While I haven't personally done this, I can speak from experience in having run a full square set on my E38 for years for this very reason.

Since you know there are square sets of 19" rims, and you also know the front 214 rim has the proper backspacing to fit the front, therefore it must fit the back too. Assuming you're not talking about an X5M, then you have issues with different center bores.

This can all be confirmed by running numbers and figuring out what the exact backspacing is, by comparing wheel width with the offset and doing some math.

The issue you'll probably run into is aesthetic - do you have wide wheel flares, or stock fender arches? I'm guessing the stock arches may look better with a square set of rims on there.

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2009 E70 N62 Titansilber Sport
2000 E38 M62 Schwartz II Sport
1997 E36/5 Schwartz II M-Sport (gone)