Properties of Transition Metals Coordination Chemistry

Outline

• Properties of Transition Metals

• Coordination Chemistry

• Naming Coordination Compounds

• Structures of Complexes

• Theories in Coordination Chemistry

Properties of Transition Metals

1. Electron Configurations

Transition Metal Atoms

4s orbital filled before 3d orbital

Sc [Ar]4s23d1 Ti [Ar]4s23d2 V [Ar]4s23d3

Cr [Ar]4s13d5 Cu [Ar]4s13d10 ... are exceptions!

Transition Metal Ions

4s e−’s removed before 3d e−’s

Ti3+ [Ar]3d1 V3+ [Ar]3d2 Cr3+ [Ar]3d3

2. Atomic Radii

except Sc thru V, little variation

of radii in first series (3d)…

d-electrons shield nuclear charge!

metals in the second series (4d) and third series (5d) have similar radii: lanthanide contraction

f-electrons don’t shield the increasing nuclear charge…

electrons are drawn inward by increasing charge!

3. Catalytic Activity

transition metals are often key elements in catalysts

Ni hydrogenation of oils

Pt, Pd, Rh catalytic converter in automobiles

Fe3O4 synthesis of NH3

V2O5 manufacture of H2SO4

4. Color and Magnetism

partially filled d-sublevels imparts color and magnetic properties to transition metal compounds

Coordination Chemistry

Coordination Compounds are compounds with a metal atom or ion surrounded by 2 or more atoms or ions

[Pt(NH3)6]Cl4

Pt4+ ion surrounded by 6 NH3 molecules

Pt(NH3)64+

Complex Ions (or Coordination Complexes) are ions with a metal atom or ion surrounded by 2 or more atoms or ions:

Counterions are anions or cations added to produce a neutral compound

Cl−

Properties of coordination compounds and complex ions vary…

1. Color Co(NH3)63+ yellow

Co(NH3)5NCS2+ orange

Co(NH3)5H2O3+ red

Co(NH3)5Cl2+ purple

The color depends on the chemical groups attached to the transition metal

The attached chemical groups are called ligands

2. Coordination Number

The number of ligands surrounding a central atom or ion

Ox. No. Typical Co. No. Example

+1 2 Ag(NH3)2+

+2 4, 6 Pt(NH3)42+

+3 6 Co(NH3)63+

3. Magnetism

Some are diamagnetic (no unpaired e−’s), and some are paramagnetic (1 or more unpaired e−’s)

Frog levitated in very strong magnetic field…

… as a result of the diamagnetic behavior of water!

4. Influence of d-electrons

d0 very stable, difficult to produce moving to right

Sc3+ (III)

TiF62− (IV)

CrO42− (VI) Stable when

MnO4− (VII) bonded to oxygen!

K2CrO4 KMnO4

Color of CrO42− and MnO4

− due to charge transfer… electrons on O2− are excited to empty d-orbitals by visible light

d5 and d10 very stable

Mn2+ and Fe3+ (d5)

Ag+ and Zn2+ (d10)

d3, d6, d8 form inert (stable) complexes

d3 Cr(H2O)63+

d6 Co(NH3)63+

d8 PtCl42−

d4 very unstable

Cr2+ (d4) → Cr3+ (d3)

Naming Coordination Compounds

1. name cation first, anion second

2. ligands precede the central metal atom in the complex ion

3. negatively charged ligands end in –o

4. number of ligands indicated with a prefix (no mono-)

5. ligands are ordered alphabetically (ignore prefixes)

6. the name of the metal ion ends in –ate if the complex is negatively charge

7. a roman numeral (or 0) in parentheses indicates the ox. no. of the metal atom

[Pt(NH3)6]Cl4 hexaammineplatinum(IV) chloride

[Pt(NH3)5Cl]Cl3 pentaamminechloroplatinum(IV) chloride

Pt(NH3)2Cl4 diamminetetrachloroplatinum(IV)

K[Pt(NH3)Cl5] potassium amminepentachloroplatinate(IV)

Fe(CO)5 pentacarbonyliron(0)

K4[Fe(CN)6] potassium hexacyanoferrate(II)

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

Structures of Complexes

The coordination number determines the structure…

If CN = 2, then linear

CN = 4, then tetrahedral (square planar if d8)

CN = 6, then octahedral

Structural Isomers are compounds with the same chemical formula, but with the atoms bonded differently

H – C = C – C – H

H

_H H H__

_

C

C C___

H

H

H

H

H

H

Both are C3H6...

