A goal three minutes from the end of an at times scrappy encounter at Fir Park cruelly ended United hopes of a place in the semi-final of the Co-op Cup at Fir Park.
The tie looked to be heading for extra time until three minutes from the end of the regulation 90 minutes until disaster struck.
Buaben's clearance from Hateley's corner came off the post and hit Gomis and the ball fell to Gow who delivered a killer strike from ten yards.
Compounding the misery was the denial of a first half penalty when Scott Robertson appeared to be on the point of scoring when impeded by Steve Jennings in the unlucky 13th minute.
With Manager Peter Houston having called for the side to hit the ground running, there was no repeat of the poor opening period performances which had proved so injurious in the previous games against Celtic and Motherwell in the league.
Indeed, it was United who almost struck first through David Goodwillie, but his prodded effort was straight at Darren Randolph.
But Motherwell had their moments none more so seven minutes from the interval when Dusan Pernis denied Nick Blackman at close range.
The second half failed to provide anything so clear cut until that late Gow blow.
And in the little time that was left, a stramash on the goaline from a Cadamarteri shot was as close as United came to salvaging something.
Teams:
Motherwell:Randolph, Hammell, Reynolds, Craigan (c), Hateley, Humphrey , Jennings (Forbes 88), Murphy (Sutton 20), Saunders, Blackman, Gow (Page 89)
Subs not used:
Hollis, McHugh, Page
United:Pernis, Watson, Dillon, Kenneth, Dixon (Douglas 46), D Robertson (Shala 88), S Robertson, Buaben, Gomis, Daly (c) (Cadamarteri 79), Goodwillie
Subs not used:
Banks, Dow
Referee: Willie Collum
Attendance: 4838
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