in the nineteen sixties, ralph pearson developed the type a and and type b logic by explaining the...
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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