Magnetic exchange interactions in transition metal dimers. 11. Structural and magnetic characterization of [Ni2(tren)2(C6O4Cl2)](BPh4)2 and [Cu2(Me5dien)2(C6O4Cl2)](BPh4)2. Magnetic exchange interactions propagated by the dianions of 2,5-dihydroxy-1,4-benzoquinones