Darcy Tucker and Mike Peca had Toronto up 2-0 in the first period and, after a scoreless second, Metropolit scored and then set up Vyacheslav Kozlov for two goals. Marian Hossa added a breakaway goal and Ilya Kovalchuk put in an empty-netter with 56 seconds left to complete the onslaught, earning Atlanta a fifth straight win and sent the Leafs to a fifth straight defeat.
Boos rained down on the Leafs as the final seconds evaporated.
The Ottawa Senators moved ahead of Toronto into third place in the Northeast Division with a 4-2 win over the New York Islanders.
The Leafs play in Boston on Thursday and in Detroit on Saturday.
Tucker opened the scoring with his 17th goal, and league-leading 12th on the power play, at 8:41 of the first period. Bobby Holik had taken a slashing penalty at 8:30. Tucker swept in a cross-crease pass from Jeff O'Neill.
Leafs fans among the 19,507 in Air Canada Centre - just about everybody other than a vocal group yelling encouragement to Metropolit - had reason for optimism: Toronto was 9-2-1 when scoring first in its previous 28 games.
Peca made it 2-0 at 19:44 when he planted the puck in the far top corner of the net with a wrist shot from the circle to the right of Johan Hedberg, who got his first start since Nov. 22, while No. 1 goaltender Kari Lehtonen lounged on the end of the Atlanta bench.
The Leafs had an 8-0-1 mark when leading after 20 minutes so there was no reason to expect a collapse.
Atlanta failed to score during three consecutive Toronto penalties - two to Hal Gill - in the opening eight minutes of the middle period. Andrew Raycroft made huge saves on point-blank shots by Jon Sim and Kozlov - the closest anybody came to scoring in the second period.
The Leafs were being outshot but, the way Raycroft was playing, a win seemed probable given that the Leafs were 10-0-3 when leading after two periods.
Metropolit's goal on a power play 3:56 into the third period started the Thrashers rally. He deflected in a long shot by Greg de Vries with Ian White off for delay of game - his second consecutive penalty for lifting the puck into the seats in his own end.
Metropolit then fed a pass from the side boards to Kozlov, who slapped a rocket past Raycroft from the middle of the Toronto zone at 7:14. The Gill-White tandem was on the ice.
Metropolit dug the puck off the back boards and fed Kozlov on Raycroft's doorstep for the go-ahead goal at 11:27. Gill and White again were on the ice.
Gill then took his third penalty of the game to give Atlanta a ninth power-play chance. Hossa scored on a breakaway at 14:21 for his 21st goal of the season.
It was Atlanta's fourth goal of the period - on only six shots.
Notes: Atlanta outshot Toronto 37-19 ... Alex Steen hasn't scored in 23 games since netting his only goal of the season on Oct. 14 ... Bryan McCabe leads all defencemen in fan voting to decide the starting Eastern Conference lineup for the Jan. 24 all-star game in Dallas. McCabe had 245,976 votes as of Tuesday. Tomas Kaberle, who has played better than McCabe so far this season, was sixth with 194,850. The Toronto forward with the most votes on his team is Mats Sundin with 78,574 for 14th place ... Alexei Ponikarovsky's plus-six is the best plus-minus rating among Leafs ... The average for Toronto players is a height of six-foot-two, weight of 206 and age of 28 . . . Come Dec. 20, it'll be three years since Wade Belak scored a goal.