Transition-metal-promoted reactions of boron hydrides. 7. Platinum(II) bromide catalyzed cage growth and dehydrocoupling reactions of diborane with small polyhedral carboranes and boranes: synthesis of a new arachno carborane, 5,6-C2B6H12, and the diborane-coupled compounds 2:1',2'-[1,6-C2B4H5][B2H5