Chemistry · Coordination Compounds · NEET
The coordination number (CN) is the number of donor atoms that are directly bonded to the central metal ion inside the coordination sphere (the square brackets). Important: you count donor ATOMS, not ligand molecules and not charges. In [Co(NH3)6]3+, six NH3 molecules each donate one N atom, so CN = 6. Only look inside the square brackets. Ions written outside the brackets (counter ions) are NOT counted.
Step 1: Write the overall charge of the complex ion (inside the brackets). Step 2: Add up the charges of all the ligands (neutral ligands like NH3 and H2O are 0; Cl- is -1; CN- is -1; OH- is -1). Step 3: Let the metal charge be x, then solve x + (sum of ligand charges) = charge of the complex ion. Example: in [Fe(CN)6]3-, x + 6(-1) = -3, so x = +3. Iron is in the +3 oxidation state.
No. These are three different things. Coordination number = how many donor atoms are attached (a counting number, always positive). Oxidation state = the charge on the metal (can be +, - or 0). They are often different. In [Co(NH3)6]3+, the coordination number is 6 but the oxidation state is +3. Do not mix them up. NEET often tests both in one question, so keep them separate.
No. Only donor atoms inside the coordination sphere (the square brackets) count toward the coordination number. Ions written outside the brackets are counter ions (they balance the total charge only). Example: in [Cr(H2O)6]Cl3, the three Cl- are outside, so they do NOT add to the coordination number. Chromium is bonded only to six water molecules, so CN = 6, and its oxidation state is +3.
A bidentate ligand donates TWO atoms to the metal, so it counts as 2 toward the coordination number. Example: ethylenediamine (en) has two nitrogen donor atoms. In [Co(en)3]3+, there are 3 en ligands but 3 x 2 = 6 donor atoms, so CN = 6. Always count donor atoms, not the number of ligand molecules. This is a very common NEET trap.
Water written outside the coordination sphere (as .xH2O) is water of crystallisation. It is neutral and outside the brackets, so it does NOT affect the coordination number and does NOT change the ligand charge sum. Only count the ligands inside the square brackets. Example: in [Cr(H2O)4Cl2]Cl.2H2O, only 4 H2O and 2 Cl- (inside) matter: CN = 6, and Cr charge: x + 4(0) + 2(-1) = +1 (charge of complex ion) gives x = +3.
Type of isomerism exhibited by the compounds [Cr(H2O)6]Cl3, [Cr(H2O)5Cl]Cl2.H2O and [Cr(H2O)4Cl2]Cl.2H2O, and the value of the coordination number (CN) of the central metal ion in all these compounds, respectively, is:
Among the species below, the spin-only magnetic moment is highest for: (Atomic numbers: Ti = 22, Mn = 25, Fe = 26, Co = 27)
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Coordination number is how many donor atoms are bonded to the metal (a positive counting number). Oxidation state is the charge on the metal (can be +, -, or 0). For example, in [Co(NH3)6]3+, CN = 6 and oxidation state = +3.
K is +1 (four K = +4). Each CN- is -1 (six CN = -6). The whole compound is neutral, so +4 + x + (-6) = 0, giving x = +2. Iron is in the +2 oxidation state, and its coordination number is 6.
No. Neutral ligands carry zero charge, so they do not change the metal's oxidation state calculation. But they DO count toward the coordination number because they still donate a lone pair to the metal.
For NEET, the most common coordination numbers are 4 and 6, with 6 (octahedral) being the most frequent. You may occasionally see 2 (like in [Ag(NH3)2]+). Coordination number 6 is the one to expect most often.
The coordination number decides the geometry (shape) and hybridisation of the complex, which then decides isomerism, magnetic behaviour, and colour. Many NEET questions cannot be started without first finding the coordination number and oxidation state, so this is a base skill for the whole chapter.
