Description: This article discusses mechanistic studies of ethylene hydrophenylation catalyzed by bipyridyl Pt(II) complexes. Abstract: Cationic platinum (II) complexes [(ᵗbpy)Pt(Ph)-(L)]⁺[ᵗbpy=4,4'-di-tert-butyl-2,2'-bipyridyl; L = THF, NC₅F₅, or NCMe] catalyze the hydrophenylation of ethylene to generate ethylbenzene and isomers of diethylbenzene. Using ethylene as the limiting reagent, an 89% yield of alkylarene products is achieved after 4 h at 120 ˚C. Catalyst efficiency for ethylene hydrophenylation is diminished only slightly under aerobic conditions. Mechanistic studies support a reaction pathway that involves ethylene coordination to Pt(II), insertion of ethylene into the Pt-phenyl bond, and subsequent metal-mediated benzene C-H activation. Studies of stoichiometric benzene (C₆H₆ or C₆D₆) C-H/C-D activation by [(ᵗbpy)Pt(Ph-dn)-(THF)]⁺ (n = 0 or 5) indicate a kʜ/kᴅ = 1.4(1), while comparative rates of ethylene hydrophenylation using C₆H₆ and C₆D₆ reveal kʜ/kᴅ = 1.8(4) for the overall catalytic reaction. DFT calculations suggest that the transition state for benzene C-H activation is the highest energy species along the catalytic cycle. In CD₂Cl₂, [(ᵗbpy)Pt(Ph)(THF)][BAr'₄] [Ar' = 3,5-bis(trifluoromethyl)phenyl] reacts with ethylene to generate [(ᵗbpy)Pt(CH₂CH₂Ph)(ɳ²-C₂H₄)][BAr'₄] with kobs = 1.05(4) x 10⁻³ s⁻¹ (23 ˚C, [C₂H₄] = 0.10(1) M). In the catalytic hydrophenylation of ethylene, substantial amounts of diethylbenzenes are produced, and experimental studies suggest that the selectivity for the ...