Bb Minor 9b5, 4th inversion (B# in the bass)
Also written as Bbm9b5/B#.
Notes, from the bass up
B# (bass)A#C#EG#
The same notes as Bb Minor 9b5 (B#, A#, C#, E and G#), rearranged so B# is the lowest note. Use it to move between chords with smaller jumps across the keyboard.