REACTIONS OF ACETYLENES WITH NOBLE-METAL CARBONYL HALIDES .11. SYNTHESIS OF [PT(CME=C(H)COOME)(CO)CL] VIA POLYNUCLEAR PLATINUM CARBONYLS - X-RAY CRYSTAL-STRUCTURE DETERMINATION OF THE SIGMA-ALKENYL PLATINUM COMPLEX [N(PPH3)2][PT(CME=C(H)CO2ME)(CO)CL2]