Zumdahl’s Chapter 20

Transition Metals

Chapter Contents e– configuration Oxidation #s & IP Coordination

Compounds Coordination # Ligands Nomenclature

Isomerism Structural Isomerism Stereoisomerism

Bonding in Complex Ions Crystal Field Theory

Octahedral Tetrahedral

Electronic Configurationsd – block transition metals

ns2 (n–1)d X where n = 4,5,6,7 Potential for high spin (Hund’s Rule) Ions lose s electrons first.

f – block transition elements ns2 (n–1)d0,1 (n–2)f X where n = 6,7 Lanthanides & Actinides are even more

similar than members of d – block.

Oxidation StatesOften lose e– to Rare Gas configuration.

But beyond Mn, transition metal ions do not achieve that high.

Because the 8th IP is prohibitively expensive!

Sc Ti V Cr Mn Fe Co Ni Cu Zn

3 2,3

41,2,3

4,51,2,

3,4,

5,6

1,2,3,4,5,

6,7

2,3,4,5,6

1,2,3,4

1,23,4

1,2 2

Coordination CompoundsOften complex ions (both cat– and an–)

But neutrals possible if ligands exactly balance metal ion’s charge.

Often highly colored Since MO energy separations match visible

light photon energies, absorb visible light.

Often paramagnetic Duhh! These are transition metals, no?

Dative bonded by e– donating ligands.

Coordination NumberThe number of ligand bonds

Usually 6 (octahedral) but as few as 2 (linear) and as many as 8 (prismatic or antiprismatic cube).

Here’s Gd bonding

to a ligand called

DOTA 6 ways …

But to only one

of many solvent

water molecules.

For a bizarre

7 coordination.

Sane Coordination Numbers 6-coordinated

metals like cobalt sepulchrate : C12H24N8Co2+

Or the one we used in lab, MgEDTA2– C10H12O8N2Mg2–

Ligands From Latin ligare, “to bind”

Must be a Lewis base (e– donor)Could, as does EDTA, have several

Lewis base functionalities: polydentate! If monodentate, should be small enough

to permit others to bind.Relative bonding strengths:

X– < OH– < H2O < NH3 < en < NO2– < CN–

halides ethylene diamine

Naming Anionic NamesAnions that electrically balance cationic

coordination complexes can also be present as ligands in that complex! So they need different names that identify

when they’re being used as ligands:

Species Cl– NO2– CN–

As ion: chloride nitrite cyanide

As ligand: chloro nitro cyano

Naming Neutral NamesBut ligands needn’t be anions; many

neutral molecules are Lewis bases. And they too get new names appearing as

ligands in coordination complexes:

Species H2O NH3 CO

Normal: water ammonia carbon monoxide

As ligand: aqua ammine carbonyl

Name That Complex, Oedipus

[ Cr Br2 (en)2 ] Br Anion, bromide, is named last (no surprise) chromium(III) is named next-to-last Ligands named 1st in alphabetical order:

Number of a ligands is shown as Greek prefix: dibromo …

Unless it already uses “di” then use “bis” Dibromobis(ethylenediammine) …

Dibromobis(ethylenediammine)chromium(III) bromide

Charge OverrunSince ligands are often anions, their

charge may swamp the transition metal, leaving the complex ion negative!

Na2 [ PbI4 ] (from Harris p. 123)

Sodium tetraiodoplumbate(II) While lead(II) is the source, the Latin root is

used for the complex with “ate” denoting anion.

Li [ AgCl2 ], lithium dichloroargentate

Isomeric Complicationsdichlorobis(diethylsulfide)platinate(II)

would appear to be the name of the square planar species above, but The square planar configuration can have

another isomer where the Cl ligands are on opposite sides of the platinum, so it’s really

cis-dichlorobis(diethylsulfide)platinate(II) and this is not the only way isomers arise!

Complex Isomerization Simplified

Stereoisomers preserve bonds Geometric (cis-trans) isomers Optical (non-superimposable mirrors)

Structural isomers preserve only atoms Coordination isomers swap ligands for

anions to the complex. Linkage isomers swap lone pairs on the

ligand as the bonding site.

Coordination IsomersUnique to coordination complexes [ Pb (en)2 Cl2 ] Br2

bis(ethylenediammine)dichlorolead(IV) bromide

Only 1 of 3 possible coordination isomers The other 2 are

[ Pb Br (en)2 Cl ] Br Cl bromobis(ethylenediammine)chlorolead(IV)

bromide chloride

[ Pb Br2 (en)2 ] Cl2 dibromobis(ethylenediammine)lead(IV) chloride

Optical IsomersWe need to compare the mirror image

of a sample complex to see if it can be superimposed on the original.

These views of cobalt sepulchrate and its

Mirror image demonstrate non-superimposition.

They are optical isomers.

Colorful ComplexesColors we see everywhere are due, for

the most part, to electronic transitions. Most electronic transitions, however, occur

at energies well in excess of visible h. d-electrons transitions ought not to be

visible at all, since they are degenerate. But, in a complex, that degeneracy is

broken! Transition energies aren’t then 0.

Breaking Degeneracy5 d orbitals in a tetrahedral charge field

split as a doublet (E) and a triplet (T).

Td E 8 C3 3 C2 6 S4 6 d h=24

A1 1 1 1 1 1 x2+y2+z2

A2 1 1 1 –1 –1

E 2 –1 2 0 0 (2z2–x2–y2, x2–y2)

T1 3 0 –1 1 –1

T2 3 0 –1 –1 1 (xy, xz, yz)

Symmetry Tells Not AllWhile the symmetry tables assure us

that there are now 2 energy levels for d orbitals instead of 1, we don’t know the energies themselves. That depends upon the field established by

the ligands and the proximity of the d s. See Zumdahl’s Fig. 20.26 for a visual

argument why dxy,dxz,dyz are lower energy.

Other Ligand SymmetriesOctahedral, Oh, (6-coordinate, Fig. 20.20)

Eg symmetic species for (2z2–x2–y2, x2–y2)

T2g symmetric species for (xy, xz, yz)

Square Planar, D4h (Fig. 20.27a)

A1g symmetric species for z2

B1g symmetric species for x2–y2

B2g symmetric species for xy

Eg symmetric species for (xz, yz)

ConsequencesDegeneracies work in Hund’s favor to

separate e– pairs and maximize spin.With high enough energy separations,

, Aufbau (lowest level) wins instead. High field case, large, e– pairs in lower

energy states. Low field case, small, e– unpaired as

much as feasible.

Symmetry and tetrahedral = (4/9) octahedral (same ligands)

As a consequence of symmetry. If some ligand was 9/4 as strong as the

weakest to give octahedral strong field, then strong field (low-spin) tetrahedral might exist. But none does.

Field strengths of ligands vary as: X– < OH– < H2O < NH3 < en < NO2

– < CN–