Formation of Complex Compounds

What a complex compound is

A complex (or coordination) compound is one in which a metal ion binds a number of anions or neutral molecules — called ligands — around itself, giving rise to a complex species with its own distinct set of properties, different from the simple metal ion. Some representative examples are:

Why transition metals form so many of them

Transition metals form a strikingly large number of such complexes, far more than the typical main-group metal. Three features of transition-metal ions work together to make this possible:

Comparatively small size of the metal ion, which lets ligands approach closely.

High ionic charge on the metal ion, which attracts electron-rich ligands strongly.

Availability of vacant d orbitals of suitable energy, which can accept the lone pairs of electrons donated by the ligands.

It is this last point that is distinctive to the d-block: the empty d orbitals give the metal ion somewhere to place the electron pairs offered by the ligands, over and above what the s and p orbitals alone could accommodate — which is why complex formation is so much more extensive here than among non-transition metals. …