∵点C在直线AB上,即在直线y= 1 2 x-2上,点C的纵坐标为-1,∴代入得:-1= 1 2 x-2,解得,x=2,即C(2,-1),∴OM=2,∵CD∥y轴,S△OCD= 5 2 ,∴ 1 2 CD×OM= 5 2 ,∴CD= 5 2 ,∴MD= 5 2 -1= 3 2 ,即D的坐标是(2, 3 2 ),∵D在双曲线y= k x 上,∴代入得:k=2× 3 2 =3.故答案为:3.