Question

how benzene has coordination no. 6

Answer 1

Hapticity is the coordination of a ligand to a metal center via an uninterrupted and contiguous series of atoms.[1] The hapticity of a ligand is described with the Greek letter η('eta'). For example, η2 describes a ligand that coordinates through 2 contiguous atoms. In general the η-notation only applies when multiple atoms are coordinated (otherwise the κ-notation is used). In addition, if the ligand coordinates through multiple atoms that are not contiguous then this is considered denticity [2] (not hapticity), and the κ-notation is used once again.[3] When naming complexes care should be taken not to confuse η with μ ('mu'), which relates to bridging ligands.[4][5]