Regarding Non Linear Regression in Matlab

Hello there,

I have a question regarding non-linear regression fit in matlab.I need to fit the following model and optimize the coefficients:

f(x) = 4*0.035523978*((1.380622E-23*293.15)/(3*pi*u*d))*(1-(9/16)*(d/(d+ 2*x))+(1/8)*(d/(d+2*x))^3-(45/256)*(d/(d+2*x))^4-(1/16) *(d/(d+2*x))^5)

I need to find the value of the coefficients u & d in this equation provided a set of 'x' and 'f(x)' data.I tried to use 'cftool' command with custom equation but the algorithm somehow not being able to optimize the coefficients with 95% confidence intervals.The results are giving exact the same value as whatever I am putting as my initial guess.Any help would be highly appreciated.