Bb 7b13, 2nd inversion (E# in the bass)
Also written as Bb7b13/E#.
Notes, from the bass up
E# (bass)G#F#A#D
The same notes as Bb 7b13 (E#, G#, F#, A# and D), rearranged so E# is the lowest note. Use it to move between chords with smaller jumps across the keyboard.