The substitution of a terminal hydride ligand in the complexes [Ir-2(mu -H)(mu -Pz)(2)H-3(L)(PPr3)-Pr-i)(2)] (L = NCCH3 (1) or pyrazole (3)) by chloride provokes a significant change in the lability of the L ligand, despite the fact that the substituted hydride and the L ligand lie in opposite extremes of the diiridium(III) complexes. Detailed structural studies of complex 3 and its chloro-trihydride analogue [Ir-2(mu -H)(eta -Pz)(2)H2Cl(HPz)((PPr3)-Pr-i)(2)] (4) have shown that this behavior is a consequence of the transmission of ligand trans effects from one extreme of the molecule to the other, with the participation of the bridging hydride. Extended Huckel calculations on model diiridium complexes have suggested that such trans effect transmissions are due to the formation of molecular orbitals of a symmetry extended along the backbones of the complexes. This is also an expected feature for metal-metal bonded complexes. The feasibility of the transmission of ligand trans effects and trans influences through metal-metal bonds and its relevance to the understanding of both the reactivity and structures of metal-metal bonded dinuclear compounds have been substantiated through structural studies and selected reactions of the diiridium(II) complexes [lr(2)(mu -1,8-(NH)(2)naphth)I(CH3)(CO)(2)((PPr3)-Pr-i)(2)] (isomers 6 and 7) and their cationic derivatives [lr(2)(mu -1,8-(NH)(2)naphth)(CH3)(CO)(2)((PPr3)-Pr-i)(2)](CF3SO3) (isomers 8 and 9).