Avatar: The Last Airbender introduced metalbending, but The Legend of Korra explained why they can't bend platinum. While there are four main types of elemental bending in the Avatar franchise, each form has special abilities and extensions. One example was the discovery that earthbenders could control metal. Metalbending was invented by Toph Beifong when she was a teenager and she was one of the only people who could bend metal for some time.
By the time of The Legend of Korra, though, metalbending became much more popular. The police of Republic City were all metalbenders led by Toph's daughter, Lin Beifong. They mastered the ability to control metal to take down criminals, but they soon discovered a threat they couldn't stop in season 1. Future Industries founder and secret Equalist supporter Hiroshi Sato created large mechanical suits that non-benders could get into to fight benders. When Korra, Lin, Tenzin, and the rest of Team Avatar discover them, they soon learn that Lin cannot stop them using metalbending because they are made out of platinum.
The limits of metalbending were never explored in Avatar: The Last Airbender, so the reveal that platinum is somehow impervious to metalbending came as a surprise. However, this is a known weakness of metalbenders in the Avatar world at this time. The explanation given for metalbenders not being able to control platinum is because it is too pure of a metal. The core of metalbending is based around the idea of finding the unpurified pieces of earth within the metal and bending the material that way. But, since platinum is one of the purest forms of metal, there is not enough earth for benders to manipulate.
It is a shame for metalbenders that they can't manipulate platinum, as it could be one of their best resources. Platinum is a very malleable and ductile metal that would make it an ideal substance for metalbenders to use. While metalbenders are already plenty strong using whatever metal is at their disposal, platinum would make them even tougher. It is also able to maintain shape at high temperatures, which could allow it to help protect metalbenders from firebenders. Platinum is quite rare too, so any metalbender that could've had access to it could've had a real advantage.
Even though The Legend of Korra establishes that bending platinum is not possible, there are some theories that this changes by the end of the show. One example of Korra controlling platinum chains against Zaheer has been debunked as an example of metalbending. There are also some fans who believe she used metalbending to free herself from chains. This has yet to be proven to be Korra metalbending platinum. But if that theory is ultimately proven to be correct, Korra will be even more powerful than fans previously believed. Now, if the Avatar: The Last Airbender universe continues on at some point, perhaps more information on what can and can't be done with metalbending and platinum will be revealed.