Sprites overlap Unity C #

Hello I am developing a simulator in unity the problem that I have is with the Sprites that would be being tools in the game / simulator, in this case I must use about 8 sprites that simulate being objects which I have to take them and drag them to another object all this in a "touch" for SmartPhones, what happens is that when I take a sprite and pass on another sprite they are absorbed and then I can not separate them becoming a bug. then I show them the code I use for the movement hopefully can help to know what to add to solve this.

using UnityEngine;
using System.Collections;
using UnityEngine.UI;

public class movimiento : MonoBehaviour {


    public float speed = 0.1F; 
    public bool calor = false;

    // Use this for initialization
    void Start () 
    {

    }

    // Update is called once per frame
    void Update () 
    {
        if (Input.touchCount > 0)
            print(Input.touchCount);

        //Gets the world position of the mouse on the screen        
        Vector2 mousePosition = Camera.main.ScreenToWorldPoint( Input.mousePosition );

        //Checks whether the mouse is over the sprite
        bool overSprite = this.GetComponent<SpriteRenderer>().bounds.Contains( mousePosition );

        //If it's over the sprite
        if (overSprite)
        {
            //If we've pressed down on the mouse (or touched on the iphone)
            if (Input.GetButton("Fire1"))
            {
                //Set the position to the mouse position
                this.transform.position = new Vector3(Camera.main.ScreenToWorldPoint(Input.mousePosition).x,
                    Camera.main.ScreenToWorldPoint(Input.mousePosition).y,
                    0.0f);
            }
        }
    }
    void OnCollisionEnter2D(Collision2D coll) {
        if (coll.gameObject.tag == "cabeza")
            calor = true;
            Debug.Log ("ouch");
            Debug.Log (calor);
    }


}