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Notes
Coordination Compounds MODULE - 6
Chemistry of Elements
COORDINATION COMPOUNDS
You have come across compounds like Na[Ag(CN) 2 ] and Na 2 [Zn(CN) 4 ]. Such compounds are referred to as coordination compounds or complex compounds. Coordination compounds play an important role in the chemical industry and in life itself. For example, the Ziegler-Natta catalyst which is used for polymerization of ethylene, is a complex containing the metals aluminum and titanium. Metal complexes play important role in biological systems. For example, chlorophyll, which is vital for photosynthesis in plants, is a magnesium complex and heamoglobin, which carries oxygen to animal cells, is an iron complex. These are the compounds that contain a central atom or ion, usually a metal, surrounded by a number of ions or molecules. The complexes tend to retain their identity even in solution, although partial dissociation may occur. Complex ion may be cationic, anionic or nonionic, depending on the sum of the charges of the central atom and the surrounding ions and molecules.
In this lesson you will study about the complexes including their nomenclature and nature of bonding in them.
OBJECTIVES
After reading this lesson, the learner will be able to,
z state the postulates of Werner’s theory;
z define ligands, coordination number and coordination sphere;
z name simple complexes by IUPAC system;
z explain valance bond theory;
z apply VB theory to explain hybridization, shape and magnetic behavior of the following complexes [Fe(CN) 6 ]4–, [Fe(CN) 6 ]3–, [Cr(NH 3 ) 6 ]2+, [NiCl 4 ]2–, [Ni(CO)4] and [Ni(CN) 4 ]2–^ and
Notes
MODULE - 6 Coordination Compounds Chemistry of Elements z explain Crystal Field Theory (CFT); z explain the colour and magnetic behaviour of coordination compounds on the basis of CFT. z explain the isomerism in coordination compounds; z explain the applications of coordination compounds in extraction of metals, medicine and qualitative analysis.
22.1 WERNERS’ COORDINATION THEORY
Coordination compounds were known in eighteenth century. It was a mystery for the chemist, of those days to understand as to why a stable salt like CoCl 3 reacts with varying number of stable molecules or compounds such as ammonia to give several new compounds: CoCl 3 .6NH 3 , CoCl 3 .5NH 3 and CoCl 3 .4NH 3 ; and what are their structures? These compounds differed from each other in their chloride ion reactivity. Conductivity measurements on solutions of these compounds showed that the number of ions present in solution for each compound are different. Several theories were proposed, but none could satisfactorily explain all the observable properties of these compounds and similar other series of compounds which had been prepared by then. It was only in 1893 that Werner put forward a set of ideas which are known as Werner’s coordination theory, to explain the nature of bonding in complexes. His theory has been a guiding principle in inorganic chemistry and in the concept of valence. The important postulates of Werner’s theory are:
- Metals exhibit two types of valence: (a) Primary valence (ionizable) (b) Secondary valence (non-ionizable). Primary or ionizable valence is satisfied by negative ions and corresponds to oxidation state of the metal. The secondary or non-ionizable valence, which is satisfied by negative, positive or neutral groups, is equal to the coordination number of metal ion.
Every metal tends to satisfy both its primary and secondary valence.
- The secondary valence is directed toward fixed positions in space i.e. this has spatial arrangement corresponding to different coordination number.
For the complexes CoCl 3 .6NH 3 CoCl 3 .5NH 3 and CoCl 3 .4NH3, the number of ionizable ions in these complexes are three, two and one, respectively. It has been proved by precipitation reactions and conductivity measurements. On the basis of Werner’s postulate these compounds are formulated as:
Notes
MODULE - 6 Coordination Compounds Chemistry of Elements of as Lewis acid-base reaction. As you know a Lewis base is a substance capable of donating one or more electron pairs, every ligand has at least one unshared pair of valence electron. Few examples are shown below:
: (^) O: N
:
H H H H H
[ Cl ]:^ : –
: :
The atom in the ligand that is bound directly to the metal atom is known as the donor atom. For example, nitrogen is the donor atom and Cu 2+^ is the acceptor atom in the [Cu(NH 3 ) 4 ]2+^ complex ion. Depending on the number of the donor atoms present, ligands are defined as monodentate, bidentate or polydentate. H 2 O and NH 3 are monodentate ligands with only one donor atom in each. Ethylenediamine (en) is a bidentate ligand.
