Bb Minor Add9, 3rd inversion (B# in the bass)
Also written as Bbmadd9/B#.
Notes, from the bass up
B# (bass)A#C#E#
The same notes as Bb Minor Add9 (B#, A#, C# and E#), rearranged so B# is the lowest note. Use it to move between chords with smaller jumps across the keyboard.