A distorted trigonal-bipyramidal phosphorane with a diequatorial four-membered ring; X-ray crystal structure of 4-p-bromophenyl-2′,2′,3′,4′,4′-pentamethyl-3,3,6,6-tetrakistrifluoromethyl-2,7-dioxa-1-phospha(V)bicyclo[3,2,0]hept-4-ene-1-spiro-1′-phosph(V)etan