Chemistry 223 Chapter 26Coordination Complexes

d-block elements a.k.a. transition metals

d-block elements are: • all metals• all have partially filled d subshells• exhibit horizontal & vertical similarities• alloys & compounds are important components of materials in modern world • most first-row transition metals are essential for life

General Trends among Transition Metals

General Trends among Transition Metals

4th row Horizontal Periodic Trends

General Trends among Transition Metals

Going across row from left to right , e-’s are added to 3d subshell

to neutralize increase in (+) charge of nucleus as atomic # increases.

General Trends among Transition Metals

3d subshell fill based on aufbau principle & Hund’s rule with two important exceptions:

Reactivity:

Size of neutral atoms of d-block elements gradually decreases

left to right across a row.Why?

Due to increase in Zeff with increasing atomic #

Atomic radius increases going down a column.Why?

Transition metals become less reactive (more “Noble”)going from left to right across a row

Trends in Transition Metal Oxidation States:

Transition metals form cations by initial loss of ns e-’s, even though ns orbital is lower in energy than (n–1)d subshell in the neutral atom.

d-electron configuration for di-cations of 1st row of transition metals

Trends in Transition Metal Oxidation States:

Small E difference btwn ns and (n-1)d plus screening effect means

less E losing ns e-’s before (n-1)d e-’s

All transition-metal cations possess dn valence e- configurations for 2+ ions of 1st row.

Trends in Transition Metal Oxidation States:

Electronegativities of first-row transition metals increase (somewhat) smoothly from Sc to Cu

Sc Ti V Cr Mn Fe Co Ni Cu Zn1.36 1.54 1.63 1.66 1.55 1.83 1.88 1.91 1.90 1.65

Trends in Transition Metal Oxidation States:

max oxid states for 2nd & 3rd row transition metals in Groups 3 thru 8

increase from +3 for Y and La to +8 for Ru and Os

Trends in Transition Metal Oxidation States:

Going farther to right, maximum oxidation state decreases,

reaching +2 for elements of Group 12,

Scandium [Ar]4s23d1 +3 Titanium [Ar]4s23d2 +4 Strong, light, corrosion-resistant, steel alloys, white

pigments, ore is rutile

Vanadium [Ar]4s23d3 +2, +3, +4, +5 Catalysts, steel alloys

Chromium [Ar]4s13d5 +2, +3, +6 Colorful, Cr2O72− OA, stainless steel, chrome plating

Manganese [Ar]4s23d5 +2, +4, +7 MnO4− OA, MnO2 catalyst, Mn steels

Iron [Ar]4s23d6 +2, +3 Ores are hematite, magnetite, and pyrite (fool’s gold), steel, hemoglobin, blast furnace, magnetic

Cobalt [Ar]4s23d7 +2, +3 Blue cobalt glass, , AlNiCo, magneticNickel [Ar]4s23d8 +2 Coins, AlNiCo, Monel, magneticCopper [Ar]4s13d10 +1, +2 Coins, brass, bronze, Statue of Liberty, patina,

electric wires, ores are chalcocite, chalcopyrite and malachite, unreactive w/ HCl and H2SO4 but very reactive w/HNO3

Zinc [Ar]4s23d10 +2 Coins, brass, biochemistry, RA Gold [Xe]6s14f145d10 +1, +3 Coins, jewelry, soft as pure metal, alloys are harder,

CN− used to extract Au from ores

Silver [Kr]5s14d10 +1 Coins, jewelry, most electrically conductive of all metals

Mercury [Xe]6s24f145d10 +1, +2 Quicksilver, poisonous, “mad as a hatter”, Minimata

Descriptive Chemistry of 3d Transition Metals:

Compounds of Mn in +2 to +7 oxidation states

Different # of d electrons = different colorsWhy is that?

Coordination Compounds

Metallic elements act as Lewis acids form complexes with various Lewis bases.

Metal complex:

Coordination Compounds

Central metal atom (or ion) bonded to one or more ligands.

Ligands:

Ligands

Coordination Compoundsmetal & ligand complexes as ions:

Coordination CompoundsCoordination compounds & complexes are

distinct chemical species properties & behavior diff from metal atom / ion or the ligands

History of Coordination Compounds

Coordination compounds used since ancient times, but chemical nature unclear.

