On the high efficiency of cis-dichloro[(S)-a-methylbenzylamine](ethylene)platinum(II) as Chiral Derivatizing Agent for the determination of the enantiomeric composition of chiral unsaturated ethers by 195Pt NMR spectroscopy: a spectroscopic conformational and configurational characterization in solution of diastereoisomeric complexes cis-dichloro[(S)-?-methylbenzylamine][(S)- and (R)-3-phenyl-3-methoxybut-1-ene]platinum(II)