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). 1.1.1 Types of Ligand
- Monodentate ligands: It has one atom that can bind to a central atom or ion. Water (H2O), ammonia (NH3) are examples of neutral monodendate ligands.
- Bidentate ligands: these are lewis bases that donate two pairs (bi) of electrons to a metal atom. Bidentate ligands are often reffered to as chelating ligands because they can grab a metal atom in two places. Examples, ethylenediamine (en), oxalate ion (ox).
- Polydentate ligands: it has more than two donor site. A complex with apolydendate ligands is called a CHELATE.
- Ambidentate ligands: it is a monodendate ligands that can bind in two possible places. For example , the thiocyanate ion, SCN-, can bind to the central metal at either the sulfur or the nitrogen (Anne, 2018).
Fig 1.1 Structure of Bidendate Ligands
Fig 1.2 Structure of Polydendate Ligand
Benzene-1,2,4,5-tetracarboxylic acid Dimethyl-4,4-biphenyldicarboxylate
Fig 1.3 Structure of Ligands Used for the Synthesis
1.2 Co-ordination Number
The coordination of a number of an atom in a molecule is the number of atoms bonded to the central atom. In chemistry and crystallography describes the number of neighbor atoms with respects to a central atom. The term was originally defined in 1893 by Alfred. The value of the co-ordination number is determined differently for crystals and molecules. The co-ordination number can vary as low as 2 to as high as 16.
The value depends on the relative sizes of the central atom and ligands by the charge from the electronic configuration of an ion. The co-ordination number of an atom in a molecule or poly atomic ion is found by counting the number of atoms bound to it (note, not by counting the number of chemical bonds).
It is more difficult to determine chemical bonding in solid-state crystals, so the co-ordination number in crystals is found by counting the number of neighboring atoms. Most commonly, the co-ordination number looks at an atom in the interior of a lattice, with neighbors extending in all directions. However, in certain contexts crystal surfaces are in important (example, heterogeneous catalysis and material science), where the co-ordination number of an interior atom is the bulk co-ordination number and the value for a surface atom is the surface co-ordination number.
In co-ordination complexes, only the first sigma bond between the central atom and ligands count (Anne, 2018)
1.3 Complex Ion
A complex ion has a metal ion at its center with a number of other molecules or ions surrounding it. These can be considered to be attached to the central ion by co-ordinate ( dative covalent) bonds (although in some cases, the bonding is actually more complicated than that.) the molecules or ions surrounding the central metal ion are called ligands. Simple ligands include water, ammonia and chloride ion. All ligands are lone pair donors. In other words, all ligand function as lewis bases (Jim,2018).
1.4 Transition Metals
This is defined as the metals with inner d or f orbitals being filled (orbitals; describe ways that electron can be organized around a nucleus. There are four types of orbitals: s, p, d, and f.).
Elements that lose electron easily, that are lustrous and malleable, that are good conductors of heat and electricity are known as metals. Metal element can be broken down into several categories, one of which is the category of transition metals.
The transition metals consist of the 40 elements located in columns 3 – 12 on the periodic table and the 28 element comprising the lanthanide and the actinide series. Element in the lanthanide and actinide series are often considered to be inner transition metals(Elizabeth, 2003).
1.4.1 Properties of Transition metals
- They are lustrous, silvery, hard and good conductors of heat and electricity.
- All, except mercury (which is liquid at room temperature), appear as high melting point and boiling point lustrous solids.
- All form alloys with one another, and with metallic main group element
- All have high enthalpy atomization.
- Many of them are sufficiently electropositive to react with mineral acid to form salt, though some of them are rather inert in this respect.
- Most of them show more than one oxidation state (variable valence). (SatyaVihar et al)
Table 1.1 Electronic Configuration of the First Transition Metal
Elements Symbol Atomic Number Electronic Configuration Scandium Sc 21 [Ar]4s23d1
Titanium Ti 22 [Ar]4s23d2
Vanadium V 23 [Ar]4s23d3
Chromium Cr 24 [Ar]4s13d5
Manganese Mn 25 [Ar]4s23d5
Iron Fe 26 [Ar]4s23d6
Cobalt Co 27 [Ar]4s23d7
Nickel Ni 28 [Ar]4s23d8
Copper Cu 29 [Ar]4s13d10
Zinc Zn 30 [Ar]4s23d10
1.4.2 Coloured Ion Formation
Many transition metal ions are coloured due to d-d transitions. The energy absorbed in excitation of an electron from a lower energy d orbital to a higher energy d orbital corresponds to the frequency which generally lies in the visible region. The colour observed correspond to the complimentary colour of the light absorbed. The frequency of the light absorbed is determined by the nature of ligands. The colour of some 3d transition metal ion in aqueous solution, where water molecule co-ordinate with the metal ion.
1.4.3 Alloy formation
The transition metals readily form alloys with each other because of similar radii. The alloys so formed are hard and have often high melting point. The best known are ferrous alloys; vanadium, chromium, molybdenum, tungsten and manganese are used for the production of variety of steels and stainless steel. Industrially important alloys of transition metals with non – transition metals are bras (copper-zinc), bronze (copper-tin) (Cotton et al., 2003).
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.
- 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.
Characterization of the synthesized complexes using FTIR, UV, and solubility.