Yes, 135.101 requires an SIC for IFR pax ops. 135.105 allows the ops specs work around. And 61.51 says you can log it.
(2) Holds the appropriate category, class, and instrument rating (if an instrument rating is required for the flight) for the aircraft being flown, and more than one pilot is required under the type certification of the aircraft or the regulations under which the flight is being conducted.