Stereochemistry at the Chiral Ruthenium Atom in Half-Sandwich Compounds of the Type (.eta.6-Ar)Ru(LL*)L'X and (.eta.6-Ar)Ru(LL*)-Hal. X-ray Structure Analysis of (.eta.6-cymene)Ru(LL*)(4-Mepy)PF6 and Appeal to the Proper Use of Stereochemical Terminology