A Counterintuitive Structural Effect of Metal–Metal Bond Protonation and Its Electronic Underpinnings

Abstract

Protonation across the metal-metal bond in the complexes [(CO)(2)M(mu-dppm)(mu-PtBu(2))(mu-H)M(CO)(2)] (M=Fe or Ru, dppm=Ph(2)PCH(2)PPh(2)) induces M-M bond shortening of up to about 0.05 A. DFT calculations on simplified iron models reproduce this trend well. Conversely, the computations show that the M-M distance in the dimer [{Cp*Ir(CO)}(2)] lengthens with two consecutive protonations, but there are no crystal structure determinations to highlight the effects on the Ir-Ir bond. DFT calculations and the analogous cobalt system confirm that the transformation of a two-electron, two-center (2e-2c) bond into a 2e-3c bond is accompanied by the predicted elongation. An MO analysis indicated similar nature and evolution of the M-M bonding these cases. In particular, the HOMOs of the mono-hydrido cations [Cp(CO)M(mu-H)M(CO)Cp](+) (M=Ir, Co) have evident M-M bent-bond character, and hence subsequent protonation invariably causes a decrease in the bond index. The Fe(2) and Co(2) systems have also been analyzed with the quantum theory of atoms in molecules (QTAIM) method, but in no case was an M-M bond critical point located unless an artificially shorter M-M distance was imposed. However, the trends for the atoms-in-molecules (AIM) bond delocalization indexes delta(M-M) confirm the overall M-M bond weakening on protonation. In conclusion, all the computational results for the iron system indicate that the paradigm of a direct correlation between bond strength and distance is not always applicable. This is attributable to a very flat potential energy surface and various competing effects imposed by the bridging ligands.

Comparison of the One-Electron Oxidations of CO-Bridged vs Unbridged Bimetallic Complexes: Electron-Transfer Chemistry of Os2Cp2(CO)4 and Os2Cp*2(μ-CO)2(CO)2 (Cp = η5-C5H5, Cp* = η5-C5Me5)

“…Interestingly, even though removal of an electron from the HOMO of anti - 2 decreases the Os–Os bond order, the Os–Os bond distance actually decreases in going from 34 e – anti - 2 to 33 e – anti - 2 + (Table ). Certainly, the smaller atomic radii of the more positively charged osmium atoms in 2 + become a factor, but similar nonintuitive relationships between bond orders and bond distances have been noted in other transition metal systems. , It is important to note that the present complex retains a substantial Os–Os bond order (0.52) in the radical cation, suggesting reasonable kinetic stability for anti - 2 + .…”

“…The more unusual species found in the present study is 33 e − 1 + , which is an uncommon example of a verified radical cation derived from a metal−metal bonded system unsupported by bridging ligands. The few predecessors of this type of which we are aware are [Co 2 Cp 2 (CO) 4 ] + , 3, Detailed computations such as those reported 52,53 for [Co 2 Cp 2 (CO) 2 ] 0 and [(μ-H)M 2 Cp 2 (CO) 2 ] + can reveal a more nuanced picture of the relationship between metal− metal bonding, electron count, and charge in unsupported dimetallic complexes. Such is the case for 1 and 1 + .…”

Carbonyl substitution chemistry of some trimetallic transition metal cluster complexes with polyfunctional ligands

“…The overall structure adopted by [Ru 3 (CO) 5 (dppm)(l 2 -PMe(C 6 H 4 CH 2 OMe)) 4 ] (6), appears to be unique with no examples of a trinuclear, open-triangular cluster that contains six phosphine/phosphide donors found in the CSD [30]. The individual phosphides bridge the metal-metal bonds, a motif that is well known in the literature [32][33][34][35][36][37][38][39][40]. Much more common is a motif in which a triruthenium cluster has each Ru-Ru vector symmetrically bridged by phosphides, as exemplified by [Ru 3 (CO) 9 (l-H)(l-P(C 6 H 11 ) 2 ) 3 ] [38].…”

Synthesis of Iron and Cobalt Complexes of a Ferrocene-Linked Diphosphinoamide Ligand and Characterization of a Weak Iron–Cobalt Interaction

“…Because of the scarcity of weak M–M bonds between 3d metals that have been characterized using the Mayer bond order, direct comparison is difficult. The carbonyl cluster Fe 3 (CO) 12 and dimeric [CpCoH­(CO)] 2 display bond orders of 0.42 (ref ) and 0.342 (ref ), respectively, and are described as single bonds. However, other complexes of 1,1′-substituted ferrocene-based ligands, which display a Fe–M interaction have been studied.…”