Werner: modern theory of coordination chemistry - based on studies of several series of metal halide complexes with ammonia

History of Coordination Compounds

Werner postulated that metal ions have 2 different kinds of valence:

1. primary valence (oxidation state) =

2. secondary valence (coordination #)

Alfred Werner (1866-1919)

Same chemical composition, same # of groups of same types attached to same metal. What made the two different colors?

More on this later!

Structures of Metal Complexes

Coordination #’s of metal ions in metal complexes

can range from 2 to 9.

Differences in E btwn different arrangements of ligands greatest for complexes w/ low coordination #’s

& decrease as coordination # increases.

Structures of Metal Complexes

Only one or two structures possible for complexes w/ low coordination #’s.

Several different energetically = structures are possible for complexes with

high coordination #’s (n > 6)

Structures of Metal Complexes

Coordination # 2 = linearRare for most metals; common for d10 metal ions,especially: Cu+, Ag+, Au+, and Hg2+

Coordination # 4Two common structures: tetrahedral & square planar

Tetrahedral: all 4-coordinate complexes of • non-transition metals & • d10 ions and first-row transition metals,

Coordination # 4Two common structures: tetrahedral & square planar

Square planar: 4-coordinate complexes of 2nd & 3rd row transition metals with d8 e- configurations, e.g. Rh+ , Pt2+ and Pd2+, also encountered in some Ni2+ & Cu2+ complexes.

Structures of Metal Complexes

Coordination # 6Most common: six ligands at vertices of an octahedron or a distorted octahedron.

We will focus primarily on octahedral

Other Structures of Metal Complexes Possible:

Coordination # 3Encountered with d10 metal ions e.g.Cu+ & Hg2+

trigonal planar structure

Coordination # 5 geometries

… and 2nd & 3rd row transition metals

7, 8 & 9 coordination #’s,

give other geometries:

Metal-ligand interaction is an example of Lewis acid-base interaction.

Lewis acid Lewis base

Lewis bases Must have

Transition metal ions tend to form coordination complexes

which we encountered back in Chapter 22.

e.g. AgCl is more soluble in 0.10 M NH3 than it is in pure water because Ag+ forms a complex with NH3

with a very large formation constant: 

Ag+ + 2NH3 Ag(NH3)2+

The complex ion Ag(NH3)2+

that forms is called diamminesilver(I) (review rules on pp. 1055-1056).

Why does it form?It forms because each NH3 is a Lewis base and forms a coordinate covalent bond with the silver ion, Ag+, in solution

The complex has a linear geometry.

to purify the Ag(NH3)2+ complex ion

& store it in a bottle it would need an anion to neutralize the charge

e.g. diamminesilver(I) chloride, [Ag(NH3)2

+]Cl or

diamminesilver(I) nitrate: [Ag(NH3)2+]NO3.

[Ag(NH3)2+]Cl or [Ag(NH3)2

+]NO3.

In these compounds, silver is ____________NH3 is ______________

and Cl or NO3 is ____________________.

Ligands are attached by ___________ bonds

Counterions are attached by _______ bonds!

Another complex formation reaction is:Co3+ + 6 NH3 Co(NH3)6

3+

Kf = [Co(NH3)63+] = 2.3 x 1033

[Co3+][NH3]6

This complex ion is called:

This complex has an octahedral geometry.

Another example is:Cu2+ + 4 CN Cu(CN)4

2

Kf = [Cu(CN)42] = 1.0 x 1025

[Cu2+][CN]4

This complex ion is called

This complex has a tetrahedral geometry.

When a bidentate ligand binds to a metal,

A polydentate ligand is a chelating agent,

complexes containing polydentate ligands:

Ethylenediaminetetraacetate ion: hexadentate ligand

chelate effect: metal complexes

of polydentate ligands

are more stable than complexes

of chemically similar

monodentate ligands.

