U/s at 6 wks 2 days showed yolk sac but no embryo and measurements of only 5.5 weeks. used hcg trigger so exact dates- what are chances of viable baby?

Barely visible : if your hcg is normal, don't despair yet. The embryo can be BARELY visible at this stage. I don't know what your doctor's plan is, but the embryo and sac grows rapidly right now. An ultrasound in 2 weeks will be a definite answer.

Expect heart beat: With accurate dating, the ultrasound technology available and a skilled monographer a crown lump length should be measurable and a fetal heart beat demonstrable. If a transvaginal ultrasound in a week still does not show an embryo with a heart beat, I would be concerned that this is a non viable pregnancy.