The bis(ortho-)chelated platinum complex [PtCl(NCN)], 1 (NCN = [C6H3(CH2NMe2)(2)-2 6](-)), has been used as a novel metal precursor for C-H bond activation and subsequent cyclometalation of the potentially terdentate coordinating pincer ligand PCHP (PCP = [C6H3(CH2PPh2)(2)-2,6](-)); Depending on the:reaction temperature various intermediates, each with unique structural features, could be isolated and identified during the formation of the final, bis(ortho-)cyclometalated product [PtCl(PCP)], 2. These results provide valuable:: insight into the intimate steps of this reaction for which we like to introduce the term transcyclometalation process (see note). Ar room temperature, a sparingly soluble dimetallic complex is formed, which comprises a eta (1)-monodentate C-bound NCN ligand:and a bridging, P,P eta (2)-epsilon (2)-coordinating PCHP ligand.:Solution spectroscopy and X-ray analysis of this platinum dimer established a C-H bond which is interacting by intramolecular noncovalent H . . . Cl hydrogen bonding with the Pt-CI motif. As a further intermediate, the formation of a trans-bis(phenyl) complex [Pt(eta (3)-PCP)(eta (1)-HNCN)]Cl, 5, has been identified, which is characterized by a bis(ortho-)cyclometalated PCP ligand which is eta (3)-P, C, P' coordinated to the platinum center as well as an eta (1)-monodentate C-bound NCN ligand. The equivalent HCl, which is formally released during the formation of 5, is intramolecularly trapped by one of the basic amine groups, as is apparent from the identification of precursors of 5, i.e., compounds in which either one ([3]Cl) or two ([4]X-2) of the NMe2 groups have become protonated (see Scheme:5): Remarkably, the proton in [4]X-2 is not only bound to the nitrogen lone pair but also interacts with the filled d(z2) orbital of the nucleophilic platinum(II) center, thus disclosing complex 5 as an organometallic proton sponge that is able to sequester protons due to Pt,N-bidentate chelating properties. Prolonged exposure of the reaction mixture at 80 degreesC or performing the reaction at 110 degreesC afforded the transcyclometalated complex 2 and 1 equiv of the diaminoarene NCHN as the two ultimate products.