Question: why fei3 is not stable?
Answer: I- is much larger in size, so has higher tendency to lose electron, and hence is a good reducing agent. I- reduces Fe3+ to Fe2+ and itself gets oxidised to I2, hence FeI3 is unstable. ... So, it being an oxidising agent oxidises Fe2+ to Fe3+, which favours formation of FeCl3 and makes it stable.