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1.1       Transition Metal

The term transition metals and d block element are sometimes used as if they mean the same thing but they don’t, there is a subtle difference between the two terms.

D block element: the elements in periodic table which correspond to the d levels filling are called d block elements.

Transition metals: is the metals which forms one or more stable ions which have incompletely filled d orbitals. Not all d block elements count as transition metals. When d-block elements form ion, the 4s electrons are lost first (Khandewal, 2004).

1.1.1 Properties of transition metals

  1. All are metals with high tensile strength and good conductor of heat and electricity.
  2. All forms alloys with one another, and with metallic main group elements.
  • Many of them are sufficiently electropositive to react with mineral acids to form salts, though some of them are rather inert in this respect.
  1. Because of partly filled d orbitals some transition metal ions containing odd number of electrons form paramagnetic compounds.
  2. Many form coloured compounds in one if not in all oxidation state; the absorption of visible light being associated with the presence of partly filled d orbitals.
    • Chemical properties of transition metals
  3. Variable oxidation state: apart from scandium which forms only +3 oxidation state, transition metals shows various positive oxidation state by losing electrons from both their 4s and 3d orbitals (the energy difference between these orbitals is small). Compounds of the metals (from Ti to Cr) in the +2 state are ionic and are strong reducing agent (they are strongly oxidized).
  4. Complex ion formation: transition metal ions have small cationic size, large ionic charge and empty or partially filled 3d orbitals, hence they are capable of accepting electron pairs from either charged or neutral particles into their d-orbitals to form dative or co-ordinate bonds, resulting in complex ion formation.
  • Formation of coloured ions: Transition metal tends to be highly coloured. This is due to the easy movement of electrons between the d-orbital resulting in absorption or emission of light whose frequencies lie in the visible region. The colour is usually associated with a specific oxidation state of a given element.

1.1.3    Magnetic properties of transition metals

Many transition metal compounds are paramagnetic due to the presence of unpaired electrons. Magnetic moment of each such electron is associated with its spin angular momentum and orbital angular momentum. For the compounds of the first transition series, the orbital angular momentum quantum number is effectively quenched and hence is of no significance. for these the magnetic moment is determined by the number of electrons and is calculated by using ‘spin-only’ formula, i.e µs = √ n (n + 2).

            Where n is the number of unpaired electrons and µs is the magnetic moment in Bohr magneton (BM). A single 1s electron has a magnetic moment of 1.73BM (1BM = 9.2732 × 10-

24Am2) (Cotton et al., 2003).

  • Bonding in transition metal complexes
  1. Valence Bond Approach (VBA): According to this approach, the formation of a complex is considered to be a reaction between a lewis base (ligand) and a lewis acid (metal or metal ion) with the formation of a co-ordinate covalent bond between the ligand and the metal.
  2. Crystal Field Approach (CFA): the crystal field approach is an electrostatic model which considers the metal-ligand bond to be ionic arising purely from electrostatic interaction between the metal ion and the ligand. The transition metal (central metal ion atom) is regarded as a positive ion and is surrounded by negative or neutral ligands which have a lone pair of electrons. If the Ligands are neutral like NH3, the negative end of the dipole in the molecule is directed towards the metal atom. The electrons on the central metal atom are under repulsive forces from the ligands, hence they occupy the d-orbital furthest from the direction of the ligands.

1.2       Ligands

            A ligand is an atom, ion or molecules that donates or shares one or more of its electron through a covalent bond with a central atom or ion. It is a complexing group in coordination chemistry that stabilizes the central atom and determines its reactivity (Anne,2018).

