A good lens for nikon d800?

I need a good lens for shooting events but also good for portraits

Sporting events depends on what sport. Portraits depends on how old the majority of your subjects are!
For Portraits (of over age 45) I use 150mm or 200mm, with a Tiffen Soft X filter #3 or #4

The D800 is an excellent camera, but will show the limitations of any but the very best lenses. In other words, without the best lenses, you will not get all of the performance you could out of the D800.In fact, with marginal lenses, you might even have worse looking photos with the D800, due to the lens performance, unless you soften them a bit in post processing.

Here is the recommended lens set for the D800:

Nikon AF-S 14~24mm f/2.8
Nikon AF-S 24-70mm f/2.8
Nikon AF-S 70-200mm f/2.8

This set of three lenses will set you back $6, 300 in US dollars. If you are not prepared to spend this kind of serious money, you should consider a lower cost camera, such as the D600 - which is still a very capable camera.