Thermolytic rearrangement of bis((trimethylsilyl)methyl)platinum(II) complexes with phosphine ligands, cis-Pt(CH(2)SiMe(3))(2)L(2) [L = PMe(3), PEt(3), PPhMe(2), PPh(2)Me, PPh(3)], results in a novel migratory isomerization which yields the asymmetric organoplatinum(II) products cis-Pt(CH3)(CH(2)SiMe(2)CH(2)SiMe(3))L(2). Similarly, most analogous bis((aryldimethylsilyl) methyl) platinum(II) complexes, cis-Pt(CH(2)SiMe(2)C(6)H(4)R)(2)L(2) [R = H, L = PMe(3), PEt(3), PMe(2)-Ph, PPh(3), P-4-tol(3), PPh(2)C(6)H(4)-2-CF3; L(2) = 1,2-bis(dicyclohexylphosphino)ethane, dcpe; R = 2-Me, 3-Me, 4-Me, 4-OMe, 4-Bu(t), 4-F, 2-CF3, 3-CF3, 4-CF3, 4-NMe(2), L = PPh(3)] and cis-Pt(CH(2)SiMe(2)C(6)F(5))(2)(PPh(3))(2), react to give cis-Pt(C(6)H(4)R) (CH(2)SiMe(2)CH(2)SiMe(2)C(6)H(4)R)L(2) and cis-Pt(C6F5)(CH(2)SiMe(2)CH(2)SiMe(2)C(6)F(5)) (PPh(3))(2), respectively. In some cases, the trans-isomer is formed, either as primary product [R = H, L = PEt(3)] or after prolonged thermolysis [R = H, L = PMe(3), PPhMe(2)]. Reaction of the deuterium labeled analogues, cis-Pt[CH2Si(CD3)(3)](2)L(2) [L = PPh(3), PPh(2)Me, PEt(3)], gives only cis-Pt(CD3)[CH2Si(CD3)(2)CH2Si(CD3)(3)]L(2), establishing that it is a silicon-methyl bond which cleaves during the rearrangement, with secondary isotope effects, k(H)/k(D) = 1.4-1.7. Thermolysis of cis-Pt(CH(2)SiMe(2)C(6)D(5))(2)L(2) [L = PPh(3), PPhMe(2)] reveals similar isotopic inhibition (k(H)/k(D) = 1.2-1.4). Rearrangement of cis-Pt(CH(2)SiMe(3))(CH(2)SiMe(2)Ph)(PPh(3))(2) affords only cis-Pt(Ph)(CH(2)SiMe(2)CH(2)SiMe(3))(PPh(3))(2), confirming further that phenyl migration is more facile than methyl migration. Aryl migration is regiospecific, is accelerated by electron-withdrawing substitutents, and is retarded by electron-donating groups. Linear Hammett correlation emerges, with rho = +0.9. Where L is monodentate, lability increases with increasing steric bulk of L, and the presence of free phosphine inhibits reaction. Deuteration of PPh(3) in both cis-Pt(CH(2)SiMe(3))(2)(PPh(3))(2) and cis-Pt(CH(2)SiMe(2)Ph)(2)(PPh(3))(2) also slows isomerization (k(H)/k(D) = 1.5 and 2.1, respectively).