# Why is CO ligand higher than CN- in the spectrochemical series?

I've drawn MO diagrams for both molecules and they are isoelectronic. The main difference is that $\ce{C}$ is closer in electronegativity to $\ce{N}$ than it is to $\ce{O}$ so there will be less stabilisation of the bonding orbitals for $\ce{CO}$ due to the increased energy difference in the AO's.

Does this difference lead to $\ce{CO}$ having its $\pi^*$ orbital (LUMO) lower than that of $\ce{CN^{-1}}$ so it more readily accepts $\pi$ backdonation from a metal centre? Does this alone account for why $\ce{CO}$ is higher in the spectrochemical series? What about the difference between the $\sigma$ orbitals (HOMO) in $\ce{CO}$ and $\ce{CN^{-1}}$?