Coordination Chemistry I:
Structures and Isomers
Chapter 9
Coordination Compounds
• Coordination compounds –
compounds composed of a
metal atom or ion and one
or more ligands.
– [Co(Co(NH3)4(OH2)3]Br6
– Ligands usually donate
electrons to the metal
– Includes organometallic
compounds
Werner’s totally inorganic
optically active compound.
Werner’s Coordination Chemistry
• Performed systematic studies to understand bonding
in coordination compounds.
– Organic bonding theory and simple ideas of ionic charges
were not sufficient.
• Two types of bonding
– Primary – positive charge of the metal ion is balanced by
negative ions in the compound.
– Secondary – molecules or ion (ligands) are attached directly
to the metal ion.
• Coordination sphere or complex ion.
• Look at complex on previous slide (primary and secondary)
Werner’s Coordination Chemistry
• He largely studied compounds with four or six
ligands.
– Octahedral and square-planar complexes.
• It was illustrated that a theory needed to account
for bonds between ligands and the metal.
– The number of bonds was commonly more than
accepted at that time.
• 18-electron rule.
• New theories arose to describe bonding.
– Valence bond, crystal field, and ligand field.
Chelating Ligands
• Chelating ligands
trisoxalatochromate(III) ion or just [Cr(ox)3]3(chelates) – ligands that
have two or more points
of attachment to the
metal atom or ion.
– Bidentate, tridentate,
tetra.., penta…, hexa…
(EDTA).
A Hexadentate Ligand, EDTA
• There are six points of
attachment to the calcium
metal.
– Octahedral-type geometry
ethylene diamine tetraacetic acid
(EDTA)
ethylenediaminetetraacetatocalcium ion or just [Ca(EDTA)]2-
Nomenclature
• The positive ion (cation) comes first, followed by the
name within the coordination sphere, followed by the
negative ion (anion).
– These ions are not in the coordination sphere.
– Diamminesilver(I)chloride and potassium hexacyanoferrate
(III).
• The inner coordination sphere is enclosed in brackets in
the formula. Within this sphere, the ligands are named
before the metal, but in formulas the metal ion is
written first.
– Tetraamminecopper(II) sulfate and hexaamminecobalt(III)
chloride.
Nomenclature
• The number of ligands is
given by the following
prefixes. If the ligand
name includes prefixes
or is complicated, it is
set off in parentheses
and the second set of
prefixes is used.
– [Co(en)2Cl2]+ and
[Fe(C5H4N-C5H4N)3]2+
2
3
4
5
6
7
8
di
tri
tetra
penta
hexa
hepta
octa
bis
tris
tetrakis
pentakis
hexakis
heptakis
octakis
Nomenclature
• Ligands are named in alphabetical order
(name of ligand, not prefix)
– [Co(NH3)4Cl2]+ and [Pt(NH3)BrCl(CH3NH2)]+2
• Anionic ligands are given an ‘o’ suffix.
Neutral ligands retain the usual name.
– Coordianted water is called ‘aqua’.
– Chloro, Cl– Sulfato, SO42-
Nomenclature
• The calculated oxidation number of the metal ion is
placed as a Roman numeral in parentheses after the
name of the coordination sphere.
– [Pt(NH3)4]+2 and [Pt(Cl)4]-2
– A suffix ‘ate’ is added to the metal ion if the charge is
negative.
• The prefixes cis- and trans- designate adjacent and
opposite geometric location, respectively.
– trans-diamminedichloroplatinum(III) and cistetraamminedichlorocobalt(III)
Nomenclature
• Bridging ligands between two metal ions
have the prefix ‘’.
– -amido--hydroxobis(tetraamminecobalt)(IV)
There is an error in this picture. What is it?
Isomerism
• Our discussion of isomers will be largely
limited to those with the same ligands arranged
in different geometries. This is referred to as
stereoisomers.
Isomerism
• Four-coordinate complexes
– Square-planar complexes may have
cis and trans isomers. No chiral
isomers (enantiomers) are possible
when the molecule has a mirror
plane.
– cis- and transdiamminedichloroplatinum(II)
– How about tetrahedral complexes?
– Chelate rings commonly impose a
‘cis’ structure. Why
Chirality
• Mirror images are nonsuperimposable.
• A molecule can be chiral if it has no rotation-reflection
axes (Sn)
• Chiral molecules have no symmetry elements or only
have an axes of proper rotation (Cn).
– CBrClFI, Tetrahedral molecule (different ligands)
– Octahedral molecules with bidentate or higher chelating
ligands
– Octahedral species with [Ma2b2c2], [Mabc2d2], [Mabcd3],
[Mabcde2], or [Mabcdef]
Six-Coordinate Octahedral
Complexes
• ML3L3’
– Fac isomers have three
identical ligands on the
same face.
– Mer isomers have three
identical ligands in a plane
bisecting the molecule.
Six-Coordinate Octahedral
Complexes
• The maximum number of isomers can be
difficult to calculate (repeats).
• Placing a pair of ligands in the notation <ab>
indicates that a and b are trans to each other.
– [M<ab><cd><ef>], [Pt<pyNH3><NO2Cl><BrI>]
• How many diastereoisomers in the above
platinum compound (not mirror images)?
• Identify all isomers belonging to Ma3bcd.
Determining the Number of
Isomers
Determining the Number of
Isotopes
• Bailar method
• With restrictions (such as chelating agents)
some isomers may be eliminated.
• Determine and identify the number if
isomers.