Structural isomers!

Geometric Isomers are compounds with the same chemical formula, but with different arrangements of the atoms

Co(NH3)4Cl2+ is octahedral shape and 2 geometric isomers…

adjacent (cis) opposite (trans)

H3N

NH3

NH3

H3N Cl

Cl

____

Co

____

H3N NH3

NH3H3N

Cl

Cl____

Co

____

++

Pt(NH3)2Cl2 is square planar (Pt2+ is d8) and has 2 geometric isomers…

Cu(NH3)2Cl2 is tetrahedral (Cu2+ is d9)

and has only 1 structure…

adjacent (cis) opposite (trans)

NH3

Pt

H3N

Cl Cl

NH3

Pt

H3N

Cl

Cl

H3N

___

Cu

Cl

Cl

H3N

Co(NH3)3Cl3 is octahedral and has 2 geometric isomers…

nonplanar(facial)

or(fac-)

planar(meridional)

or(mer-)

H3N

NH3

NH3

Cl

Cl

____

Co

____

Cl

H3N

NH3

H3N Cl

Cl____

Co

____

Cl

Draw the trans-square planar coordination compounds...

[Pt(NH3)2Cl2]Br2 [Pt(NH3)2Br2]Cl2

atoms are bonded differently, so structural (coordination) isomers

Coordination isomers have different names...

trans-diamminedichloroplatinum(IV) bromide

trans-diamminedibromoplatinum(IV) chloride

Br – Pt – Br

NH3

——

NH3

2+

Cl−Cl −Cl – Pt – Cl

NH3

——

NH3

2+

Br −Br −

Draw the tetrahedral coordination complexes...

Cu(NO2)2Cl22− Cu(ONO)2Cl2

2−

atoms are bonded differently, so structural (linkage) isomers

Linkage isomers have different names...

dichlorodinitrocuprate(II)

dichlorodinitritocuprate(II)

ONO

___

Cu

Cl

ClONO

O2N

___

Cu

Cl

Cl

O2N

2− 2−

Draw the square planar isomers of...

[Pt(H2O)2Cl2]

atoms are bonded the same but arranged differently, so geometric isomers

geometric isomers have similar names...

trans-diaquadichloroplatinum(II)

cis-diaquadichloroplatinum(II)

H2O

Cl – Pt – Cl

——

H2O

H2O – Pt – Cl

H2O

——

Cl

Ligands that bind to the metal at only 1 point are monodentate ligands

Ligands that bind to the metal at more than 1 point are polydentate ligands

Bidentate:

ethylenediamine (en) oxalate (ox) – “ethanedioate”

NH2

H2N

CH2

CH2

O

O

C

C

O

O═

–

–

____

Co

____

____

Co

____

…bis(ethylenediamine)… …tris(oxalato)…

Hexadentate: ethylenediaminetetraacetate (EDTA), −4 charge

A polydentate ligand is called a chelating agent; complex that is formed is called chelate

O

O

–

____

Co

____

O

O

–N

N

O

═O

–

O

O

═

–

Consider the tris(ethylenediamine)cobalt(III) ion: Co(en)33+

Not the same... They’re mirror images of each other that are not superimposable

2 nonsuperimposable mirror images are called enantiomers, and are optical isomers (a type of stereoisomer)

____

Co

____

NN

N

N

NN____

Co

____

NN

N

N

NN

Draw the isomers of dichlorobis(ethylenediamine)cobalt(III)...

CoCl2(en)2+

Three isomers… 1 trans-chloro and 2 cis-chloro optical isomers

____

Co

____

ClN

N

N

ClN

____

Co

____

NCl

N

N

NCl

____

Co

____

NN

NN

Cl

Cl + + +

Theories in Coordination Chemistry

They attempt to explain:

geometries (shapes)

magnetism (paired or unpaired e-’s)

color (electronic energy level differences)

1. Valence Bond Theory

assumes ligands are covalently bonded to the metal using hybrid metal orbitals

these orbitals overlap with a ligand orbital containing a lone pair of e-’s, forming a coordinate covalent bond

CN Shape Hybridization

2 linear s + p → 2 sp

4 tetrahedral s + 3 p → 4 sp3

square planar s + 2 p + d → 4 dsp2

6 octahedral s + 3 p + 2 d → 6 d2sp3

Cu(CN)2− CN = 2, linear sp hybridization

ZnCl42- CN = 4, tetrahedral sp3 hybridization

Pt(en)22+ CN = 4, square planar dsp2 hybridization

Co(H2O)63+ CN = 6, octahedral d2sp3 hybridization

2. Crystal Field Theory

assumes ionic bonding between the ligands (negative) and the metal cation

lone pair e−’s on ligands affect energies of the d orbitals

dz2dx2 – y2

xy

z

xy

z

xy

z

dxy

xy

z

dxz

xy

z

dyz

dx2 – y2 and dz2 affectedmost by approaching ligands

in octahedral complex!

