Radius ratio for tetrahedral is 0.225-0.414.

Unequal Coordination Number Example Titanium oxide (TiO2) is an example of a crystal structure containing anions and cations in a 2:1 ratio. So, the fundamental unit of the lattice is tetrahedral: Each Ti4+ cation is amid six O2-ions, while every O2-ion has three immediate Ti4+ neighbors.

Explanation: The cation to anion ratio for a ceramic having coordination number 2 is less than 0.155.

0.225

Length of the face diagonal =

1. Along the body diagonal AD, two spheres radius R each touch the tetrahedral void of radius r.
2. Putting the value of R from equations (i) and (ii)
3. Thus, for tetrahedral void = = 0.225.

Cation-anion radius ratio

Radius Ratio	Coordination number	Type of void
0.155 – 0.225	3	Triangular Planar
0.225 – 0.414	4	Tetrahedral
0.414 – 0.732	6	Octahedral
0.732 – 1.000	8	Cubic

the radius ratio of a crystal helps to determine the coordination number cation and anion.It is designated by (r+/r-). Limitation: This rule is applicable when all the cations and anions are perfectly sphere but in maxium cases the atoms or ions are not perfectly sphere. Hence deviation arises.

This stability of the ionic crystals can be explained on the basis of radius ratio. Therefore, radius ratio is the ratio of cation to the ratio of an anion. Here, Ratio of cation= r, Ratio of anion = R. Thus, Radius ratio = (r/R).

According to the radius ratio rule, the coordination number of six corresponds to the radius ratio of 0.414-0.732. Of the given options, the value 0.52 is in the range 0.414-732. Hence, sodium chloride crystal has the radius ratio of 0.52. Hence, the option D ) is the correct answer.

The limiting radius ratio is the minimum allowable value for the ratio of ionic radii (ρ=r+/r-) for this structure to be stable. Here, r+ is the radius of the cation and r- is the radius of the surrounding anions. Note that the anions are usually larger than cations.

Calculating the Coordination Number

1. Identify the central atom in the chemical formula. Usually, this is a transition metal.
2. Locate the atom, molecule, or ion nearest the central metal atom.
3. Add the number of atoms of the nearest atom/molecule/ions.
4. Find the total number of nearest atoms.

For molecules and polyatomic ions the coordination number of an atom is determined by simply counting the other atoms to which it is bonded (by either single or multiple bonds). For example, [Cr(NH3)2Cl2Br2]− has Cr3+ as its central cation, which has a coordination number of 6 and is described as hexacoordinate.

These drum-like structures consist of two B8 rings sandwiching a cobalt atom, which has the highest coordination number known heretofore in chemistry.

In the example you gave, the coordination number would be 4 because there are 4 molecules that form bonds with Pt. In order to find the oxidation number, you need to look at the charge of each piece of the complex. We know that each NH3 has a +1 charge so there are three NH3’s for a combined total of +3.

Writing the (Line) Formula of a Complex:

1. Identify the central metal ion.
2. Identify the oxidation state on the central metal ion (shown in Roman numerals parantheses)
3. Identify the ligands.
4. Identify the number of ligands.
5. Calculate the total charge on the ligands.
6. Calculate the charge on the complex ion.

Bidentate ligands have two donor atoms which allow them to bind to a central metal atom or ion at two points. Common examples of bidentate ligands are ethylenediamine (en), and the oxalate ion (ox).

A ligand is a molecule that binds another specific molecule, in some cases, delivering a signal in the process. Ligands can thus be thought of as signaling molecules. Ligands interact with proteins in target cells, which are cells that are affected by chemical signals; these proteins are also called receptors.

Types of Ligands

• Unidentate ligands: Ligands with only one donor atom, e.g. NH3, Cl-, F- etc.
• Bidentate ligands: Ligands with two donor atoms, e.g. ethylenediamine, C2O42-(oxalate ion) etc.
• Tridentate ligands: Ligands which have three donor atoms per ligand, e.g. (dien) diethyl triamine.

Occasionally ligands can be cations (NO+, N2H5+) and electron-pair acceptors. Examples for anionic ligands are F–, Cl–, Br–, I–, S2–, CN–, NCS–, OH–, NH2– and neutral ligands are NH3, H2O, NO, CO. A ligand is an ion or molecule, which binds to the central metal atom to form a coordination entity or complex compounds.

Monodentate ligand is a ligand that has only one atom that coordinates directly to the central atom in a complex . For example , monodentate ligands may be simple ions such as Cl− or small molecules such as H2O or NH3 . Hence water is a monodentate ligand only .

dative

Carbon monoxide has lone pairs on both the carbon and the oxygen atoms. I know that the much lower electronegativity of carbon makes it a much stronger donor. However, this difference in electronegativity exists in the cyanide ion too, albeit a little less. However, cyanide ion is ambidentate.

The strength of a ligand is determined by the amount of crystal filed energy. Since, CO causes more crystal field splitting than Cl- , it has more crystal field energy and thus is a stronger ligand than Cl -.

The more electropositive C atom in the strong field ligand CN- allows better orbital overlap and sharing of the electron pair. Note that CN- typically coordinates metal ions through the C atom rather than the N atom.

F−,I−,Cl− and H2O are weak ligands, whereas OH−, NH3, CH3COO−, en and CN− are strong lignads.

In case of H2O, it is weaker ligand as compared to NH3 as the donor atom is Oxygen which is more electronegative than Nitrogen. So as the electronegativity of central atom decrease, the ligand becomes stronger. so in NH3, pairing will occur and in H2O pairing will not occur.

EDTA is a moderately strong field, while (en ) is a strong field ligand.