Are you taking "head shots" of heads of departments - or are you shooting them "in their place of work"?
If it's the former, then maybe you can choose the location for the shoot. If it is only head & shoulders, then maybe you could use the boardroom - or CEO's office - or the canteen (depending on size of company I guess!
)
However, you need to appear professional so I'm not sure I would recommend you turning up with some sheets to hang from a coat-hook (you probably wouldn't - but you get my drift)
As I said, use a room with plenty of natural light, try not to sit them in front of a window and use a tripod if you have one.
If it's option #2 (in their place of work) then only you know what the lighting will be like (as you work there!) If it's too dark then I would decline the job - UNLESS they can be persuaded - in return for their pics - to buy you a speedlite! (now theres an idea!)
Good luck
TJ