Download Coordination Number 2

Aspects of Coordination Chemistry (Part 1)
The Electron Configuration of Transition Metal Atoms and Ions
When determining the electron configuration of an element, we begin by using the Aufbau principle.
Add electrons one at a time to the lowest energy available orbital. Through element 18 (argon) it works
in an easily predictable pattern: 1s, 2s, 2p, 3s, 3p. When we reach potassium, something quite
unexpected happens. The next electron goes into the 4s orbital instead of the 3d. Furthermore, when
electrons are removed from transition metals (ie when cations are formed), we find the 4s depopulates
first. It should first be noted that for the 4th period and lower, the ns and (n-1)d orbitals lie very close in
energy to one another. This is particularly true for the first 5 or 6 elements of each period. Consider
constructing a potassium atom from an argon atom. One proton will be added to the nucleus and one
electron to an orbital. To choose the orbital, the question that needs to be asked is “Which orbital will
result in the lowest energy atom?” and there are really only two candidates 3d and 4s. This is
because 3d is in the same subshell as the last filled orbital (3p) and 4s is the lowest energy orbital
from the next principal quantum number. The electrons in an atomic core are more tightly packed than
those of the valence shell. This is because low principal quantum number orbitals are inherently smaller
than those with larger principal quantum numbers and because added protons (as one moves through
the periodic table) pull them even closer together. The result is that as one approaches the nucleus from
a point far from the nucleus, electron-electron repulsions increase considerably as one gets close to
the nucleus. Thus, although a 3d orbital is lower in energy than a vacant 4s orbital in a one electron
atom, when they are filled in a multi-electron atom electron-electron repulsions raise the energy of the
3d more than the 4s and the latter ends up more stable. As more protons and electrons are added to
the atom, the protons pull harder on the electrons closest to the nucleus. Since, on average, electrons in
the 3d orbital are closer than those in the 4s, the energy of the 3d falls more rapidly and eventually
drops below that of the 4s. The crossover point is near vanadium. This is why the electron configuration
of chromium is [Ar] 4s1 3d5. In this configuration, there are 6 unpaired electrons whose spins will
align. The energy released from aligning all of these spins is greater than the energy difference
between the orbitals. We can conclude that the 3d orbital on scandium (for example) is higher in energy
than its 4s orbital, yet the scandium(II) ion has an electron configuration of [Ar] 3d1. We now need to
reconcile this apparent discrepancy. The error commonly made here is tying together the atom and ion.
The question here is actually which orbital is more stable in the ion, not the atom. In all transition metal
ions, the valence (n-1)d orbital is lower in energy than the ns orbital. Removing electrons has two
effects. The first is the same as adding protons, the second is reduced electron-electron repulsions. Both
of these effects favor stabilizing the (n-1)d over the ns.
Naming Coordination Compounds (Nomenclature)
A complex is a substance in which a metal atom or ion is associated with a group of neutral
molecules or anions called ligands. Coordination compounds are neutral substances (i.e.
uncharged) in which at least one ion is present as a complex. The coordination compounds are
named in the following way.
A. To name a coordination compound, no matter whether the complex ion is the cation or the
anion, always name the cation before the anion.
B. In naming the complex ion:
1. Name the ligands first, in alphabetical order, then the metal atom or ion.
Note: The metal atom or ion is written before the ligands in the chemical formula.
2. The names of some common ligands are listed in Table 1.
For anionic ligands end in "-o"; for anions that end in "-ide"(e.g. chloride), "-ate" (e.g. sulfate,
nitrate), and "-ite" (e.g. nirite), change the endings as follows: -ide
-o; -ate
-ato; -ite
ito For neutral ligands, the common name of the molecule is used e.g. H2NCH2CH2NH2
(ethylenediamine). Important exceptions: water is called ‘aqua’, ammonia is called ‘ammine’,
carbon monoxide is called ‘carbonyl’, and the N2 and O2 are called ‘dinitrogen’ and ‘dioxygen’.
Table 1. Names of Some Common Ligands
Anionic Ligands
Names
Neutral Ligands
Names
Br-
bromo
NH3
ammine
F-
fluoro
H2O
aqua
O2-
oxo
NO
Nitrosyl
OH-
hydroxo
CO
Carbonyl
CN-
cyano
O2
dioxygen
C2O42-
oxalato
N2
dinitrogen
CO32-
carbonato
C5H5N
pyridine
CH3COO-
acetato
H2NCH2CH2NH2
ethylenediamine
dppm, dppe,
dppp
bis(phosphines)
3. Greek prefixes are used to designate the number of each type of ligand in the complex ion,
e.g. di-, tri- and tetra-. If the ligand already contains a Greek prefix (e.g. ethylenediamine) or if it
is polydentate ligands (ie. can attach at more than one binding site) the prefixes bis-, tris-,
tetrakis-, pentakis-, are used instead. The numerical prefixes are listed in Table 2.
