what is p pie p pie and d pie p pie bond?

p π -p π  bonding: When electron present in p orbital of an atom forms pi(π) bond with the electron present in p orbital of another atom then it is known as p π-p π bonding. Carbon has a unique property to form p π-p π bonds i.e. double or triple bonds with itself and with other elements like nitrogen and oxygen. In these compounds e.g,alkenes, alkynes, graphite, carboxylic acids ,Acid amides,CO₂ etc there are p π-p π bonding.The following image shows  p π-p π bonding between carbon atoms.

d π -p π  bonding:­ When electron present in d orbital of an atom forms pi(π) bond with the electron present in p orbital of another atom then it is known as d π-p π bonding. Elements like silicon, germanium etc. which have vacant d-orbitals in their valence shell can undergo d π-p π bonding with elements like O and N. e.g N(SiH₃)₃.