H N 2
:^ NH (^2)
:
CH 2 CH (^2)
Ethylenediamine
The two nitrogen atoms can coordinate with a metal atom. Bidentate and polydentate ligands are also called chelating agents because of their ability to hold the metal atom like a claw (from the Greek Chele, meaning “claw”) one example is ethylenediaminetetraacetate ion (EDTA), a polydentate (hexadentate) ligand.
O
O
N CH^2 CH^2 N
CH (^2)
CH (^2) CO –
CO –
O
O
CH (^2)
Ethylenediaminetetraccetate ion
Coordination number: The coordination number in coordination compounds is defined as the number of ligand (donor) atoms/ions surrounding the central metal atom in a complex ion. For example, the coordination number of cobalt in [Co(NH 3 ) 6 ]3+^ is six. Similarly the coordination number of Ag +^ in [Ag(NH 3 ) 2 ]+^ is 2, that of Cu 2+^ in [Cu(NH 3 ) 4 ]2+^ is 4, and that of Fe 3+^ in [Fe(CN) 6 ]3–^ is 6.
Notes
Coordination Compounds MODULE - 6 Chemistry of Elements Coordination sphere: The central metal atom and the ligands which are directly attached to it are enclosed in a square bracket and are collectively termed as coordination sphere. The ligands and the metal atom inside the square brackets behave as single constituent unit.
3 4 2 Coordination sphere
[Cr(NH ) Cl ] (^) Cl
Oxidation number: Another important property of coordination compounds is the oxidation number of the central metal atom. The net charge on a complex ion is the sum of the charges on the central atom and its surrounding ligands. In the [PtCl6]2-^ ion for example, each chloride ion has an oxidation number of –1, so the oxidation number of Pt must be +4. If the ligands do not bear net charges the oxidation number of the metal is equal to the charge of the complex ion. Thus in [Cu(NH 3 ) 4 ]2+^ each NH 3 is neutral, so the oxidation number of copper is +2.
INTEXT QUESTIONS 22.
- What is the coordination number of the metal ion in the following?
(i) [Co(NH 3 ) 5 Cl]+ (ii) [Cr(en) 2 Cl2]+ (iii) [NiCl 4 ]2-
- What is the oxidation state of the metal ion in the following?
(i) [MnCl 6 ]4- (ii) [Fe(CN) 6 ]3- (iii) [Cr(NH3) 6 ] 3+ (iv) [Ni(en) 3 ]2+
- Give an example of a chelate ligand.
- Give one example of each monodentate, bidentate and polydenatate ligand.
- What is the oxidation and coordination number of Co in this [Co(en) 2 (H 2 O)CN]2+^ complex ion Which ligand is bidentate in the above complex?
22.3 RULES OF NOMENCLATURE OF COORDINATION
COMPOUNDS
We have already discussed about the ligands and oxidation number of metal, our next step is, to learn how to name these coordination compounds. The rules for naming coordination compounds as recommended by IUPAC are as follows:
Notes
Coordination Compounds MODULE - 6 Chemistry of Elements
- The oxidation number of the metal is written in roman numerals following the name of the metal. For example, the roman numeral III is used to indicate the +3 oxidation state of chromium in [Cr(NH 3 ) 4 Cl2]+^ , which is named as tetraamminedichlorochromium (III) ion.
- If the complex is an anion, its name ends in –ate. For example, in K 4 [Fe(CN) 6 ] the anion [Fe(CN) 6 ]4–^ is called hexacyanoferrate(II) ion. Note that the numeral (II) indicate the oxidation state of iron. Table given below gives tha name of anions containing metal atoms (Table 22.2).
