@Crowriver - those are former RBS buildings, but not the ones most commonly associated with it on the square.
41-42 SAS is the vacant former bank HQ next to the Bank of Scotland / British Linen Bank office. It was built as the HQ of the National Bank of Scotland. Commercial Bank of Scotland was where the Dome now is. These later merged as National Commercial Bank of Scotland, before being taken over in 1969 by RBS.
9 - 19 South St. Andrew Street are 60s or 70s offices for RBS, linked to 41-42 SAS by the walkway over West Register Street.
28 - 50 West Register Street incorporates I think that whole seemingly abandoned block between Greggs and Cafe Royal. It is was built as a paper warehouse for the Cowans (perhaps the same paper lairds of Penicuik), for which purpose it is ridiculously elaborate, must have some of the finest facade carvings in Edinburgh on it. It later became RBS's central stationary warehouse, but I don't think it's been occupied or used by them for many a year.
The first two parts of the site have been vacant since RBS decamped to Gogarburn. Vladimir Romanov attempted to turn it into a hotel at one point and got nowhere. Apparently Fred didn't want to go to Gogarburn, preferred the idea of buying New St Andrew a.k.a. St James House (which they did) and building himself a monument to delusions of grandeur there. The council made sure that never happened. Not sure if that's a good thing or not, it could have prevented building an enormous "drive to" office beyond the green belt city limits and would have gotten shot of New St Andrew House ~ 10 years ago. I suppose it depends if you think a bank HQ would be more or less offensive on the site than the current offerings of unaffordable housing, shops and boutique hotels.