1.2.1 Types of ligands

  1. Monodentate Ligands: these are ligands which bonds to the central metal atom through a single atom, example, ammonia (NH3) which bonds to the central atom through nitrogen atom. Other examples are Cl-, CN-, H2O.
  2. Bidentate Ligands: These are ligands which link to the central metal atom through two atoms, e.g. oxalato(C2O42-), acetylacetonato (acac- ), ethane -1, 2 diamine (en) etc.
  • Polydentate Ligands: These include all types of ligands with denticity higher than two. The various types of ligands included are :
  • Tridentate (denticity of 3), e.g. diethylenetriamine (dien), terpyridine (terpy) etc. (terpy) Here coordination to the metal atom can take place through the three nitrogen atoms.
  • Tetradentate (denticity of 4), e.g. triethylenetetramine (triene), Here coordination to the metal atom can take place through the four nitrogen atoms.
  • Pentadentate ligand (denticity of 5), e.g. ethylenediaminetriacetato, Here coordination to the metal atom can take place through the two nitrogen atoms and three oxygen atoms of the acetate group.
  • Hexadentate ligand (denticity of e.g. ethylenediaminetetraacetato (EDTA).

            In many bidentate to polydentate ligands, the ligands are bonded to the same central metal atom at two or more places due to which a ring structure is formed. Such a ring structure formed by bidentate (or polydentate) ligands is known as CHELATE. The term chelate, derived from the greek word chela meaning great claw of the lobster or other crustaceans, is suggested for the cliperlike groups which function as two associating units.

            Chelated complexes are known to have a higher stability as compared to ordinary complexes. Ethylenediaminetetracetic acid (H4EDTA) is an important chelating ligand. It binds the metal atom through two nitrogen and four oxygen atoms and hence act as a hexadentate ligands and forms five rings.

            Ligands which ligate to metal ions through two atoms of different elements present in it are called AMBIDENTATE LIGANDS. Examples of such ligands are NO2- and SCN- ions. NO2-ion can co-ordinate either through nitrogen or through oxygen to a central metal. Similarly, SCN- ion can co-ordinate through sulfur or nitrogen atom (Cotton et al., 2003).


Table 1.1 Structure of Monodentate Ligand


Lewis structure



Lewis structure







Fluoride ion



Carbon monoxide



Chloride ion



CNHyanide ion



Bromide ion






Hydroxide ion









Oxalate ion

Ethylenediamine (en)

Fig 1.1 Structure of Bidendate Ligands


Ethylenediaminetetraacetic acid

Fig 1.2 Structure of Polydendate Ligand



Benzene-1,2,4,5-tetracarboxylic acid                                      Dimethyl-4,4-biphenyldicarboxylate



4,4-Bipyridine                                                     4-(2-carboxyphenyl)-2(4-carboxyphenyl)benzoic                                                                                         

            Fig 1.3 Structure of Ligands Used for the Synthesis







1.3       Co-ordination compound

            The transition metals and their ion have much higher tendency to form co-ordination compound as compared to the s and p block elements. It is because of their relatively smaller sizes, higher ionic charges and the availability of d-orbital for bond formation. Coordination compounds, unlike normal compounds, retain their identity even when dissolve in water or any other suitable solvent. The properties of these compounds are totally different from those of their constituent.

            Alfred Werner show that neutral molecules where bond directly to the metals so that complex salt are correctly formulated. G.N Lewis and N.V Sidgwick proposed that a chemical bond require the sharing of an electron pair. This led to the idea that a neutral molecule with an electron pair can donate these electrons to a metal ion or other electron acceptor. Thus, I a coordination compound, the metal species act as electron acceptor (Lewis acid) and neutral molecule with lone pair of electrons or anion as electron donor (Lewis base). The number of ligands donor atoms directly bonded to the central atom is defined as the COORDINATION NUMBER (Abel, 2001).

1.4       Aim

To synthesize and characterize mixed-ligand complexes of Ni(II), Mn(II) and Cu(II) with benzene- 1,2,4,5-tetra carboxylic acid, 4,4-bipyridine, 4-(2-carboxylphenyl)-2(4-carboxylphenyl) benzoic acid, and Dimethyl-4,4-biphenyldicarboxylate.





1.5       Objectives

  1. Synthesis of mixed-ligand complexes of Ni(II), Mn(II) and Cu(II) using various metal salts of nickel, manganese and copper salt with benzene-1,2,4,5-tetracarboxylic acid, 4,4-bipyridine, 4-(2-carboxylphenyl)-2(4-carboxylphenyl) benzoic acid, and Dimethyl-4,4-biphenyldicarboxylate.
  2. Characterization of the synthesized complexes using FTIR, UV, and solubility.