Reaction of Model Nucleobases with the Diaqua(bis(N-methylimidazol-2-yl) ketone)platinum(II) Dication. Synthesis and Structure of the Head-to-Tail Isomers of Bis(9-methylguanine-N7) (bis(N-methylimidazol-2-yl) ketone)platinum(II) Perchlorate, Bis(1-methylcytosine-N3) (bis(N-methylimidazol-2-yl) ketone)platinum(II) Perchlorate, Bis(μ-1-methylthyminato-N3,O4)-bis[(bis(N-methylimidazol-2-yl) ketone)platinum(II)] Perchlorate, and Bis(μ-1-methyluracilato-N3,O4)bis[(bis(N-methylimidazol-2-yl) ketone)platinum(II)] Nitrate