Free metal ion... In octahedral complex...

The difference in energy between the d orbitals in a metal ion complex is called the splitting energy (Do)

If Do is large, the complex is called a low-spin complex

If Do is small, the complex is called a high-spin complex

___ ______ ___ ___d

x2 – y2, z2

xy, xz, yz

___ ___

___ ___ ___

DoE

Co(NH3)63+ is a low-spin complex

Co3+ has 6 d e−’s

With large Do, the 6 e-’s cannot spread through the 5 orbitals

Co(NH3)63+ is diamagnetic

___ ___

___ ___ ___large

___ ___

___ ___ ___↿⇂ ↿⇂ ↿⇂

CoF63- is a high-spin complex

Co3+ has 6 d e-’s

With small Do, the 6 e-’s can spread through the 5 orbitals

CoF63- is paramagnetic

___ ___

___ ___ ___small

___ ___

___ ___ ___↿⇂ ↿ ↿

↿↿

The stronger the electrostatic field the metal is in, the greater Do

Co(NH3)63+ large Do strong-field

CoF63- small Do weak-field

Spectrochemical series gives relative field strengths for ligands:

CO > CN− > en > NH3 > H2O > OH − > F − > Cl − > Br − > I −

strong-field ligands weak-field ligands

Predict the number of unpaired electrons and magnetism in Cr(CN)6

4−.

Cr2+ has 4 d e−’s (ignore ligand e−’s)

CN− is a strong-field ligand... so large Do

2 unpaired e−’s (paramagnetic)

Predict the number of unpaired electrons and magnetism in FeCl6

4−.

Fe2+ has 6 d e−’s

Cl− is a weak-field ligand... so small Do

4 unpaired e−’s (paramagnetic)

___ ___

___ ___ ___↿⇂ ↿ ↿

___ ___

___ ___ ___↿⇂ ↿ ↿

↿↿

Crystal field theory can be used to explain colors of coordination compounds (complexes)

e−’s are promoted from low d-orbitals to high d-orbitals with absorption of photons of visible light

color we observe is the complement (or leftovers) of absorbed light

A solution of... absorbs... and appears...

Co(NH3)63+ blue (450 nm) orange

Co(NH3)5H2O3+ green (500 nm) red

trans-Co(NH3)4Cl2+ red (680 nm) green

Red

Green

Blue Orange

Wavelength of light absorbed determined by Do

h = 6.626 x 10-34 J s

c = 2.998 x 108 m s-1

l = wavelength absorbed (m)

What is the splitting energy of Co(CN)63- which absorbs 290 nm

light?

l

c h o =D

J 10 x 6.8 m 10 x 290

)s m 10 x s)(2.998 J 10 x (6.626

c h 19-

-9

1-834-

o ===Dl

What’s Do (in kJ/mol) of Co(NH3)63+ which absorbs 440 nm light?

What wavelength of light does CoF63- absorb if Do = 155 kJ/mol?

J 10 x 4.5 m 10 x 440

)s m 10 x s)(2.998 J 10 x (6.626

c h 19-

-9

1-834-

o ===Dl

kJ/mol 270 mol 1

10 x 6.022 x

J 1000

kJ 1 x J 10 x 4.5

2319- =

J 10 x 2.57 10 x 6.022

mol 1 x

kJ 1

J 1000 x

mol

kJ 155 19-

23=

m 10 x 773 J 10 x 2.57

)s m 10 x s)(2.998 J 10 x (6.626

c h -9

19-

1-834-

o

==D

=l

nm) 773(or

d-Orbital energies are split for other geometries...

tetrahedral

ligands point at dxy, dxz, and dyz

so they have higher energies

square planar

ligands point right at dx2 – y2,

near dxy, and donut of dz2

___ ___

___ ___ ___ xy, xz, yz

z2, x2 – y2

___

___

___ ___

___

x2 – y2

xy

z2

xz, yz