Why do transition metals form complex compounds?

In simple terms please :)

any help would be much appreciated!
Answers:
The main-block metals do form complexes but not as readily (e.g. magnesium and calcium next to edta in water softening), because...
(a) transition metals tend to be smaller and/or more importantly charged and so have a higher charge/size ratio which attracts the elevated electron density on ligands rather more.
(b) Transition elements have disused orbitals of suitable energy that can accept the non-bonding pairs from the ligands.

Related Questions:
Why GOLD is the just form of metal explicitly expensive of adjectives the other form of metals?   Do Metals, Nonmetals, or Metalloids form cations surrounded by solutions, same interrogate for forming anions. HELP!?   How do metals form?   Why do transition metals near d6 low spin electron configuration form adjectives complexes?   How are metals formed?  
  • Whats the diffrence form the current Full Metal Alchemist anime nd the strange one base on the manga?
  • What type of ions are the metals contained by the column possible to form?
  • What is designed by pi complex.how CO forms pi complex near metals?
  • I'm a metal soloist. What's the best method to find a group to blend or form a group?
  • How to determine if a metal forms one stable cation?