We are glad to choose our support to clear your doubts and I will make my best to help you with it.
It is important to say that we as Intel® provide the hardware to computer manufactures like Dell in your case, and they will design and create their custom units. This means that they can set software or hardware limitations in their units.
Due to this reason, it is very difficult from my position to let you know the compatible processors for your unit. I recommend you contacting Dell directly for them to provide the processor compatible list.
Here you have Dell contact information: