Resolution of a planar-chiral platinum(II) complex. Crystal and molecular structure of [SP-4-2-A]-(+)589-[PtCl(achiraphos)(R-PMenPh2)]PF6[achiraphos = (R*,S*)-2,3-bis(diphenylphosphino)butane; R-PMenPh2 = [1(R)-(1.alpha.,2.beta.,5.alpha.)]-diphenylmenthylphosphine]