Reactions of Acetylenes with Noble-Metal Carbonyl Halides. Part 7. Synthesis and Chemical Characterization of Cationic and Neutral Tetrasubstituted Cyclobutadiene Complexes of Platinum(II). X-Ray Structure of Dichloro (eta4-,2-dimethyl-3,4-diphenylcyclobutadiene) - (triphenylphosphine) platinum(II)