Bb Minor Major 9th, 1st inversion (C# in the bass)
Also written as Bbm(maj9)/C#.
Notes, from the bass up
C# (bass)E#AB#A#
The same notes as Bb Minor Major 9th (C#, E#, A, B# and A#), rearranged so C# is the lowest note. Use it to move between chords with smaller jumps across the keyboard.