Nomenclature (IUPAC) rules for

Naming coordination compounds:

• Cation named before anion (as usual); but, transition metal atom in the complex

is named last

with oxidation state in roman numerals in parentheses

Nomenclature (IUPAC) rules for

Naming coordination compounds:• Cation named before anion (as usual), no D;

• anion ending for transition metal will be “ate”

e.g. Cobalt anion =

[Ni(NH3)6] (NO3)2 cation complex

K3 [Co(Cl)6] anion complex

Anionic complex metal ending:Scandium = ScandateTitanium = TitanateVanadium = VanadateChromium = ChromateManganese = ManganateIron = FerrateCobalt = CobaltateNickel = NickelateCopper = CuprateZinc = Zincate

Special names for some transition metals in an anion complex

Nomenclature (IUPAC) rules for

Naming complexes:

Ligands named 1st (alphabetically)• Greek prefixes for counting di, tri, tetra, penta, hexa, etc.

Nomenclature (IUPAC) rules for

Naming anionic ligands:

• Use suffix “o” if ending in “ide” (e.g. chloride chloro; cyanide cyano

hydroxide hydroxo; oxide oxo)

• Use suffix “ito” if ending in “ite” (e.g. nitrite nitrito)

• Use suffix “ato” if ending in “ate” (e.g. oxalate oxalato; sulfate sulfato

carbonate carbonato

Neutral ligands:Usual name: e.g. ethylenediamineExceptions:

Nitrite, NO2:

Which atom on the ligand donates its lone pair D’s the name

Give the chemical formula for

Hexaaquanickel(ll) diaquatetrabromochromate(lll)

Give the chemical formula for

Give the name for

[Co(NH3)6][CoCl6]

Practice naming some complex compounds:

[Pt(Cl2)(NH3)2]

K2[PtCl4]

Practice naming some complex compounds:

[Pt(NH3)4]Cl2

[Pt(NH3)3Cl]Cl

Na[CoCl4(NH3)2]

Practice writing the complex compound formulas:

hexaaquochromium(III) chloride

diaquodichloroaurate(III) chloride

potassium hexacyanoferrate(II)

potassium hexacyanoferrate (III)

Clicker Qstn: the correct name for the complex

Na2[Ni(CN)4]

A. Disodium tetranickelcyanide

B. Sodium tetracyanidenickel(l)

C. Disodium tetracyanonickelo(lV)

D. Natrium tetranickel(Vl)cyanide

E. Sodium tetracyanonickelate(lll)

Constitutional (Structural) Isomers

1. Ionization isomers

2. Linkage isomers

Geometrical isomers of Complexes

Differ only in arrangement of ligands around metal ion.

Metal complexes that differ only in which ligands areadjacent to one another (cis)

or directly across from one another (trans).

Cis-platin isomer fights cancer, Trans-platin doesn’t

Geometrical isomers are most important for square planar & octahedral

complexes.

Square planar complexes:all vertices of a square are equivalent,

it does not matter which vertex is occupied by ligand B

in a square planar MA3B complex.

Only one geometrical isomer is possible

Only one isomer when there’s one B ligand.With two, there are other possible arrangements.

Symmetrical bidentate ligands also only have one structure

Isomers of Metal Complexes

Octahedral complexes:Only one structure possible for octahedral complexes

(if only one ligand is different from other five): (MA5B)

since all six vertices of an octahedron are equivalent.

Isomers of Metal Complexes

Octahedral complexes:If two ligands in an octahedral complex

are different from other four (MA4B2), two isomers are possible:

two B ligands can be _____________________.

Chelating agents (chelate = ________)

Bidentate (2 teeth):

Bidentate (2 teeth): e.g. ethylenediamine (en)

Octahedral isomer complexes:

Replacing another A ligand by B gives an MA3B3 complex

for which there are two isomers:

Octahedral isomer complexes:

Fac: 3 ligands of each kind occupy opposite triangular faces

of the octahedron

Mer: 3 ligands of each kind lie on the meridian

(cut across flat mid-plane)

(cut across flat mid-plane)

Some coordination complexes with mixed ligands have optical isomers and are said to be chiral.

A complex is chiral if its mirror images are different molecules.

Anything that’s linear is not chiral (achiral), i.e. mirror image is always same as original.

Anything that’s square planar is not chiral (achiral),

i.e. mirror image is always same as original.

