Bb Minor 9b5, 1st inversion (C# in the bass)
Also written as Bbm9b5/C#.
Notes, from the bass up
C# (bass)EG#B#A#
The same notes as Bb Minor 9b5 (C#, E, G#, B# and A#), rearranged so C# is the lowest note. Use it to move between chords with smaller jumps across the keyboard.