Buchnevich attempted a pass across the slot, but the puck hit the stick of Pittsburgh defenseman Kris Letang and sailed over the glove of Penguins goaltender Tristan Jarry at 11:12 of the third to break a 3-3 tie.
Ondrej Pavelec, who was pegged to start in net when Henrik Lundqvist developed a flu-like illness, stopped 41 shots for the Rangers (15-10-2), who have won six of their past seven and 12 of 15.
Jarry made 25 saves but had his four-game winning streak end. He has been filling in for the injured Matt Murray.
The Penguins (15-11-3) took leads of 1-0 and 2-1 on goals by Conor Sheary and Phil Kessel, but New York answered fairly quickly both times with goals by rookie Boo Nieves and Jesper Fast before a goal by Mats Zuccarello late in the second gave the Rangers a 3-2 lead.
Pittsburgh's Patric Hornqvist forged another tie, 3-3, in the third.
Although New York dominated much of the first, including a 10-4 shot edge, the Penguins broke through first.
Sheary converted a big backhand shovel of a pass through a crowded slot from Hornqvist at 15:13 for a 1-0 Pittsburgh lead.
The Rangers tied it 1:36 later on Nieves' first NHL goal, a shot from the top of the right circle that made it through traffic and under Jarry's pads.
Kessel scored his team-leading 13th goal at 7:11 of the second for a 2-1 Penguins lead when he took a feed from Jake Guentzel and scored from the right circle.
New York tied it once more at 12:26 of the second on a Fast rebound goal.
The Rangers took their first lead, 3-2, at 17:20 of the second when Zuccarello moved in on Jarry and found a sliver of room inside the left post.
Pittsburgh went into the third trailing despite outshooting New York 23-8 in the second, and tied it 3-3 when Evgeni Malkin sprang Hornqvist for a breakaway on a power play. Hornqvist beat Pavelec to the glove side at 3:35.
That gave Hornqvist a four-game goal streak and a six-game point streak.
NOTES: Pittsburgh D Justin Schultz limped off late in the first, returned for the second but did not come out for the third. ... The Rangers recalled G Alexandar Georgiev, who backed up G Ondrej Pavelec with G Henrik Lundqvist ill. ... New York RW Michael Grabner played in his 500th NHL game. ... Pittsburgh G Matt Murray, who missed his third straight game because of a lower-body injury, has resumed skating without gear. ... In the annual Forbes Magazine financial ranking of NHL clubs, the Rangers are No. 1 with a value of $1.5 billion, up 14 percent, while the Penguins are No. 10 with a value of $650 million, up 20 percent.