Hughes500/Bob,
To fly legally in IMC a helicopter should be certificated for such flight. Requirements are contained within Appendix B of Parts 27/29 (with the mass of 3,175 kg being the boundary between them).
These appendices describe the stability, equipment and miscellaneous requirements before certification can be granted for flight on instruments. In general (with the exception of the Bell 222 - certificated to an earlier standard) the handling qualities cannot be met without the assistance of stability augmentation systems.
Because these are the minimum standard, States have operational requirements that also have to be met - these describe instruments and additional equipment for flight in IMC.
ICAO Annex 6 Part III contains additional standards for single-engine flight in IMC - these are: reliability measures for the engine; some redundancy of equipment; and, minimum flight conditions and routing - to protect the passengers and third parties. States wishing to permit such flights over their territory, should show compliance with the ICAO SARPs.
Jim