In the nineteen sixties, Ralph Pearson developed the Type A and and Type B logic by explaining the

differential complexation behaviour of cations and ligands in terms of electron pair donating Lewis bases and electron pair accepting Lewis acids:

Lewis acid   +   Lewis base            Lewis acid/base complexPearson classified Lewis acids and Lewis bases as

hard, borderline or soft. According to Pearson's hard soft [Lewis] acid base (HSAB) principle:

Hard [Lewis] acids prefer to bind to hard [Lewis] basesand

Soft [Lewis] acids prefer to bind to soft [Lewis] basesAt first sight, HSAB analysis seems

rather similar to the Type A and Type B system. However, Pearson classified a very wide range of

atoms, ions,

molecules and molecular ions

as hard, borderline or soft Lewis acids or Lewis bases, moving the analysis from traditional metal/ligand inorganic chemistry

into the realm of organic chemistry.

Hard Acids

Hard Bases

Borderline Acids

Borderline Bases

Soft Acids

Soft Bases

Most metals are classified as Hard acids or acceptors.Exceptions: acceptors metals in red box are always soft .

Solubilities: AgF(S-H) > AgCl > AgBr >AgI (S-S)

But: LiBr (H-S) > LiCl > LiI > LiF (H-H)

Green boxes are soft in low oxidation states, hard in high..

Orange boxes are soft in high oxidation states.

Log K for complex formation

softness

softhard

Chatt’s explanation: soft metals ACIDS have d electrons available for -bonding

Higher oxidation states of elements to the right of transition metals have more soft character.

There are electrons outside the d shell which interfere with pi bonding. In higher oxidation states they are removed.

For transition metals:

Soft BASE molecules or ions that are readily polarizable and have vacant d or π* orbitalsavailable for π back-bonding react best with soft metals

Model: Base donates electron density to metal acceptor. Back donation, from acid to base, may occur from the metal d electrons into vacant orbitals on the base.

low oxidation states and position to the right of periodic table are soft

high oxidation states and position to the left of periodic table are hard

Tendency to complex with hard metal ions

N >> P > As > SbO >> S > Se > Te

F > Cl > Br > I

Tendency to complex with soft metal ions

N << P > As > SbO << S > Se ~ Te

F < Cl < Br < I

The hard-soft distinction is linked to polarizability, the degree to which the electrons in a molecule or ion may be easily distorted by interaction with other

molecules or ions.

Hard acids or bases are small and non-polarizable

Hard acids are cations with high positive charge (3+ or greater),or cations with d electrons not available for π-bonding

Soft acids are cations with a moderate positive charge (2+ or lower),Or cations with d electrons readily availbale for π-bonding

The larger and more massive an ion, the softer (large number of internal electronsshield the outer ones making the atom or ion more polarizable)

For bases, a large number of electrons or a larger size are related to soft character

Soft acids and bases are larger and more polarizable

Examples

•Harder nucleophiles like alkoxide ion, R-O–, attack the acyl (carbonyl) carbon.•Softer nucleophiles like the cyanide ion, NC–, and the thioanion, R-S–, attack the "beta" alkyl carbon

S - S

H - H

Further Development

Pearson and Parr defined the chemical hardness, , as the second derivative for how the energy with respect to the number of electrons.

Expanding with a three point approximation

1

softness

Related to Mulliken electronegativity 2

AI

Energy levels for halogens and relations between , and HOMO-LUMO energies

Chemical Hardness, , in electron voltAcids Bases

Hydrogen H+ infinite Fluoride F- 7

Aluminum Al3+ 45.8 Ammonia NH3 6.8

Lithium Li+ 35.1 hydride H- 6.8

Scandium Sc3+ 24.6 carbon monoxide CO 6.0

Sodium Na+ 21.1 hydroxyl OH- 5.6

Lanthanum La3+ 15.4 cyanide CN- 5.3

Zinc Zn2+ 10.8 phosphane PH3 5.0

Carbon dioxide CO2 10.8 nitrite NO2- 4.5

Sulfur dioxide SO2 5.6 Hydrosulfide SH- 4.1

Iodine I2 3.4 Methane CH3- 4.0

Coordination Chemistry• General aspects (Ch. 9)• Bonding (Ch. 10)• Electronic spectra (Ch. 11)• Reaction mechanisms (Ch. 12)

