He said to his wife: If you do this thing you will be divorced

If a man says to his wife, “If you do such and such then you are divorced,” then this is a divorce that is dependent upon the condition, and if she does that then divorce takes place according to the majority of scholars. 

Some scholars are of the view that it depends. If the husband intended that divorce should indeed take place if the condition was fulfilled, then divorce counts as such. But if his intention was to stop her doing that thing, and he did not intend divorce, then this comes under the same rulings as an oath (yameen), so he has to offer expiation if it is broken, and it does not count as divorce. This is the view favoured by Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) and some of the scholars. 

Thus it is clear that if your wife did not do that thing, then nothing has happened, and there is no divorce or expiation. But if she does do it, then the difference of opinion mentioned above comes into play. 

See also the answer the questions no. 82400 and 105438. 

It is essential to adhere to the limits set by Allah and beware of using the word of divorce in arguments or at times of anger. Divorce is only prescribed to end the relationship between the spouses, and the husband should not use it except with wisdom and caution to end the relationship. 

And Allah knows best.