Here are another ones useful document and article about IM in a business sphere:
Could Secure Instant Messaging Software Boost Your Business?
What is the Emphasis on Choosing an IM Software For Business
Try a look at Bopup IM software that is especially designed for private business communications with strong encryption. The main key is that they offer a stand-alone IM server so nothing would be hosted outside company’s network and environment, i.e. history, accounts, user sessions, file transfers.