Table 2. Numerical Prefixes
Number
Prefix
Number
Prefix
Number
Prefix
1
mono
5
penta
(pentakis)
9
nona (ennea)
2
di (bis)
6
hexa (hexakis)
10
deca
3
tri (tris)
7
hepta
11
undeca
4
tetra (tetrakis)
8
octa
12
dodeca
4. After naming the ligands, name the central metal. If the complex ion is a cation, the metal is
named same as the element. For example, Co in a complex cation is called cobalt and Pt is
called platinum. If the complex ion is an anion, the name of the metal ends with the suffix –ate.
(See examples 5 and 6.). For example, Co in a complex anion is called cobaltate and Pt is called
platinate. For some metals, the Latin names are used in the complex anions e.g. Fe is called
ferrate (not ironate).
Table 3: Name of Metals in Anionic Complexes (so-called "ate" complexes)
Name of Metal
Name in an Anionic Complex
Iron
Ferrate
Copper
Cuprate
Lead
Plumbate
Silver
Argenate
Gold
Aurate
Tin
Stannate
5. Following the name of the metal, the oxidation state of the metal in the complex is given as a
Roman numeral in parentheses.
C. To name a neutral complex molecule, follow the rules of naming a complex cation.
Remember: Name the (possibly complex) cation BEFORE the (possibly complex) anion.
For historic reasons, some coordination compounds are called by their common names. For
example, Fe(CN)63- and Fe(CN)64- are named ferricyanide and ferrocyanide respectively, and
Fe(CO)5 is called iron carbonyl.
Examples Give the systematic names for the following coordination compounds:
1. [Cr(NH3)3(H2O)3]Cl3
Answer: triamminetriaquachromium(III) chloride
Solution: The complex ion is inside the parentheses, which is a cation. The ammine ligands are
named before the aqua ligands according to alphabetical order. Since there are three
chlorides binding with the complex ion, the charge on the complex ion must be +3 ( since the
compound is electrically neutral).
From the charge on the complex ion and the charge on the ligands, we can calculate the
oxidation number of the metal. In this example, all the ligands are neutral molecules.
Therefore, the oxidation number of chromium must be same as the charge of the complex ion,
+3.
2. [Pt(NH3)5Cl]Br3
3. [Pt(H2NCH2CH2NH2)2Cl2]Cl2
Answer: dichlorobis(ethylenediamine)platinum(IV) chloride
Solution: ethylenediamine is a bidentate ligand, the bis- prefix is used instead of di-
4. [Co(H2NCH2CH2NH2)3]2(SO4)3
Answer: tris(ethylenediamine)cobalt(III) sulfate
Solution: The sulfate is the counter anion in this molecule. Since it takes 3 sulfates to bond
with two complex cations, the charge on each complex cation must be +3. Since
ethylenediamine is a neutral molecule, the oxidation number of cobalt in the complex ion
must be +3.
Again, remember that you never have to indicate the number of cations and anions in the
name of an ionic compound.
5. K4[Fe(CN)6]
Answer: potassium hexacyanoferrate(II)
Solution: potassium is the cation and the complex ion is the anion. Since there are 4 K+ binding
with a complex ion, the charge on the complex ion must be - 4. Since each ligand carries –1
charge, the oxidation number of Fe must be +2. The common name of this compound is
potassium ferrocyanide.
6. Na2[NiCl4]
Answer: sodium tetrachloronickelate(II)
Solution: The complex ion is the anion so we have to add the suffix –ate in the name of the
metal.
7. Pt(NH3)2Cl4
8. Fe(CO)5
Answer: pentacarbonyliron(0)
Solution: Since it is a neutral complex, it is named in the same way as a complex cation. The
common name of this compound, iron carbonyl, is used more often.
9. (NH4)2[Ni(C2O4)2(H2O)2]
Answer: ammonium diaquabis(oxalato)nickelate(II)
Solution: The oxalate ion is a bidentate ligand.
