Relation between structure and magnetic exchange interactions in bis(hexafluoroacetylacetonato){N,N'-ethylenebis[2-hydroxypropiophenone iminato-N,O(2-)]copper(II)}M'(II), Cu((prp)2en)M'(hfa)2, where M' = Cu, Ni, and Mn. Crystal structures of the complexes(M((prp)2en)M'(hfa)2)