public sealed class Legend : MapSurround, ArcGIS.Desktop.Mapping.IElement, System.ComponentModel.INotifyPropertyChanged, System.IComparable, System.IDisposable, System.IEquatable<Element>  

{
  //Create a legend for an associated map frame.
  //Note: Must be on QueuedTask.Run

  //Build 2D envelope geometry
  Coordinate2D leg_ll = new Coordinate2D(6, 2.5);
  Coordinate2D leg_ur = new Coordinate2D(8, 4.5);
  Envelope leg_env = EnvelopeBuilderEx.CreateEnvelope(leg_ll, leg_ur);

  //Reference MF, create legend and add to layout
  mapFrame = layout.FindElement("New Map Frame") as MapFrame;
  if (mapFrame == null)
  {
    //TODO handle null map frame
    return;
  }
  var legendInfo = new LegendInfo()
  {
    MapFrameName = mapFrame.Name
  };
  Legend legendElm = ElementFactory.Instance.CreateMapSurroundElement(
                      layout, leg_env, legendInfo, "New Legend") as Legend;
}

{
  //Note: Must be on QueuedTask.Run

  //Build geometry
  Coordinate2D ll = new Coordinate2D(6, 2.5);
  Coordinate2D ur = new Coordinate2D(8, 4.5);
  Envelope env = EnvelopeBuilderEx.CreateEnvelope(ll, ur);

  //Reference MF, create legend and add to layout
  MapFrame mf = layout.FindElement("Map Frame") as MapFrame;
  var surroundInfo = new LegendInfo()
  {
    MapFrameName = mf.Name
  };

  var legendElm = ElementFactory.Instance.CreateMapSurroundElement(
    layout, env.Center, surroundInfo) as Legend;
  legendElm.SetName("New Legend");
}