The reaction of Pt(IV) organometallic cyclometalated complexes of type [PtXAr2(Ar'CHNCH2CH2NMe2)] to produce 5- and 7-membered Pt(II) metalacycles by a formal C-C reductive elimination/C-H oxidative addition/Ar-H reductive elimination sequence has been studied from a preparative and kinetico-mechanistic perspective. The detection and characterization of key intermediates has also been achieved via the careful selection of reaction conditions, including time, extracted from the kinetic studies. From the data collected, it is clear that a fine-tuning of the reactivity is possible with respect to the formation of the alternative 5- and 7-membered cyclometalated complexes (i.e., [PtX(Ar-Ar'CH=NCH2CH2NMe2)] and [PtX(Ar-Ar'CH=NCH2CH2NMe2)]). In all cases a common reductive elimination reaction occurs to form a non cyclometalated intermediate compound of type [PtX(Ar)(Ar-Ar'CH=NCH2CH2NMe2)}, which leads to the selective formation of the above-mentioned complexes by the actuation of an unexpected equilibrium between its cis-(X,NMe2) and trans-(X,NMe2) isomers. While the cis-(X,NMe2) isomer produces the 7-membered metallacycle, the trans-(X,NMe2) form leads exclusively to the 5-membered analogue. The isomerization process is dominated by the steric hindrance existing between the Ar-Pt and Ar-Ar'CHN-Pt moieties in the cis-(X,NMe2) isomer that forces a Z conformation of the imine, thus leading exclusively to the 7-membered ring formation only for the less hindered X = Cl systems.