The larger question is why? The button looks not only missing or unbuttoned, but it is deliberately set off to be impossible to button even if the buttonhole were a working buttonhole, which it doesn't appear to be.

https://commons.wikimedia.org/wiki/File ... kko_05.jpg
Emperor Nicholas II's coronation uniform (1896).

https://commons.wikimedia.org/wiki/File ... _photo.jpg
Emperor Alexander II of Russia (c. 1878 or 1881).
Note that this detail was not seen on the 1801 coronation uniform.
http://www.vam.ac.uk/content/articles/m ... 720s-1917/