Question

Hi all

I am having trouble finding the MO order for heteronuclear diatomic molecules:

In my textbook, heteronuclear order is listed as:
1o 2o 3o 4o 1pi 5o 2pi 6o

where o = sigma

When I first saw it, I was confused; how can the above be correct? If one were to calculate bond order, wouldn't it just keep increasing?

Now, for NO and CO, I BELIEVE MO order is:
1o 2o* 3o 4o* 1pi 5o 2pi*

But why is it like this? Is there a general order like in homonuclear diatomic molecules?
Is my textbook incorrect?

Regards