From The StarCraft 2 Wiki

The Merc Compound is a Starcraft 2 terran buildin which upgrades marines,unlocks reapers and trains mercenaries, but only in story mode.

Upgrades researched: Ballistic Alloy Combat Shield

Abilities researched: Stim Pack, D-8 Charge

[edit] Changes

The Merc Compound was called Merc Haven until November 2008. The merc haven used to train reapers. Training reapers worked similarly to hiring units from mercenary camps in WarCraft III. There was a maximum number of units "in stock" at any one time with that number slowly regenerating over time. Reapers were inexpensive with a build time of one or two seconds. Four reapers could be bought from a single merc haven in quick succession with another reaper becoming ready to "hire" every thirty seconds.

[edit] Trivia

• In the pre-alpha build shown at the Zerg reveal in March 2008, the little hologram on top of the Merc Haven will do the jumping animation that Night Elves in World of WarCraft do when idle.

Navigation - Protoss - Terran - Zerg [e]

Terran Units (StarCraft II) [e]
Ground Units:	SCV | Marine | Marauder | Ghost | Reaper | Hellion | Siege Tank | Thor
Air Units:	Viking | Medivac Dropship | Raven | Banshee | Battlecruiser
Basic Buildings:	Command Center | Supply Depot | Refinery | Barracks | Bunker | Engineering Bay | Missile Turret | Planetary Fortress | Radar Tower
Advanced Buildings:	Ghost Academy | Factory | Merc Haven | Orbital Command | Armory | Starport | Deep Space Relay
Support Buildings:	Tech Lab | Reactor
Hero ground units:	Jim Raynor | Tychus Findlay
Cancelled Units:	Cobra | Firebat | Predator | Medic | Dropship
Cancelled Buildings:	Sensor Tower | Munitions Depot | Starbase | Shadow Ops

Terran Units (StarCraft I & Brood War) [e]
Ground Units:	SCV | Marine | Firebat | Medic | Vulture | Goliath | Siege Tank | Ghost
Air Units:	Dropship | Wraith | Valkyrie | Battlecruiser | Science Vessel |
Basic Buildings:	Command Center (ComSat Station & Nuclear Silo) | Supply Depot | Engineering Bay | Barracks | Refinery | Missile Turret | Academy | Bunker
Advanced Buildings:	Factory | Machine Shop | Armory | Starport (Control Tower) | Science Facility (Physics Lab & Covert Ops)
Hero ground units:	Gui Montag (fb) | Terran Civilian | Sarah Kerrigan (g) | Samir Duran (g) | Alexei Stukov (g) | Alan Schezar (gol) | Jim Raynor (vul, mrn) | Edmund Duke (st)
Hero air units:	Tom Kazansky (wrt) | Magallan (sv) | Hyperion (bc) | Norad II (bc)
(fb) = Firebat -- (g) = Ghost -- (gol) = Goliath -- (v) = Vulture -- (m) = Marine -- (st) = Siege Tank -- (wrt) = Wraith -- (sv) = Science Vessel -- (bc) = Battlecruiser