With three games remaining in the season, the D-backs still need one more win to ensure a .500 record.
"It's tough, we played a very sloppy game," D-backs manager Kirk Gibson said. "They scored seven runs and only three were earned, that's tough."
The club led by a run entering the sixth inning, but a Mike Jacobs fielding error at first base allowed the Cubs to load the bases with one out.
After Mike Zagurski struck out Luis Valbuena, the D-backs appeared as if they would escape from the inning without surrendering any damage when Matt Albers induced a grounder to first.
But running hard out of the box, Dave Sappelt beat Albers to the bag and the Cubs tied the game on the play.
Albers proceeded to hit the next batter to force in the go-ahead run before serving up a two-run single to Bryan LaHair to blow the game open, 5-2.
An inning later, two more D-backs fielding errors in the infield let the Cubs tack on another run.
"We had a full bullpen, and we thought we could get through it that way," Gibson said. "It didn't work out today."
The late collapse left Collmenter with a no-decision to finish his season after he tossed five innings of one-run, six-hit work.
In two starts since rejoining the rotation, the 26-year-old has only given up two runs over 10 innings.
"It was good to be able to get some spot starts and throw a couple times to end on a good note," Collmenter said. "That's a positive to take into next year."
The season certainly didn't start as well as it ended for Collmenter. Beginning his 2012 campaign in the rotation, the right-hander ended April with a 9.82 ERA, leading to his relegation to the bullpen.
"It started off pretty terrible," he said. "But I learned to stay true to myself. I know what I can do. Once I got back to throwing strikes down in the zone, it was good. Earlier on, I was all over the place."
Since his disappointing launch to the season, Collmenter has been tremendous for the D-backs the last five months, boasting a 2.13 ERA since May over 72 innings in 24 appearances, including seven additional starts.
"I had to look at some film, I wasn't getting up and over," he said. "Once I got that worked out, it all rolled. Take out the four starts and everything went well the rest of the season."
Once a large part of the reason for the club's poor start to the season, Collmenter's versatility in moving back and forth from the bullpen proved to be one of the most valuable assets the D-backs possessed in 2012.
"He's been great, he found it and he's got a good work ethic," Gibson said. "He's got a great confidence about himself. He's been really steady with what we've asked him to do."
Moving forward into 2013, the D-backs aren't sure what they plan to do with Collmenter. If it were up to Collmenter, though, he'd prefer to start.
"It doesn't matter a whole lot, whatever I can do to help the team, but from a personal standpoint, I'd rather start," he said. "I enjoy starting and being the guy the team counts on every five days. In this role, too, I relish it, because I know what a long man means to the bullpen."
The Cubs jumped on Collmenter early on Sunday, producing the first run of the game on first inning doubles from David DeJesus and Anthony Rizzo to go ahead, 1-0.
"I guess if you're going to win one, it might as well be the last one on the West Coast," said Cubs manager Dale Sveum, whose club moved to 1-18 on the road against the National League West this season. "It's nice we got some breaks."
In the bottom of the first, the D-backs responded with the bases loaded and two outs when Cody Ransom hit a dribbler down the third-base line for an infield single to tie the game.
The play didn't end there, however. Justin Upton, running from second base, never stopped and beat home a throw from Cubs pitcher Chris Rusin as the ball got away from the catcher and the D-backs took a 2-1 advantage.
On the season, Upton is now second in the NL in runs scored with 106.
"It's just good stuff, that's what he has the ability to do," Gibson said. "That's the reason why that happens, and that's one of the tools he possesses."
The club's offense didn't show much life throughout the rest of the game, tallying just four hits over the final eight innings.