When you complain to another person about a problem employee, you're automatically talking to the wrong person. It may seem pretty obvious, but it's the problem employee you should be talking with.
Complaining to colleagues -- especially ones you don't know that well -- or to the employee's co-workers, usually erodes credibility. Consequently, people stop being as open with you for fear that you can't keep anything confidential. The more you complain, the more people see you as someone who is interested only in talking about problems, not in solving them.
Complaining doesn't move you closer to solving a problem; it simply makes you feel better because you're getting support or your feelings (assuming the person gives you a sympathetic ear). Or it fools you into thinking you're doing something constructive.
Source: Problem Employees: How to Improve Their Performance, by Peter Wylie and Mardy Grothe, published by Pitman Learning Inc., Belmont, Calif.