Anything that’s tetrahedral is chiral only if all four groups are different.

octahedral is chiral with monodentate groups only if:(a) all six groups are different (ABCDEF) or

(b) two groups are the same and cis (AABCDE) or (c) three groups are the same and fac,

i.e. none trans (AAABCD) or

(d) there are two pairs of identical groups and both are cis (AABBCD)

ORsome possibilities with bidentate ligands,

cis-dichlorobisethylenediaminecobalt(III)

If any pair of identical groups is trans, there is no chirality!

any octahedral molecule with a mirror plane is achiral. (any single pair of identical trans ligands

guarantees a mirror plane)

Crystal Field Theory

Crystal Field Theory

Bonding model explaining many important properties of

transition-metal complexes:

Crystal Field Theory

Central assumption of CFT:

metal-ligand connections are electrostatic interactions btwn

a central metal ion

and a set of negatively chargedligands (or ligand dipoles) arranged around metal ion.

d-Orbital Splittings

five d orbitals are initially degenerate (same energy).

When the 6 (-) charges are distributed uniformly over

surface of a sphere, d orbitals remain degenerate.

d-Orbital Splittings

But! Their energy will be higher

due to

d-Orbital Splittings

If the 6 (-) charges are placed at vertices of an octahedron,

avg energy of d orbitals does not change.

d-Orbital Splittings

But! It does remove their degeneracy

and the 5 d orbitals split into two groups

d-Orbital Splittings

The dx2 – y

2 and dz

2 orbitals (eg orbitals)

point directly at the six (-) charges,

which increase their Energy compared with a spherical distribution of negative charge.

The dxy, dxz, & dyz (t2g orbitals) are all oriented at a 45º angle to the coordinate axes

and point between the 6 (-) charges,

which decreases their Energy compared with a spherical distribution of charge

AsAs LP’s on ligands approach along x, y, and z axes.

d-Orbital Splittings

Difference in E btwn the two sets of d orbitals is

crystal field splitting energy.

d-Orbital Splittings

Magnitude of the splitting depends on:

Splitting of d orbitals in a crystal field does not D total energy

of the five d orbitals

Electronic Structures of Metal Complexes

Using d-orbital energy-level diagram:electronic structures & some properties of transition-metal complexes can be predicted.

Start with Ti3+ ion, (contains a single d electron),

proceed across first row of transition metals by adding a single e- at a time.

Additional e-’s placed in lowest-E orbital available while keeping their spins parallel

For d1-d3 systems, e-’s successively occupy the 3 degenerate t2g orbitals

with their spins parallel (paramagnetic)giving one, two, and three unpaired electrons.

Electronic Structures of Metal Complexes

d4 configuration: two possible choices for 4th e-: enter one of the empty eg orbitals or

enter one of the singly occupied t2g orbitals

D < P D > P

Spin Pairing Energy (P) is an increase in Energy(due to electrostatic repulsions)

when an e- is put into an occupied orbital.

If o is < P, then lowest-energy arrangement has 4th e-

in an empty eg orbital.

Electronic Structures of Metal Complexes

If o is > P,

lowest-energy arrangement has 4th e- in one of the occupied t2g orbitals,

Metal ions with d4, d5, d6, or d7 e- configurations can be either high spin or low spin,

depending on magnitude of o

magnitude of o

Large o =Smaller o =

Only one arrangement of d electrons is possible for metal ions with d8–d10 e- configurations

Factors That Affect the Magnitude of o

magnitude of o dictates whether a complex with 4, 5, 6, or 7 d e-’s

is high spin or low spin:

1. Large values of o yield a low-spin complex

2. Small values of o a high-spin complex

Which affects its:

• Magnetic properties

• Structure

• Reactivity

Factors That Affect the Magnitude of o

Nature of the ligands

For a series of chem similar ligands, magnitude of o decreases

as size of donor atom increases

because smaller, more localized charges interact

Factors That Affect the Magnitude of o

Nature of the ligands

Nature of the ligands

experimentally observed order of the crystal field splitting energies

produced by different ligands is called: the spectrochemical series

Nature of the ligands

1. Strong-field ligands interact strongly with d orbitals of metal ions and give a large o

2. Weak-field ligands interact more weakly and give a smaller o

The Spectrochemical Seriessplitting of d orbitals in crystal field model

not only depends on geometry of the complex

also depends on nature of the metal ion, charge on this ion,

and the ligands that surround the metal.

