Coordination chemistry of 7,9-disubstituted 6-oxopurine metal compounds. 3. Platinum(II) coordination at N(1). Molecular and crystal structure of (diethylenetriamine)(7,9-dimethylguanine)platinum(II) hexafluorophosphate and (diethylenetriamine)(7,9-dimethylhypoxanthine)platinum(II) hexafluorophospha