Not quite equivalent, since System.Tuple is immutable. So, at minimum:

  public class Complex {
    private readonly double _i;
    private readonly double _j;
    public double I {get {return _i;}}
    public double J {get{return _j;}}
    public Complex(double i, double j){
      _i = i;
      _j = j;
    }
  }

But System.Tuple is also IComparable, IStructuralComparable, IStructuralEquatable. I haven't had enough coffee yet to add all the boilerplate for that to the above, which only reinforces the point about verbosity.

Although with c# 6.0 (current release) you can simplify that to:

  public class Complex {
    public double I { get; } 
    public double J { get; }
    public Complex(double i, double j) {
      I = i;
      J = j;
    }
  }

Even before C# 6.0, you could simplify it this way:

  public class Complex {
    public double I { get; private set; } 
    public double J { get; private set; }
    public Complex(double i, double j) {
      I = i;
      J = j;
    }
  }