I own an i5-750 and I would say that 100C is definitely too hot, but from a safety point of view, modern CPUs (built in the 2000s more or less) will self-throttle before they fry themselves so it is not unsafe. You will see performance degredation and ultimately a forced shutdown of your computer before your chip destroys itself or poses a safety hazard to you.
That's only partially true, modern chips may indeed throttle/shut down before reaching a point of immediate failure. However running at such high temperatures absolutely degrades electronics. Operating non-ruggedized hardware at such high temperatures for sustained periods will absolutely ruin them.
A general rule of thumb is: Every 10c over recommended operating temp = half the life.
If recommended is 72c and you're running at 100c, good chance you'll reduce the remaining life of the chip about 85%.
There's other things to consider, running a CPU at a sustained 100c will certainly heat up and stress the MOBO as well.
I think it's safe to say for most CPUs the danger zone is anything above 80c. Continuously above 90c is unacceptable.
Occasional short spikes up to these temperatures can be expected, and should be fine (that's why the limits are set high).