- If the complex is either a cation or is neutral, no change is required in the name of the central metal ion. For example [Co(NH 3 ) 6 ]3+^ and [Ni(CO) 4 ] are named as hexaamminecobalt(III)ion and tetracarbonyl nickel(0), respectively.
Table 22.2: Some anions containing metal atoms Metal Name of metal in anionic state Copper Cuperate Zinc Zincate Aluminum Aluminate Chromium Chromate Tin Stannate Cobalt Cobaltate Nickel Nickelate Gold Aurate Silver Argentate Lead Plumbate Rhodium Rhodate Iron Ferrate Manganese Manganate
A. few examples are given below:
[Co(H 2 O)6]Cl 3 hexaaquacobalt(III) chloride
K 2 [PtCl 6 ] potassium hexachloroplatinate(IV)
[Pt(NH 3 ) 2 Cl 4 ] diamminetetrachloroplatinum(IV)
[Co(en) 2 Cl2]Cl dichlorobis (ethylenediamine)cobalt(III) chloride.
Notes
MODULE - 6 Coordination Compounds Chemistry of Elements
INTEXT QUESTIONS 22.
- Write down the name of the following complexes: (a) [Co(NH 3 ) 4 Cl 2 ] + (b) (NH 4 ) 3 [Cr(NCS) 6 ] (c) Ni(CO) 4 (d) K 4 [Fe(CN) 6 ] (e) [Cr(en) 3 ]Cl
- Write down the formula of the following: (a) Tetrachloronickelate(II) (b) Pentaamminenitrocobalt(III) ion (c) Potassium hexacyanoferrate(III) (d) Dichlorobis(ethylenediammine) chromium(III) ion
22.4 VALENCE BOND THEORY
Linus Pauling of the California Institute of Technology developed the valance bond theory. He was awarded the Nobel prize in chemistry in 1954. Pauling’s ideas have had an important impact on all areas of chemistry. He applied valence bond theory to coordination compounds. This theory can account reasonably well for the structure and magnetic properties of metal complexes. The basic principles, which are involved in the valence bond treatment of coordination compounds are: (a) Hybridization of valance orbitals of the central metal/ ion (b) Bonding between ligand and the metal ion/atom. (c) Relation between the type of bond and the observed magnetic behaviour. Six Coordinate Complexes
Let us explain by taking simple examples such as [CoF 6 ]3-^ and [Co(NH^3 )^6 ]3+. Although in both the complexes, the oxidation state of cobalt is +3, but [CoF 6 ]3- is paramagnetic and [Co(NH 3 ) 6 ]3+^ is diamagnetic, why? The formation of a complex may be considered as a series of hypothetical steps. First the appropriate metal ion is taken e.g. Co 3+. Cobalt atom has the outer electronic configuration 3d 7 4s 2. Thus Co 3+^ ion will have the configuration 3d 6 and the electrons will be arranged as: 3 d (^) 4 s^4 p
Notes
MODULE - 6 Coordination Compounds Chemistry of Elements About such complexes you will study later. Let us illustrate six coordinate complexes with more examples:
- Cr(NH 3 ) 6 ] 3+ The electronic configuration of only 3d, 4s and 4p orbitals are taken into account. The following steps are involved. The electronic configuration of Cr atom and Cr3+^ ion are given in (i) and (ii) below:
(i) Cr ground state:
3 d 4 s^4 p
(ii) Cr3+ 3 d (^) 4 s^4 p
(iii) [Cr(NH 3 ) 6 ]3+
3 d 4 s^4 p ** **^ **^ **^ **^ ** NH 3 NH 3 NH 3 NH 3 NH 3 NH 3 d^2 sp^3 (inner orbital)
The 12 electrons for bond formation come from six ligands, each donating a lone pair of electrons. The resulting complex will be paramagnetic because it has three unpaired electrons. Its magnetic moment will be:
n n ( + 2) = 3( 3 + 2) = 15 =3.87B.M
- [Fe(CN) 6 ]4- (i) Fe 3 d (^) 4 s^4 p
(ii) Fe2+ 3 d 4 s^4 p
Notes
Coordination Compounds MODULE - 6 Chemistry of Elements (iii) [Fe(CN) 6 ] 4– 3 d (^) 4 s^4 p ** ** ** ** (^) ** ** CN–^ CN –^ CN^ – CN^ –^ CN^ – CN^ –
d^2 sp^3 The resulting complex is inner orbital, octahedral and due to the absence of unpaired electron, it will be diamagnetic.
