imine insertion into metal–hydride bonds [60]. The same concept was applied to
alkyne insertions leading to bridging alkenyls, in this case aiming to explain
unusual stereoselectivities in stoichiometric [61] or catalytic [62] reactions.
3 Trans Effect in Bimetallic Complexes
The trans effect/influence in binuclear complexes brings about two main phenomena that are not observed in mononuclear complexes: (i) the position of the bridging
ligands is determined by the trans influences of ligands in both metals, and (ii) the
trans effect/influence of a terminal ligand in one of the metal centers can be
transmitted to the adjacent metal center via a bridging hydride or an
intermetallic bond.
3.1 Position of Bridging Ligands
Hydrides are one of the most commonly found bridging ligands in iridium
homobimetallic complexes. The symmetry or asymmetry of the hydride bridge is
usually determined by the nature of the trans-ligands in both metal centers. For
example, the complexes depicted in Fig. 1 feature asymmetric hydride bridges in
solution and in the solid state. Although the position of the bridging hydrides
obtained from X-ray diffraction must be examined with care due to the limited
accuracy of the technique for these situations, this assumption has been also
supported by NMR studies in solution [63, 64].
The asymmetry of the bridge has been attributed to the different trans influence
of the ligands in trans positions to the bridging hydrides. For example, the μ-H
ligand in complex [Ir 2 ((CH 3 ) 2 CO)(CO) 2 (H)(μ-H)(μ-1,8-(NH) 2 naphtha)(P
i
Pr 3 ) 2 )]
[CF 3 SO 3 ] 2 presents two trans-ligands that possess very different trans influences,
namely, an acetone and a hydride ligand. For that reason, the μ-hydride is closer to
the metal that contains the trans-ligand with the weakest trans effect, in this case
acetone.
3.2 Intermetallic Trans Effect/Influence
The intermetallic trans influence in L–Au–Au–L
0 complexes has been postulated in
order to explain the Au–L
0 bond length, which changes depending on the trans
influence of L, while the Au–Au distance remains unaltered [65, 66]. Focusing on
the chemistry of homobimetallic iridium complexes, several illustrative cases of
intermetallic trans effect can be found. For example, complexes [Ir 2 (CH 3 CN)
(H) 3 (μ-H)(μ-Pz) 2 (P
i
Pr 3 ) 2 ] and [Ir 2 (CH 3 CN)(Cl)(H) 2 (μ-H)(P
i
Pr 3 ) 2 (μ-Pz) 2 ] (Fig. 2)
36
M. Iglesias et al.
alkyne insertions leading to bridging alkenyls, in this case aiming to explain
unusual stereoselectivities in stoichiometric [61] or catalytic [62] reactions.
3 Trans Effect in Bimetallic Complexes
The trans effect/influence in binuclear complexes brings about two main phenomena that are not observed in mononuclear complexes: (i) the position of the bridging
ligands is determined by the trans influences of ligands in both metals, and (ii) the
trans effect/influence of a terminal ligand in one of the metal centers can be
transmitted to the adjacent metal center via a bridging hydride or an
intermetallic bond.
3.1 Position of Bridging Ligands
Hydrides are one of the most commonly found bridging ligands in iridium
homobimetallic complexes. The symmetry or asymmetry of the hydride bridge is
usually determined by the nature of the trans-ligands in both metal centers. For
example, the complexes depicted in Fig. 1 feature asymmetric hydride bridges in
solution and in the solid state. Although the position of the bridging hydrides
obtained from X-ray diffraction must be examined with care due to the limited
accuracy of the technique for these situations, this assumption has been also
supported by NMR studies in solution [63, 64].
The asymmetry of the bridge has been attributed to the different trans influence
of the ligands in trans positions to the bridging hydrides. For example, the μ-H
ligand in complex [Ir 2 ((CH 3 ) 2 CO)(CO) 2 (H)(μ-H)(μ-1,8-(NH) 2 naphtha)(P
i
Pr 3 ) 2 )]
[CF 3 SO 3 ] 2 presents two trans-ligands that possess very different trans influences,
namely, an acetone and a hydride ligand. For that reason, the μ-hydride is closer to
the metal that contains the trans-ligand with the weakest trans effect, in this case
acetone.
3.2 Intermetallic Trans Effect/Influence
The intermetallic trans influence in L–Au–Au–L
0 complexes has been postulated in
order to explain the Au–L
0 bond length, which changes depending on the trans
influence of L, while the Au–Au distance remains unaltered [65, 66]. Focusing on
the chemistry of homobimetallic iridium complexes, several illustrative cases of
intermetallic trans effect can be found. For example, complexes [Ir 2 (CH 3 CN)
(H) 3 (μ-H)(μ-Pz) 2 (P
i
Pr 3 ) 2 ] and [Ir 2 (CH 3 CN)(Cl)(H) 2 (μ-H)(P
i
Pr 3 ) 2 (μ-Pz) 2 ] (Fig. 2)
36
M. Iglesias et al.