WHILE CRUISING ON AN IFR FLT AT 10,000 FT, THE PLT INFORMED ATLANTA CENTER THAT HE HAD AN ENG PROBLEM & WAS DECLARING AN EMERGENCY. HE WAS PROVIDED A HEADING TO THE BLUEFIELD ARPT WHICH WAS NEARBY. WHEN ASKED ABOUT THE PROBLEM, HE SAID THE LEFT ENG WAS PROVIDING ONLY PARTIAL POWER. SHORTLY THEREAFTER, THE PASSENGER (HIS WIFE) SAW A BREAK IN THE CLOUDS & THE PLT SAID HE WOULD CIRCLE & TRY TO GET DOWN. HE WAS INFORMED THAT THE HIGHEST OBSTACLE IN THE AREA WAS AT 3800 FT. HE REPORTED GOOD GROUND CONTACT, BUT SAID HE STILL COULD NOT SEE THE ARPT. RADAR CONTACT WAS LOST AT LOW ALT, BUT THE PLT FLEW TO THE BLUEFIELD VOR, ABOUT 1/2 MI FROM THE ARPT. HE THEN ENTERED A TURN AWAY FROM THE ARPT, & SHORTLY AFTER THAT, THE ACFT COLLIDED WITH TREES ON HILLY TERRAIN. THE SURFACE TEMP WAS 30 DEG. ICY SLUSH WAS FOUND IN THE LEFT GASCOLATOR. THE FUEL SELECTORS WERE FOUND IN THE 'AUX' POSITIONS. NO PREIMPACT MECHANICAL MALFUNCTION OR FAILURE WAS FOUND. THE BLADES OF THE LEFT PROP WERE FOUND IN A FLAT ANGLE (NOT FEATHERED) & BENT AFT WITH LITTLE ROTATIONAL DAMAGE.