- [Fe(CN) 6 ]3-
(i) Fe 3 d (^) 4 s^4 p
(ii) Fe3+
3 d 4 s^4 p
(iii) [Fe(CN) 6 ] 3–
3 d 4 s^4 p ** **^ **^ ** ** ** CN–^ CN^ –^ CN^ –^ CN^ –^ CN^ – CN^ – d^2 sp^3 The resulting complex is inner orbital, octahedral. Due to presence of one unpaired electron, it will be paramagnetic.
Four coordinate complexes:
- [NiCl 4 ]2-
(i) Ni
3 d (^) 4 s^4 p
(ii) Ni2+
3 d 4 s^4 p
Notes
Coordination Compounds MODULE - 6 Chemistry of Elements (iii) [Ni(CN) 4 ]2-
3 d (^) 4 s^4 p ** ** (^) ** CN –
** CN –^ CN–CN–
dsp^2
The resulting complex is square planar and diamagnetic.
22.5 CRYSTAL FIELD THEORY
Although valence bond theory explains the bonding and magnetic properties of complexes, it is limited in two important ways. First, the theory cannot easily explain the color of complexes. Second, the theory is difficult to extend quantitatively. Consequently, another theory—crystal field theory—has emerged as the prevailing view of transition-metal complexes. This theory has been given by Bethe and van Vlack.
Crystal field theory is a model of the electronic structure of transition-metal complexes that considers how the energies of the d orbitals of a metal ion are affected by the electric field of the ligands. According to this theory, the ligands in a transition-metal complex are treated as point charges. So a ligand anion becomes simply a point of negative charge. A neutral ligand, with its electron pair that it donates to the metal atom, is replaced by a partial negative charge, representing the negative end of the molecular dipole. In an electric field of these negative charges, the five d orbtails of the metal atom no longer have exactly the same energy. The result, as you will see, explains both the paramagnetism and the color observed in certain complexes.
Effect of an Octahedral Field on the d Orbitals
All five d orbtails of an isolated metal atom have the same energy. But if the atom is brought into the electric field of several point charges, these d orbitals may be affected in different ways and therefore may have different energies. To understand how this can happen, you must first see what these d orbitals look like. You will then be able to picture what happens to them in the crystal field theory of an octahedral complex.
Figure 22.1 shows the shapes of the five d orbitals. The orbital labeled dz^2 has a dumbbell shape along the z -axis, with a collar in the x – y plane surrounding this dumbbell. Remember that this shape represents the volume most likely to be occupied by an electron in this orbital. The other four d orbitals have “cloverleaf” shapes, each differing from one another only in the orientation of
Notes
MODULE - 6 Coordination Compounds Chemistry of Elements the lobes in space. The “cloverleaf” orbital dx^2 – y^2 has its lobes along the x - axis and the y -axis. Orbitals dxy , dxz , and dyz have their lobes directed between the two sets of axes desinated in the orbital label. Orbital dxy , for example, has its lobes lying between the x - and y -axes.
y (^) y y
z z^ z
x (^) x x
L 2 L 3 L 3
L 5 L 5 L 5
L 2 L 2 L 2
L 6 L 4 L 1 L 4 L^1 L 4
L (^6) L 6 L 6
3 dxy 3 dyz 3 dxz
L 5 L^5 z z L 2 L 2 x x
L 1 L 1
L 3 L^3
L (^6) L 6
3 dx (^2) – y 2 3 dz^2
y
L 4
Fig. 22.1: Shapes of d orbitals
A complex ion with six ligands will have the ligands arranged octahedrally about the metal atom to reduce mutual repulsion. Imagine that the anionic ligands are replaced by point negative charges and the neutral legands are replaced by the partial negative charge from the molecuar dipoles. The six charges are placed at equal distances from the metal atom, one charge on each of the positive and negative sides of the x -, y -, and z -axes.
