There should be no adaptor.
I recently bought the exact same part. It is the 2nd O2 sensor, correct?
It should fit, unless it's different for the 220 as opposed to the 280, which could be the problem also! Make sure there is no difference.
And make sure that the sensor is for the rear, 2nd, and/or feedback sensor.

__________________
2006 E350 w/ 155k miles (Daily Driver)

Previous:
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)