 In this video, we are going to look at a few important terms and definitions related to coordination compounds. We need to familiarize ourselves with these terms as they keep appearing throughout the context of coordination complexes. So let's first talk about coordination entity. A coordination entity consists of a central metal atom or an iron that is bonded to a fixed number of ions or molecules. The coordination entity is enclosed within brackets as you can see here. For example, CONS3-3Cl3 is a coordination entity where cobalt metal ion is bonded to three chloradions and three ammonia molecules. Similarly, the coordination entity in K4-FECN6 is the entity that is enclosed within the square brackets, which is nothing but FECN6-4, so this would be FECN6-4, this would be the charge of the entity here. Let's look at the next term which is central metal atom or ion. Now the central metal atom or ion once again lies within the coordination entity, that is the metal also lies inside the square brackets. The metal atom or ion is coordinated to a fixed number of groups via secondary bonding. They are also bound in a definite geometrical arrangement. For example, the central metal ion in these coordination entities are Ni2+, CO3+, and Fe3+. As these ions are positively charged, they are electron deficient in nature, and accept electrons from other neutral molecules or negatively charged ions, that is other donor species. Hence, these are also referred to as or the central metal ion is also referred to as Lewis acids, because they are capable of accepting electron pairs from electron drift species. Let's now talk about one of the most important terms which is ligands. The ions or molecules that are bound to the central metal atom or the ion within the coordination entity are called ligands. Now the ligands can be negatively charged ions like Cl-, Br-, NO2- or even neutral species like water or ammonia. The common factor being these ions or molecules are electron drift species and can donate electrons to the central metal atom. Now even larger molecules like NH2, CH2, CH2, NH2 which is nothing but ethylene diamine, or even larger macromolecules like proteins can also act as ligands as long as they have a donor atom like let's say the oxygen or nitrogen, and can act as a lowest base. Now when a ligand is bound to a metal atom through a single donor atom such as Cl- or oxygen or water or nitrogen of ammonia, then the ligand is referred to as monodentate. Yes, all of these given here are examples of monodentate ligands. And when a ligand can bind through two donor atoms it is said to be bidentate or also sometimes called didentate ligands. For example, ligands like ethylene diamine NH2, CH2, CH2, NH2 or oxalate ions as you can see here have two donor atoms that is two nitrogen in the case of ethylene diamine and two oxygen atoms, electron rich oxygen atoms in the case of oxalate ions which can donate electron pair to the electron deficient metal atom. So as you can see here in the cases where the ligands bind through two donor atoms such ligands are called bidentate ligands. And when we have several donor atoms present in a single ligand like in the case of N, CH2, CH2, NH2 thrice. So here the ligand is said to be polydentate. For example, EDTA which is a polydentate ligand or more specifically a hexadentate ligand can bind to a metal atom through two nitrogen as well as four oxygen atoms as you can see here. One, two, three and four. So as you can see here this ligand is a polydentate ligand and it can bind to the metal atom through six donor atoms. Now here's an interesting thing. When a bidentate or a polydentate ligand uses its two or more donor atoms to bind with a central metal atom simultaneously then it is also called a chelate ligand. And such complexes are called chelate complexes. Now because of the structural integrity the chelate ligands are much more stable than similar complexes that has unidentate ligands bonded to it. For example, here we have metal M bonded to six ammonia molecules and we know that ammonia is a unidentate ligand right? If the metal M is same in both the complexes the metal EDTA complex would be much more stable than the metal ammonia complex because of the structural stability offered by the chelation effect. Let's now look at a different type of ligand which is the ambidentate ligands. Ambidentate ligands are those ligands that contain two different donor atoms and can coordinate with either of the atoms with the central metal ion. For example, in NO2- the ligand can coordinate with the metal ion either through the nitrogen atom or through the oxygen atom. When it coordinates with the nitrogen atom we call nitrite ON complex and when it coordinates with the oxygen atom we call nitrite O complex. Similarly, thiocyanate ion SCN- can also coordinate with the central metal ion through sulphur in which case we call it thiocyanate OS or through nitrogen where we call it thiocyanate ON. So this is what happens in the case of ambidentate ligands where the ligand can coordinate through either of the donor atoms. So let's talk about a few more important terms related to coordination compounds in the next video.