Drain chicken and rinse through cold water. Drain well. In small mixing bowl, combine chicken, mayonnaise, celery, and pecans; mix well with fork. Add salt and pepper to taste. Gently fold in the grapes.
On toasted whole wheat bread or toasted cocktail bread, spread desired amount of chicken mixture onto one of the slices of bread. Top with a slice of tomato and the other slice of bread.
Store any leftover chicken salad in the refrigerator.