Most surface online sources (i.e. wikipedia) seem to quote HMPA as coordinating through the nitrogen atom. Fine, nitrogen is less electron negative, and thus more basic, I get that.
But in the case of HMPA, wouldn't the oxygen be a more powerful coordinator/donor, due to ionic contributions? The donating effects of all those nitrogen lone pairs, combined with the withdrawing effect of the oxygen atom, would that not effectively make the oxygen quite polarised and thus give it stronger coordinating power to a cation due to charge based interactions? I'm probably making this up but I can't actually find any references or evidence to back up either possibility (if coordinating through oxygen even is a possibility...). What I'm trying to say is that, is the oxygen not more nucleophilic and/or basic than the nitrogen, due to the inductive effects of those nitrogens and the withdrawing effect of the oxygen?
Any help much appreciated..