The Spectrochemical Series

When geometry and ligands are held constant, this splitting decreases in the following order:

Metal ions at one end are called strong-field ions, because splitting due to crystal field is

unusually strong. Ions at other end are known as weak-field ions.

Pt4+ > Ir3+ > Rh3+ > Co3+ > Cr3+ > Fe3+ > Fe2+ > Co2+ > Ni2+ > Mn2+

strong-field ions weak-field ions

CO CN- > NO2- > NH3 > -NCS- > H2O > OH- F- -SCN- Cl- > Br-

strong-field ligands weak-field ligands

The Spectrochemical SeriesWhen geometry & the metal are held constant,

splitting of d orbitals decreases in the following order:      

Strong Field Ligands: (strongest) CN−, CO > NO2

− > en > NH3

Weak Field Ligands: H2O > ox > OH− > F− > SCN−, Cl− > Br− > I− (weakest)

tetrahedral crystal field:imagine 3 ligands lying at alternating corners of a cube

The dx2

-y2 & dz

2 orbitals on metal ion at center of the cube

lie between the ligands, and dxy, dxz, & dyz orbitals point toward the ligands.

Tetrahedral Complexes

Splitting of energies of orbitals in tetrahedral complex, o, is smaller than in an octahedral

complex for two reasons:

1. d orbitals interact less strongly with ligands in a tetrahedral arrangement.

2. Only four negative charges rather than six, which decreases electrostatic interactions

tetrahedral crystal field:

the splitting observed in a tetrahedral crystal field is opposite of splitting in octahedral complex.

With square planar splittings, energy level for the x2-y2 orbital is very high

so this is an especially good geometry for d8 complexes, e.g. Pt(II), Ni(II), Pd(II), Au(III)

Factors That Affect the Magnitude of o

Charge on the metal ionIncreasing charge on a metal ion has 2 effects:

1. Radius of metal ion decreases2. Neg charged ligands are more strongly attracted

to it.

Both factors decrease metal-ligand distance, which causes (-) charged ligands

to interact more strongly with the d orbitals.

magnitude of o increases as charge on metal ion increases

Factors That Affect the Magnitude of o

Principal quantum # of the metal

For a series of complexes of metals from same group in periodic table

with same charge and same ligands:

magnitude of o increases with increasing quantum #:

Factors That Affect the Magnitude of o

Principal quantum # of the metal

o (3d) << o (4d) < o (5d)

Increase in o w/ increasing principal quantum # is due to: larger radius of valence orbitals

going down a column.

Repulsive ligand-ligand interactions are important for smaller metal ions,

which results in shorter M–L distances and stronger d-orbital-ligand interactions

Colors of Transition-Metal Complexes

Striking colors exhibited by transition-metal complexesare caused by the excitation of an e- from

a lower-lying d orbital to a higher-energy d orbital,

which is called a d-d transition

For a photon to affect the d-d transition,

its E must be = to the difference in E btwn the two d orbitals,

which depends on the magnitude of o

which depends on the structure of the

complex.

The energy of a photon of light is inversely proportional to its wavelength

E = hc = hu l

Colors of Transition Metal Complexes

CFT helps explain diff colors observed for complexes

A transition metal complex absorbs specific l of light

Color observed is complimentary to what was absorbed

Observed color is due to transmitted or reflected light that is complementary in color to light that is absorbed

Rubies & Emeralds both contain Cr3+ impurities

in octahedral 6-oxide environment.

Host lattice causes differences in distances of d-orbital-to-ligand lengths.

Applications:Chelating Agents:EDTA used to treat victims of heavy metal poisoning

Chemical Analysis:Dimethylglyoxime turns red in presence of Ni(II) and yellow in the presence of Pd(II).

Thiocyanate blood-red in presence of Fe(III) and blue in the presence of Co(II).

Applications:Coloring Agents:e.g. Iron blue - found in ink, paint, cosmetics (eye shadow) and blueprints. mixture of hexacyano complexes of Fe(II) & Fe(III).

Drug Therapy: Cisplatin is a cancer chemotherapeutic agent - the two chlorine ligands get replaced by donor atoms on the DNA double helix.

Biomolecules: Hemoglobin and cytochrome c contain Fe-heme complexes. Chlorophyll contains a Mg-porphyrin complex.