[Lynda92]

π bonds are formed from the overlap of parallel p orbtials that are in phase with each other, but why does this overlap take place when the pi electrons in the p orbtials should repel each other?
Also, π* antibonding orbitals form when the p orbitals are out of phase.could someone please explain why this would lead to antibonding orbitals,where there is no electron density?

Thankyou 

[MethylNeptuneAcetate]

π bonds are formed from the overlap of parallel p orbtials that are in phase with each other, but why does this overlap take place when the pi electrons in the p orbtials should repel each other?
Also, π* antibonding orbitals form when the p orbitals are out of phase.could someone please explain why this would lead to antibonding orbitals,where there is no electron density?

Thankyou 

It is better (at least in this case) to think of bonds not as overlapping orbitals, but single orbitals themselves. Take molecular hydrogen for example. In H2, are the electrons in 1s orbitals? No. There are 2 electrons of opposite spins in one σ orbital, which is of lower energy than the two 1s orbitals it is made from.

Antibonding orbitals come from the fact that the number of orbitals is always conserved: to use the H2 example, the two 1s orbitals mix to form two new orbitals, called σ and σ*. One new orbital is of lower energy than the 1s orbitals: the σ orbital. σ* is of higher energy (and the average of their energies is the same as that of the 1s orbitals).

Now why does this happen? Well, the shape of the orbitals is determined by the square of the wavefunction (Ψ^2). When the two atoms come together, the wavefunctions add together (Ψ1+Ψ2)^2, which does not equal Ψ1^2+Ψ2^2. Why is this important? because Ψ has both positive and negative components, while Ψ^2 is all positive due to the nature of the square function. if Ψ1 and Ψ2 are of opposite sign (out of phase), they interfere deconstructively, creating an antibonding orbital. if Ψ1 and Ψ2 are the same sign, they interfere constructively, creating  a bonding orbital.

Now, the bonding orbital is of lower enegy than the antibonding orbital, so the electrons go into the bonding orbital first. In the case of the two p orbitals forming a π bond, there are only two electrons, so there are no electrons in the π* orbital. Why talk about the π* orbital then, if it's empty? Well, often times, it is the LUMO or lowest unoccupied molecular orbital, which means any new electrons coming into the system will go into it, and if the existing electrons are excited, they are most likely to jump to it.