Complexes of Platinum(II) Containing Neutral and Deprotonated 9-Methyladenine. Synthesis, X-Ray Structures and NMR Studies on the Cyclic Trimer cis-[L2Pt{9-MeAd(-H)}](3)(NO3)(3) and the Dinuclear cis-[L2Pt(ONO2){9-MeAd(-H)}PtL2](NO3)(2) (L = PMePh2)