Mechanism of Silver-Promoted Ligand Metathesis in Square-Planar Complexes of d 8 Ions. Kinetics of Formation and Molecular Structures of a Trinuclear Intermediate [(Me)(N−N)Pt(μ-Cl)Ag(μ-Cl)Pt(N−N)(Me)] + and Its Dinuclear Evolution Product [(Me)(N−N)Pt(μ-Cl)Pt(N−N)(Me)] + (N−N = ArNC(Me)C(Me)NAr, Ar = 2,6-( i -Pr) 2 C 6 H 3 )