Fundamentally, the bonding in this model of a complex is due to the attraction of the positive metal ion for the negative charges of the ligands. However, an electron in a d -orbital of the metal atom is repelled by the negative charge of the ligands.
When ligands approach along the x , y , and z axes, electrons in the 3 d orbital will be repelled but as can be seen from above diagrams the effect will be greater for the 3 dz^2 and 3 dx^2 – y^2 orbitals since these two orbitals have lobes lying along the line of approaching ligands. The net result is that the energy of the 3 dz^2 and 3 dx^2 – y^2 orbitals is raised relative to the energy of the 3 dxy , 3 dxz , and 3 dyz orbitals i.e. the degeneracy of the 3d orbitals is now destroyed.
Notes
MODULE - 6 Coordination Compounds Chemistry of Elements Similar considerations apply to complexes in which the central transition metal ion has more than one 3d electrons although needless to say the presence of more than one electron in the 3d orbitals leads to slight complications. It is found experimentally that for a given transition series (in the case of first transition series) the value of Δ depends upon (a) the charge carried by the central transition metal ion, (b) the nature of the ligand and, (c) the transition metal ion itself. In general, for a given ligand, the crystal field spilitting is greater for M3+^ octahedral complexes compared to that in Mn 2+^ octahedral complexes, while for transition metal ions carrying the same charge, the value of Δ increase in the order, I –^ < Br–^ < Cl–^ < F–^ < H2O < NH 3 < ethylenediamine < NO 2 –^ < CN–, where the above ions and neutral molecules are the ligands which may surround the transition metal ion. This order is known as the spectro chemical series.
Since small changes in the values of Δ can significantly affect the colour of the light absorbed by transition metal ions, it is not surprising that transition metal ions can show a wide range of colour in different environment.
Magnetic Properties In order to explain why the same transition metal ion can often display two widely different degrees of paramagnetism in different environments. It is necessary to consider the spectrochemical series. For instance, the CN –^ ion produces a greater crystal field spilitting than other ligands.
Consider, for example, the octahderal complexes [FeF 6 ]3–^ and [Fe(CN) 6 ]3– (Figure ). The electron configuration of Fe3+^ is [Ar]3 d^5 , and there are two possible ways to distribute the five d electrons among the d orbitals.
Energy
Fe 3+ion
dx (^2) – y 2
dx (^2) – y 2 dz 2
dz 2
dxy dxy
dyz dyz
dxz [FeF ] dxz (high spin)
6 3– [Fe(CN) ] (low spin)
6 3–
Fig. 22.4: Energy-level digrams for the Fe 3+^ ion and for the [FeF 6 ] 3–^ and [Fe(CN) 6 ] 3–
As discussed earlier the d orbitals are split into two groups i.e. t 2 g and e (^) g. If the value of Δ is small then high-spin complex is formed but if value of Δ is large then complex will be of low spin type.
Notes
Coordination Compounds MODULE - 6 Chemistry of Elements High-spin complexes are more paramegnetic than low-spin complexes. d^4 , d^5 , d^6 and d^7 electronic configuration form low and high spin complexes. It is not possible to differenciate d^1 d^2 d^3 d^8 and d^9 systems on the basis of magnetic moments.
dx (^2) – y 2 dz 2
dxy dyz dxz
dx (^2) – y 2 dz 2
High spin Low spin
dxy dyz dxz
d^4
d^5
d^6
d^7
Fig. 22.5: Orbital diagrams for the high spin and low spin octahedral complexes corresponding to the electron configuration d 4 , d^5 , d^6 , and d^7_._
INTEXT QUESTIONS 22.
