Coordination Polyhedron

Coordinate compounds have a central metal atom surrounded by ligands via a coordinate or dative bond. In this case, the central metal functions as a Lewis acid, accepting electrons from ligands that function as Lewis bases. Alfred Werner was the first to offer his perspective on coordination compounds. These compounds retain their identity in crystal lattice and solution or molten state. 

[Ni(NH3)6]2+ is a well-known example in which nickel acts as the central metal atom, and ammonia serves as the ligand.

What is the coordination polyhedron?

The term ‘coordination polyhedron’ describes ligands’ geometric pattern or spatial arrangement directly linked to the centre atom or an ion. 

• The most frequent coordination polyhedra are tetrahedral, octahedral, and square planar geometries. 

For example:

1. The geometry of [Ni(CO)4] is tetrahedral
2. The geometry of [Ni(CN)4]2- is square planar
3. The geometry of [Fe(CO)5] is trigonal bipyramidal
4. The geometry of [Pt(Cl6)]2- is octahedral

• According to Werner, primary and secondary valency are two types of valency found in coordination compounds.

Primary valence:

A dashed line represents the oxidation state or charge on the complex in primary valency, which is ionisable.

Secondary valence:

Secondary valency is non-ionisable and is indicated by a solid line. It determines the geometry or form of the molecule.

•  The primary valency is non-directional, whereas the secondary valency is directional.

1. [Ni(NH3)6]Cl2:

1.  Solid lines (secondary valency) represent ammonia, while dotted lines represent chlorine (primary valency). Coordination entities or complexes are the species inside the square bracket, while counter ions are the species outside the square bracket.
2. 1st, 2nd, or 3rd period transition elements are commonly used as the central metal atom. All molecules with a lone pair or negative charge to donate can be used as ligands.

2. Monodentate ligand-

Only one donor site is present in a monodentate ligand, ammonia (NH3), carbonyl (CO), and phosphine (PH3) groups.

• Bidentate ligand- 

There is a two-donor site in the ligand. Oxalate (C2O4)2- is a good example.

• Polydentate ligand-

More than two donor atoms are present in a polydentate ligand. EDTA (Ethylene diamine tetra acetate) is an example of a hexadentate ligand.

• Ligands are typically donors; however, pi bonding in the molecular orbital or vacant d orbital, also known as pi acid ligand and pi base Metal, allows ligands to absorb electrons from metal.
• Homoleptic and heteroleptic complexes are two types of complexes. Metal surrounded by the same type of ligands is considered homoleptic, whereas metal surrounded by two or more types of ligands is considered heteroleptic.

Types of coordination polyhedron on the basis of shape

1. Tetrahedral
2. Square planar
3. Trigonal bipyramidal
4. Octahedral

• The coordination number, which can range from 2 to 16, but is usually 6, is the total number of attachment points to the centre element. In simple terms, the relative sizes of the metal ion and the ligands and electronic factors like charge, which is reliant on the metal ion’s electronic configuration, determine the coordination number of a complex.
• According to the radius ratio, the higher the charge on the central ion, the greater the attraction for negatively charged ligands; however, the larger the charge, the smaller the ion becomes, limiting the number of groups that can coordinate. It is vital to remember that while every geometry has a unique coordination number, every complex with that number will have a variety of geometries to choose from (i.e., there is not a one-to-one correspondence between coordination number and geometry).

Coordination No 3

• Trigonal planar Geometry: This geometry, well known for main group species such as CO32-, contains the four atoms in a plane with the ligand bond angles at 120 degrees.

Coordination No 4

There are two possible geometries. The tetrahedron is more common, but the square planar is nearly exclusively associated with metal ions with a d8 electronic structure.

• Tetrahedral Geometry: Organic chemistry courses examine the chemistry of molecules centred on a tetrahedral C atom. Tetrahedral Cobalt(II) complexes exist in huge numbers.
• Square Planar Geometry: This is a fairly rare shape that is only mentioned since it contains several really significant compounds.

Coordination No 5

• Square pyramidal form is common in oxovanadium salts (Vanadyl, VO2+), such as VO(acac)2. Notably, vanadium (IV) is coordinatively unsaturated, resulting in the production of an octahedral complex when pyridine is added.
• In 1968, the structure of [Cr(en)3][Ni(CN)5] 1.5 H2O was described as a remarkable example of a complex showing both types of geometry in the same crystal.

Conclusion

Coordination compounds are molecular addition compounds that maintain their identity in both solid and liquid states. The central metal atom or ion in these compounds is linked by ions or molecules with coordinate bonds. 

• For example, in the case of potassium ferrocyanide, K4[Fe(CN)6].