One thing to consider is that you can not only help your alliance by donating gold but also in various other ways, which can be as important/effective as donations. So if you give titles to valuable contributors to alliance success, don’t make it all gold based. Sure, spending real money for the alliance is great and helpful, but not the only way to show support.
Furthermore, dividing between players that earned a donation title and those who didn’t might lower the team spirit inside alliances and create a “we, the useful members and donators” versus “the others that are not worth a title” feeling. Not good!
Also, some players have a lot more money to spend on a game than others do, so just giving titles for a certain amount of spending is unfair to all those “poorer” kings and queens.
Last but not least let’s not forget that alliance tower level mostly determines how much gold you can spend, and that is tied to your ingame progress. Obviously, top players that get tons of gems from diamond league wins can level up their tower quicker/further than players that just started the game. But are those new comers or lower level players automatically worth less? With a title based on donations, a lower (tower) level player has no chance to get rewarded for his effort - and don’t forget, early on = lower loot = even a lower donation means a lot for those players.
Oh and of course, I have to agree with what mischreiber said above.