- Name the type of hybridization present in: [Co(NH 3 ) 6 ]3+.^.
- [Fe(CN) 6 ]3–^ is diamagnetic or paramagnetic?
- [NiCl 4 ]2–^ and Ni(CO) 4 have sp 3 or dsp^2 hybridization?
- Which one is diamagnetic: [Ni(CN) 4 ]2–^ or [NiCl 4 ]2–?
- What type of hybridization is shown by (i) inner and (ii) outer orbital complexes?
- [FeF 6 ]^4 is paramagnetic but [Fe(CN) 6 ]4–^ is diamagnetic. explain by CFT.
- Which is strong field ligand F–^ or CN–.
Notes
Coordination Compounds MODULE - 6 Chemistry of Elements Cl
Cl
NH 3
NH 3
Pt
trans (^) Cl
Cl
NH 3
NH 3
Pt
cis
Cl
Cl
NH 3
NH 3
NH 3
NH 3
M
trans
Cl
Cl
NH 3
NH 3
NH 3
NH 3
M
cis Two examples of cis - trans isomers
N
NH (^2)
CH (^2)
CH (^2) CH (^2)
N
N
N
N N
N N
Co
N
N
N
N
N
N
Co
Fig. 22.6: The enantiomers of [Co(en) 3 ] 3+^ (en = ethylenediamine), also called 1,2-di-aminoethane.
22.7 APPLICATIONS OF COORDINATION COMPOUNDS
Coordination compounds are found in living systems and have many uses in the home, in industry and in medicines. A few examples are given below:
Notes
MODULE - 6 Coordination Compounds Chemistry of Elements Extraction of metals: cyanide ions are used for the for the extraction of gold and silver. The crushed ore is heated with an aq. cyanide solution in the presence of air to dissolve the gold by forming the soluble complex ion [Au(CN) 2 ]–^.
4Au(s) + 8CN–(aq) + O 2 (g) + 2H2O(l) → 4[Au(CN) 2 ]–(aq) + 4 OH–(aq)
Zn(s) + 2[Au(CN) 2 ]–(aq) → [Zn(CN) 4 ]2–^ (aq) + 2Au(s)
Complex formation is also useful for the purification of metals. Nickel is purified by converting the metal to the gaseous compound Ni(CO) 4 and then decomposing the latter to pure nickel.
Medicines: EDTA is a chelating agent which is used in the treatment of lead poisoning. Cis platin cis [Pt(NH 3 ) 2 Cl 2 ] is used in the treatment of cancer. Sodium nitroprusside, Na 2 [Fe(CN) 5 NO] is used to lower blood pressure during surgery.
Qualitative Analyses: complex formation is useful for qualitative analyses.
(a) Separation of Ag +^ from Pb2+^ & Hg2+
Ag+^ + 2NH (^) 3(aq.) → [Ag(NH 3 ) 2 ]+
Soluble
(b) Separation of IIA and IIB groups: The cations of IIB group form soluble complex with yellow ammonium sulphide.
(c) Cu2+^ ion forms complex on addition of ammonia [Cu(NH 3 ) 4 ]2+. (d) Fe2+^ forms a blue complex with K 3 Fe(CN) 6 , i.e. K FeII[FeIII(CN) 6 ].
(f) Cobalt(II) gives color with HCl due to the formation of complex [CoCl 4 ]2–.
(g) Nickel forms a red complex [Ni(DMG) 2 ] with dimethylglyoxime (H 2 DMG).
INTEXT QUESTIONS 22.
- Name two elements which are extracted by complexation.
- What is the use of EDTA in medicine?
- Name the compound of platinum which is used as anticancer agent?
- Give two uses of complexes in qualitative analyses
- Write down geometrical isomers of Pt(NH 3 )Cl 2.
- [Co(NH 3 ) 5 SCN] 2+^ and [Co(NH 3 ) 5 NCS)2+^ is the example of ............