– [Ma2b2cd] and [M(AA)bcde]
Combinations of Chelate Rings
• Propellers and helices
– Left- and right-handed propellers
• Examine the movement of a propeller required to
move it in a certain direction.
– For a left-handed propeller, rotating it ccw would cause
it to move away ().
– For a right-handed propeller, rotating it cw would cause
it to move away ().
This is called ‘handedness’. Many molecules possess it.
Tris(ethylenediamine)cobalt(III)
• This molecule can be treated like a threebladed propeller.
• Look down a three fold axis to determine
the ‘handedness’ of this complex ion.
– The direction of rotation required to pull the
molecule away from you determines the
handedness ( or ).
• Do this with you molecule set and rubber
bands.
Determining Handedness for
Chiral Molecules
• Complexes with two or more nonadjacent chelate
rings may have chiral character.
– Any two noncoplanar and nonadjacent chelate rings can
be used.
– Look at Figure 9-14 (Miessler and Tarr).
• Molecules with more than one pair of rings may
require more than one label.
– Ca(EDTA)2+
• Three labels would be required.
• Remember that the chelate rings must be noncoplanar,
nonadjacent, and not connected at the same atom.
Linkage (ambidentate) Isomerism
• A few ligands may bond to the metal through
different atoms.
– SCN- and NO2-
• How would you expect hard acids to bond to the
thiocyanate ligand?
• Solvents can also influence bonding.
– High and low dielectric constants.
• Steric effects of linkage isomerism
• Intramolecular conversion between linkages.
– [Co(NH3)5NO2]+2, Figure 9-19.
Separation and Identification of
Isomers
• Geometric isomers can be separated by fractional
crystallization with different counterions.
– Due to the slightly different shapes of the isomers.
– The ‘fit’ of the counterion can greatly influence
solubility.
• Solubility is the lowest when the positive and negative
charges have the same size and magnitude of charges
(Basolo).
Separation and Identification of
Chiral Isomers
• Separations are performed with chiral
counterions. The resulting physical properties
will differ allowing separation.
• Rotation of polarized light will be opposite for
two chiral isomers at a specific wavelength.
– The direction of optical rotation can change with
wavelength.
Circular Dichroism Meaurement
• The difference in the absorption of right and left
circularly polarized light is measured.
Circular dichroism   l   r
– Where l and r are the molar absorption
coefficients for left and right circularly polarized
light.
• The light received by the detector is presented
as the difference between the absorbances.
Figure 9-20.
Plane-Polarized Light Measurement
• The plane of polarization is rotated when passing
through a chiral substance.
– Caused by a difference in the refractive indices of the
right and left circularly polarized light.

l   r

– The optical rotation illustrates positive value on one
side of the adsorption maximum and negative side on
the other. This is termed as the Cotton effect.
Coordination Numbers and
Structures
• Factors considered when determining structures.
– The number of bonds. Bond formation is
exothermic; the more the better.
– VSEPR arguments
– Occupancy of d orbitals.
– Steric interference by large ligands.
– Crystal packing effect.
It may be difficult to predict shapes.
Low Coordination Numbers (C.N.)
• C.N. 1 is rare except in ion pairs in the gas phase.
• C.N. 2 is also rare.
– [Ag(NH3)2]+, Ag is d10 (how?)
– VSEPR predicts a linear structure.
– Large ligands help force a linear or near-linear arrangment.
• [Mn(N[SiMePh2]2)2] in Figure 9-22.
• C.N. 3 is more likely with d10 ions.
– Trigonal-planar structure is the most common.
– [Cu(SPPh3)3]+, adopts a low C.N. due to ligand crowding.
Coordination Number 4
• Tetrahedral and square planar complexes are the
most common.
– Small ions and/or large ligands prevent high
coordination numbers (Mn(VII) or Cr(VI)).
• Many d0 or d10 complexes have tetrahedral
structures (only consider bonds).
– MnO4- and [Ni(CO)4]
– Jahn-Teller distortion (Chapter 10)
Coordination Number 4
• Square-planar geometry
– d8 ions (Ni(II), Pd(II), and Pt(III))
• [Pt(NH3)2Cl2]
– The energy difference between square-planar
and tetrahedral structures can be quite small.
• Can depend on both the ligand and counterion.
• More in chapter 10.
Coordination Number 5
• Common structures are trigonal bipyramid and
square pyramid.
– The energy difference between the two is small. In
many measurements, the five ligands appear identical
due to fluxional behavior.
– How would you modify the experiment to differentiate
between the two structures?
• Five-coordinate compounds are known for the full
range of transition metals.
– Figure 9-27.
Coordination Number 6
• This is the most common C.N. with the
most common structure being octahedral.
– If the d electrons are ignored, this is the
predicted shape.
• [Co(en)3]3+
• This C.N. exists for all transition metals (d0
to d10).
Distortions of Complexes
Containing C.N. 6
• Elongation and compression (Fig. 9-29).
– Produces a trigonal antiprism structure when the angle
between the top and bottom triangular faces is 60.
– Trigonal prism structures are produced when the faces
are eclipsed.
• Most trigonal prismatic complexes have three bidentate ligands
(Figure 9-30).
•  interactions may stabilize some of these structures.
The Jahn-Teller effect (Ch. 10) is useful in predicting
observed distortions.
Higher Coordination Numbers
• C.N. 7 is not common
• C.N. 8
– There are many 8-coordinate complexes for
large transition elements.
• Square antiprism and dodecahedron
• C.N.’s up to 16 have been observed.