You will probably also have to offer similar ones to your own employees as well as the acquired firm. You are sending a message for the future. Employees who were going to leave will learn to stretch their stay in the hopes of a retention bonus. Unfortunately their productivity levels are likely to be very low during their wait. Paying people for staying sends a message that performance doesn't matter.
Money is an interesting but insufficient cause for working hard.
Retention bonuses do not require unhappy workers to be quiet. Expect whining and bitching. Think of the analogy of paying your teenage son to stay with you in your house as you remarry and your new spouse move in. Target IT people that are at risk. Their value changes rapidly and they also tend to be less tolerant when career growth slows.