10. [Ag(NH3)2][Ag(CN)2]
Answer: diamminesilver(I) dicyanoargentate(I)
You can have a compound where both the cation and the anion are complex ions. Notice how
the name of the metal differs even though they are the same metal ions.
Can you give the molecular formulas of the following coordination compounds?
1. hexaammineiron(III) nitrate
2. ammonium tetrachlorocuprate(II)
3. sodium monochloropentacyanoferrate(III)
4. potassium hexafluorocobaltate(III)
Can you give the name of the following coordination compounds?
5. [CoBr(NH3)5]SO4
6. [Fe(NH3)6][Cr(CN)6]
7. [Co(SO4)(NH3)5]+
8. [Fe(OH)(H2O)5]2+
Answers:
Coordination numbers
The total number of points of attachment to the central element is termed the coordination
number and this can vary from 2 to as many as 16, but is usually 6.
Coordination Number 1
Transition metal complexes with the coordination number 1 are rare. In the case of main group
elements this coordination number is more common, e.g. in HF, IF. It can be also observed in
the gas phase. In the following reference (this journal is in the library) a compound had been
reported which is one-coordinate. How did the researchers achieve this? Reference: R. Lingnau
and J. Strähle, Angew. Chem. Int. Ed. Engl., 27, 1988, page 436.
Coordination Number 2
This arrangement is not very common for first row transition metal ion complexes and some of
the best known examples are for silver(I) and gold(I).
For instance, a method often employed for the detection of chloride ions involves the
formation of the linear diamminesilver(I) complex.
The first step is:
Ag+ + Cl-
→ AgCl (white ppt)
and to ensure that the precipitate is really the chloride salt, two further tests must be done:
AgCl
+ 2 NH3 (as NH4OH) → [Ag(NH3)2]+
and
[Ag(NH3)2]+ + HNO3 → AgCl (re-ppts)
The reaction of a bidentate ligand such as 1,2-diaminoethane with Ag+ does not lead to
chelated ring systems, but instead to linear two coordinate complexes. One reason for this is
that bidentate ligands can NOT exist in trans arrangements that is can NOT span 180 degrees.
Coordination Number 3
Once again, this is not very common for first row transition metal ions. Examples with three
different geometries have been identified:
Trigonal planar
Well known for main group species like CO32- etc, this geometry has the four atoms in a plane
with the bond angles between the ligands at 120 degrees.
Trigonal pyramid
More common with main group ions.
T-shaped
The first example of a T-shaped molecule was found in 1977.
Coordination Number 4
Two different geometries are possible. The tetrahedron is the more common while the square
planar is found almost exclusively with metal ions having a d8 electronic configuration.
Tetrahedral
The chemistry of molecules centred around a tetrahedral C atom is covered in organic courses.
Change all occurrences of C to Co. There are large numbers of tetrahedral Cobalt(II) complexes
known.
Square Planar
This is fairly rare and is included only because some extremely important molecules exist with
this shape, such as virtually all Pd(II) and Pt(II) and Au(III) complexes – all are d8 systems.
Coordination Number 5
Square pyramid
Trigonal Bipyramid
The structure of [Cr(en)3][Ni(CN)5] 1.5 H2O was reported in 1968 to be a remarkable example
of a complex exhibiting both types of geometry in the same crystal.
The reaction of cyanide ion with Ni2+ proceeds via several steps:
Ni2+
+ 2 CN- → Ni(CN)2
+ 2 CN-
→ [Ni(CN)4]2orange-red
[Ni(CN)4]2- + CN-
→ [Ni(CN)5]3deep red
Ni(CN)2
Oxovanadium salts (vanadyl, VO2+) often show square pyramidal geometry, for example,
VO(acac)2. Note that the vanadium(IV) can be considered coordinatively unsaturated and
addition of pyridine leads to the formation of an octahedral complex.
Coordination Number 6
Hexagonal planar
Unknown for first row transition metal ions, although the arrangement of six groups in a plane
is found in some higher coordination number geometries.
Trigonal prism
Most trigonal prismatic compounds have three bidentate ligands such as dithiolates or oxalates
and few are known for first row transition metal ions.
Octahedral
The most common geometry found for first row transition metal ions, including all aqua ions.
In some cases distortions are observed and these can sometimes be explained in terms of the
Jahn-Teller Theorem.
Coordination Number 7
Capped octahedron
Capped trigonal prism
Pentagonal Bipyramid
Coordination Number 8
Dodecahedron
Cube
Square antiprism