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-N-7) (bis(N-methylimidazole-2-yl) ketone)platinum(II) perchlorate, bis(1-methylcytosine-N-3) bis(N-methylimidazol-2-yl) ketone)platinum(II) perchlorate, bis(mu-1-methylthyminato-N-3,O-4)-bis[(bis(N-methylimidazol-2-yl ketone)platinum(II)] perchlorate, and bis(mu-1-methyluracilato-N-3,O-4)bis[(bis(N-methylimidazol-2-yl) ketone)platinum(II)] nitrate (vol 33, pg 3881, 1994)