FX ChemStruct uses a complicated algorithm to determine where the "next" element is attached to the existing structure. Sometimes this can cause problems if you want to leave a "bonding site" unbonded.
For example:
EDTA
Imagine you are trying to enter this structure into FX ChemStruct. You might start with one of the N's and move out one of the arms
nch2coo
So far so good. Now if you attempt to start another arm - by typing a C - things start to go wrong
nch2cooc
FX ChemStruct has no way of knowing that you have finished the first arm and attached the C to the remaining bonding site on the O. Oops! What we need is a way of telling FX ChemStruct to stop there. You can do this using the * key.
nch2coo*c
The * has prevented the C from bonding to the O so FX ChemStruct bonds the C to the N instead.
To enter EDTA we can use
nch2ch2nch2coo*ch2coo*ch2coo*ch2coo* or nch2ch2n(ch2coo*)4
Note: There are other possible ways of entering EDTA - but all require the use of the * key.