e.g. [PtCl2(NH3)4]Br2  [PtBr2(NH3)4]Cl2 Polymerization isomers

• Same empirical formula or stoichiometry, but different molar mass.

CHEM261HC/SS1/17

• Different compounds with similar formula

[Co(NH3)3 (NO2)3 ]° ( n = 1) [Co(NH3)6 ]3+ [Co(NO2)6 ]3− ( n = 2) [Co(NH3)4 (NO2)2 ]+ [Co(NH3)2 (NO2 )4]− ( n = 2)

e.g.

[MXx Bb ]n

• Hydration isomers exist for crystals of complexes containing water

molecules  exist in three different crystalline Hydration isomers  exist in three different crystalline forms, in which the number of water molecules directly attached to the Cr 3+ ion differs [Cr(H2O)4 Cl2]Cl·2H2O dark green e.g. CrCl3·6H2O [Cr(H2O)5 Cl]Cl2·H2O light green [Cr(H2O)6 ]Cl3 gray-blue

• In each case, the coordination number of the chromium cation is 6

6/4/2011 14

Coordination isomers

[Co(NH3)6]3+ [Cr(CN)6]-3 and [Cr(NH3)6]+3 [Co(CN)6]-3

• In compounds, both cation and anion are complex, the distribution of

ligands can vary, giving rise to isomers. [ (

3)6]

[ ( )6] [ (

3)6]

[ ( )6]

Linkage isomers

e.g. Nitro and nitrito

(a) [Co(NO2)(NH3)5]2+

Yellow

• How

the ligands arrange themselves and attach to the central metal

CHEM261HC/SS1/18

N or O coordination possible

(b) [Co(ONO)(NH3)5]2+

Red

Geometric isomers

• Formula

is the same but the arrangement in 3‐D space is different.

e.g. square planar molecules give cis

and trans isomers.

CHEM261HC/SS1/19

6/4/2011 15

For hexacoordinate systems

Purple Green

CHEM261HC/SS1/20

For M(X)3(Y)3 systems (e.g. octahedral) there is facial and meridian

F i l

CHEM261HC/SS1/21

• When three identical ligands occupy one face of an octahedron
• any two identical ligands are adjacent or cis to each other

Facial

• If these three ligands and the metal ion are in one plane

Meridian

6/4/2011 16

Co – Octahedral geometry

Exam ple

cis-[CoCl2(NH3)4]+ trans-[CoCl2(NH3)4]+ fac-[CoCl3(NH3)3] mer-[CoCl3(NH3)3]

• Are “stereoisomers” also possible?
• An analogy to organic chirality.
• molecules that have the same molecular formula and sequence

Stereoisomer

• f bonded atoms (constitution), but which differ only in the

three-dimensional orientations of their atoms in space

• Molecules which can rotate light.
• Enantiomers

– non-superimposable

CHEM261HC/SS1/22

non superimposable mirror images

6/4/2011 17

Complex Stabilities

• Generally in aqueous solution, for a given metal and

ligand, complexes where the metal oxidation state is +3 are more stable than +2

• Generally the stabilities of complexes of the first row of
• Generally the stabilities of complexes of the first row of

transition metals vary in reverse of their cationic radii MnII < FeII < CoII < NiII > CuII > ZnII

• Hard and soft Lewis acid-base theory
• small atomic/ionic radius

CHEM261HC/SS1/23

 Hard acids and bases tend to have:

• high oxidation state
• low polarizabilty
• high electronegativity
• hard bases - energy low-lying HOMO
• hard acids - energy high-lying LUMO
• Chelate effect -

is the additional stability of a complex containing a chelating ligand, relative to that of a complex containing monodentate ligands with the same type and number

• f donors as in the chelate.

CHEM261HC/SS1/24

[Cu(H2O)4(NH3)2]2+ + en [Cu(H2O)4(en)]2+ + 2 NH3

6/4/2011 18

Cu(H2O)4(NH3)2]2+ + en = [Cu(H2O)4(en)]2+ + 2 NH3

• Mainly an entropy effect.
• When ammonia molecule dissociates ‐ swept off in solution and

the probability of returning is remote.

• When one amine group of en dissociates from complex ligand

retained by end still attached so the nitrogen atom cannot move away – swings back and attach to metal again.

CHEM261HC/SS1/25

away swings back and attach to metal again.

• Therefore the complex has a smaller probability of dissociating.

Exam ple

CHEM261HC/SS1/26

6/4/2011 19

Metal carbonyl

• Compounds

that have the metal bonded to the carbon monoxide, giving a general formula of M(CO)n

M + CO M(CO)n

C O M

∏-orbitals in CO are very empty

Molecular orbital diagram ( CO)

Therefore the bond order is: 4 – 1 = 3 Bond order:

• No. of e- pairs in the bonding orbital — No. of e- pairs in

the anti-bonding orbital

6/4/2011 20

Back-bonding (back donation)

 Formation of ∏-bonding as a result of the overlap of metal d ∏-

• rbitals and the ligand, CO, ∏* orbitals

Eff t Effects:

• It enhances the bonding strength between the metal and the ligand.
• The metal-ligand bond is shortened (M

CO)

• The

becomes longer, weaker and the bond order decreases Evidence and extent

C O

• IR spectra – Vibration frequency

– The greater the extent of back bonding the lower the stretching frequency (bond order decreases) Free ≈ 2143 cm-1 M CO ≈ 1900 - 2125 cm-1

C O

C O

Effect of replacing the CO ligands

Non- ∏ accepting ligands (donor ligands)

Cr(CO) Cr(triens)(CO)3 Trien Cr(CO)6 Cr(triens)(CO)3

2100 cm-1 2000 cm-1 1985 cm-1 1900 cm-1 1760 cm-1

• Replacement of the 3 x (CO) groups with donor ligands (trien) increases ∏-

acidity of the remaining ligands (CO) so as to counter the accumulation of the negative charge on the metal centre

6/4/2011 21

Effect of introducing a positive charge on metal complex

V(CO)6

• 1 proton

1860 cm-1 2000 cm-1

V(CO)6 V(CO)6

+

1 proton

2090 cm-1

• Introducing a +ve charge on the metal inhibits shift of electrons from metal

to empty ∏*- orbital of the CO ligands – This weakens ∏-bonding or decrease stretching frequencies of M-C while the

• increases. (wave number or frequency increases)

1860 cm 1 2000 cm 1 2090 cm 1

C O

Thought

• V(CO)- and Cr(CO) are isoelectronic yet

stretching frequencies of CO in V(CO)6 is lower than that of CO in Cr(CO)6 ?

The origin of colour - absorption

CHEM261HC/SS1/27

6/4/2011 22

The colour can change depending on a number of factors e.g.

• Metal charge

Colours on coordination com pounds

• Ligand

Physical phenomenon

CHEM261HC/SS1/29

6/4/2011 23

Are there any simple theories to explain the colours in transition metal complexes?

• There is a simple electrostatic model used by chemists to

ti li th b d lt rationalize the observed results This theory is called Crystal Field Theory

• It is not a rigorous bonding theory but merely a simplistic

approach to understanding the possible origins of photo-

CHEM261HC/SS1/30

pp g p g p and electrochemical properties of the transition metal complexes