The nucleus is an organelle found in the cytoplasm of the cell - but it is not part of the cytoplasm! There are small pores in the nuclear membrane called nuclear pores where molecules can leave and enter the nucleus - so for example, mRNA would leave the nucleus and go into the cytoplasm where the mRNA would be translated into protein which in some cases go back into the nucleus to function (like DNA binding proteins - transcription factors, etc.). The nucleus and cytoplasm basically work together to keep a cell alive. So, in reality it is a symbiotic relationship - one can't survive without the other!