The complex Lr(CO)(mes)dppe (1) (mes = 1,3,5-trimethylphenyl, dppe = 1,2-bis(diphenylphosphino)ethane) has been synthesized from the reaction of IrBr(CO)(dppe) and mesitylmagnesium bromide. Complex 1 crystallizes in the monoclinic space group, P2(1)/n (No. 14), with Z = 4, a = 8.953(3) Angstrom, b = 22.752(6) Angstrom, c = 16.212(6) Angstrom, and beta = 99.00(3)degrees, and displays a slight tetrahedral distortion from square planarity. A benzene solution of 1 undergoes an unusual transformation when heated to 90 degrees C under ethylene to form the five-coordinate dimethylbenzyl complex Lr(eta(2)-C2H4)(CH2C6H3(CH3)2)(CO)(dppe) (2) via o-methyl C-H activation. An X-ray determination of 2 shows that it crystallizes in the orthorhombic space group, Pbca (No. 61), with Z = 8, a = 19.113(8) Angstrom, b = 16.243(9) Angstrom, c = 23.174(7) Angstrom. Complex 1 under CO undergoes aryl migration to form the dicarbonyl adduct Lr(Co)(2)(C(O)C6H2(CH3)(3))(dppe) (6). After a benzene solution of complex 6 is heated, an equilibrium of trans-IrH(CO)(CH2C6H2(CH3)(2)C(O))(dPPe) (7), cis-IrH(CO)(CH2C6H2(CH3)(2)C(O))(dppe) (8), and 6 is formed. Complexes 7 and 8 are formed by o-methyl C-H activation. A crystal structure determination of 8 shows that it crystallizes in the monoclinic space group P2(1)/n (No. 14), with Z = 4, a = 10.701(3) Angstrom, b = 14.554(3) Angstrom, 20.760(6) Angstrom, and beta = 90.19(3)degrees. Solutions of complex 1 undergo stereospecific oxidative additions of HY (Y = SiHPh(2), H) to form IrH(Y)(CO)(mes)(dppe) (9) (Y = SiHPh(2)) and 10 (Y = H), respectively. The complex LrI2(C(O)C6H2(CH3)(3))(dppe), 11, is formed when a benzene solution of 1 is treated with a benzene solution of I-2. Under HCl gas, a CD2Cl2 solution of 1 is readily converted to IrHCl2(CO)(dppe) (12) and free mesitylene.