How to fit a complex function

I have a complex function that I want to fit to some real data. The function is of the form: Y = -10*imag((A-B)/(A+2*B))

where "imag" means imaginary part of the equation A = a - (b/X)*1i B = c - (d/X)*1i

Y has both imaginary and real parts. But it's the imaginary part that is relevant to my problem. How do I derive the values of a,b,c and d? I have tried to use "lsqcurvefit" but it keeps returning the same starting values I put in-I guessed I must have done wrong. NB: I am new at MATLAB. Thanks