Acids and bases (the Lewis concept)

A base is an electron-pair donor An acid is an electron-pair acceptor

Lewis acid-base adducts involving metal ionsare called coordination compounds (or complexes)

acid baseadduct

Coordination complexes

L

MLL

L

L

L

+n

[A-]n

Central metal atom

Coordinated ligands

counteranionInner coordination sphere

L

L

ML

L

Solv

Solv

Solv

SolvSolv

Inner coordination sphere

The metal cation is the Lewis acid, the ligands are the Lewis bases

Naming coordination complexes

General nomenclature rules in coordination chemistry

• Cation first, then anion (as for simple salts) (K3[Fe(CN)6], potassium hexacyanoferrate)• Inner coordination sphere in square brackets in formula. Ligands named before the metal

Hexaaminecobalt(III) chloride: [Co(NH3)6]Cl3

• Number of ligand indicated by prefix (di,tri,tetra or bis, tris, tetrakis if ligand in parenthesis)tris(bipyridine)iron(II) chloride: [Fe(bipy)3]Cl2

• Ligands named in alphabetical order ignoring prefix • Anionic ligands are given the suffix -o (chloro-, sulfato-, nitrato-) while neutral ligands retain name (except aqua for H2O and ammine for NH3)• Metal named after ligands with oxidation state in roman numerals or give overall charge of

coordination sphere Ex. Fe(III), tetrachloroplatinate(-2)

• Cis (adjacent)-trans (opposite) or fac (C3v) –mer (C2v) isomers are indicated with prefix• Bridging ligands are indicated with (greek mu) -oxo for M-O-M• If complex is anionic, use ending “-ate”

-cobaltate, ruthenate, but note ferrate for Fe, argentate for Ag, plumbate for Pb, stannate for Sn and aurate for Au

Isomerism

• Stereoisomers (enantiomers, diastereomers, cis/trans, mer/fac, conformational) have same metal ligand bonds but different 3D arrangement.

• Hydrate (solvate) isomers, ionization, linkage, coodination isomers have different metal-ligand bonds.

Examples of Four Coordinate Stereoisomers

Pt

NH3

NH3

Cl Cl Pt

NH3

Cl

Cl NH3

trans cis

stereoisomers

planar

Tetrahedral, chirality now possible.

Four different monodentate ligands.

Chirality in tetrahedral complexes

(2 enantiomers if all ligands different)

Very common

L4

L1

M

L3

L2L4

L1

M

L3

L2

Examples of Six coordinate Stereoisomers

How many stereoisomers are there of formula Mabcdef?

For the six sites in the octahedron there are 6! = 6 * 5 * 4 * 3 * 2 * 1 ways of positioning the ligands.

However some of these ways are the same structure; simply rotated.

An octahedron has many rotations which simply interchange ligands: 8 C3, 6 C2, 6 C4 and 3 C2. Thus there are 23 rotated structures to be generated from an original structure. 6!/(23+1) = 30 stereoisomers.

For some complexes with multidentate ligands there are geometry constraints which reduce the number of isomers.

Examples of Six coordinate Stereoisomers

How many stereoisomers are there of formula Maabbcc?

For the six sites in the octahedron there are 6! = 6 * 5 * 4 * 3 * 2 * 1 ways of positioning the ligands.

However some of these ways are the same structure; simply rotated.

An octahedron has many rotations which simply interchange ligands: 8 C3, 6 C2, 6 C4 and 3 C2. Thus there are 23 rotated structures to be generated from an original structure. 6!/(23+1) = 30 stereoisomers.

For some complexes with multidentate ligands there are geometry constraints which reduce the number of isomers.

