How should buffer value at N/2 get initialized prior to performing an inverse FFT on a buffer, size N, that contains a complex frequency response in only values 0 to N/2-1? It is pretty clear the buffer needs to be made symmetrical for prior to calculating the inverse FFT per table 12-1 of http://www.dspguide.com/ch12/1.htm. What is not clear is how to initialize the value at N/2. Is it related to value at 0?
Normally a buffer of N points of data in the frequency domain covers, much like a two's complement number, -N/2 to +N/2-1. So when you say that it doesn't cover N/2, it probably doesn't matter. If it does matter and there is a "place" for N/2, just do as Paul suggested and set it to 0.