Apologise for geeky post- CHEMISTRY HELP PLEASE

Little bit confused with this- and have test on friday. I am generally grasping molecular orbital diagrams, but this puts a spin on things. (last page of the section too- brilliant !)
Could someone please tell me,
- where each of the molecular orbitals have arisen from and therefore where have the electrons come from
- why there has been no reference to antibonding

And why the Chemistry3 book says the following...Without s-p mixing the 4sigma orbital in CO is antibonding, and the 5sigma orbital is bonding. s-p mixing stabilizes the antibonding orbital making it more bonding in character, and destabilizes the bonding orbital, making it more antibonding. In some treatmentss of the bonding in CO, the 4sigma orbital is regarded as bonding whereas the 5sigma is regarded as antibonding.

Any help will be very appreciated.

Thank-you
Yours,

COMOdiagram.png39.1KB