Chirality in octahedral complexes

b

c b

c

a

a

b

b c

c

a

aa

c b

c

a

b

non-chiral

a

b c

c

a

b

c

c b

a

a

b

a

b c

b

a

c

a

b b

c

a

c

c

b b

a

a

c

chiral

Maabbcc

Have two trans ligands the same.

Do not have two trans ligands the same.

Multidentate ligands and isomer count.

Let AA be a multidentate ligand which must bond cis.

For octahederal complex MAAbcde how many stereoisomers?

Permutation count is not 6! but

6 * 4 *4!

M

A

Only four spots for the second A to enter.

# stereoisomers = 6 * 4 *4!/(24*2)

Due to rotations

Since both ends of the AA are the

same.For a complex MAABCde

For a complex MAABCde with multidentate ligands A – A and B - C

Number of stereoisomers = 6 * 4 * (2 *2 * 2! + 2*3 *2!)/(24 * 2) = 10 stereoisomers

Assign first A and second A in cis position

M

B

A

A MA B

A

Rotation factor

Due to A-A symmetric ligand

Chirality in octahedral complexes with chelating ligands

NCo

N N

N

Cl

Cl

non-chiral

NCo

N Cl

Cl

N

N

ClCo

Cl N

N

N

N

chiral

Several chelate rings and chirality

M

NN

N

N

N

NM

NN

N

N

N

N

isomer

Left hand screw

isomer

Right hand screw

Conformational Isomers

The chelate rings can have alternative conformations.

Constitutional Isomers• Hydrate Isomers: in crystal structure is water

part of the first ligand shell or a hydrate– [Cr(H2O)6]Cl3, violet– [CrCl(H2O)5]Cl2.H2O, blue-green– [CrCl2(H2O)4]Cl.2H2O, dark green– [CrCl3(H2O)3].3H2O, yellow green

H2OCr

H2O OH2

OH2

OH2

H2O

3+

3Cl-

H2OCr

H2O OH2

OH2

Cl

H2O

2+

2Cl-H2O

CrH2O OH2

OH2

Cl

Cl

+

Cl-

violet green green

Constitutional Isomers• Ionization isomerization: different ions produced

in solution– [Co(NH3)5SO4]NO3 & [Co(NH3)5NO3] SO4

• Coordination Isomers: More than one ratio of ligand can exist but maintaining overall ratio– [Pt(NH3)2Cl2]

– [Pt(NH3)3Cl] [Pt(NH3)Cl3]

• Linkage (ambidentate) isomerism– Thiocyanate, SCN-, can bind through either

the N (to hard acids) or through S (to soft acids).

– Nitrite, NO2-, can bond through either the N or

the O

Typical coordination numbers and structuresof coordination complexes

and isomerism

Coordination number 1

Very rare, bulky ligands, linear structures, no possible isomers

Coordination number 2

Also rare, typical of d10, linear structures, no possible isomers

Coordination number 3

Also typical of d10, trigonal planar structures (rarely T-shaped), no possible isomers

Coordination number 4

L4

L1

M

L3

L2

L2

M

L1 L2

L1

L1

M

L1 L2

L2

cis

transTetrahedral(2 enantiomers if all ligands different)

Square planar(2 geometrical isomer

for two types of ligands)typical of d8

Very common

Tetrahedral

Square planar

Coordination number 5

Trigonal bipyramidal (tbp) Square-based pyramidal sbp)

Very similar energies, they may easily interconvert in solution (fluxionality)

Le M

Le

Le

La

La

Lb

MLb Lb

Lb

La

Coordination number 6

M M

Octahedralmost common

Trigonal prismless common

Some possible isomers in octahedral complexes

B

M

A B

B

A

B

B

M

B B

B

A

A

cis-MA2B4 trans-MA2B4

B

MB A

A

A

B

B

MB B

A

A

A

fac-MA3B3 mer-MA3B3

Some examples of trigonal prismatic structures

Coordination number 7

M M

Pentagonal bipyramidal

Capped octahedral Cappedtrigonal prismatic

M

Examples of coordination number 7