Bb 7sus2, 1st inversion (B# in the bass)
Also written as Bb7sus2/B#.
Notes, from the bass up
B# (bass)E#G#A#
The same notes as Bb 7sus2 (B#, E#, G# and A#), rearranged so B# is the lowest note. Use it to move between chords with smaller jumps across the keyboard.