Unsaturated Dimetal Cyclopentadienyl Carbonyl Complexes

Geometric and Electronic Structure Analysis of the Three-Membered Electron-Transfer Series [(μ-CNR)2[CpCo]2]n (n = 0, 1–, 2−) and Its Relevance to the Classical Bridging-Carbonyl System

“…Due to the discrepancy between the M–M separations of [(μ‑ EO) 2 ­[CpCo] 2 ] n dimers (E = C, n = 0, 1–; E = N, n = 0) as a function of reduction level and the predicted M–M bond multiplicity based on the 18-electron rule, the electronic structure properties of these species have received considerable attention. Early reports based on qualitative MO considerations influenced by the 18-electron rule argued that Co–Co multiple bonds were indeed present and that the monoradical monoanion, [(μ‑CO) 2 ­[CpCo] 2 ] 1– possessed an unpaired electron in a Co–Co π* orbital (for an overall formal bond order of 1.5). ,, However, the consensus arising from later studies using more quantitative computational methods converged on an electronic structure environment where direct metal–metal bonds are not present for any [(μ‑EO) 2 ­[CpCo] 2 ] n dimer, irrespective of charge state or identity of the bridging ligand. ,,− , Instead, these analyses have concluded that the neutral d 8 -d 8 dimer, (μ‑CO) 2 ­[CpCo] 2 possesses completely filled (δ) 4 /(δ*) 4 and (σ) 2 /(σ*) 2 Co–Co orbital levels, while both the in-plane (relative to the bridging ligands) and out-of-plane Co–Co π-type orbitals are filled. , However, these π-type orbitals also have significant contributions from the bridging carbonyl ligands in the form of Co→π*­(CO) π-back-donation interactions. Importantly, it is these multicentered interactions between the two Co centers and the bridging CO/NO ligands that mitigate any direct Co–Co bonding. ,,, Notably, this view is supported experimentally by a series of photoelectron spectroscopic studies on [(μ‑EO) 2 ­[CpM] 2 ] n dimers, which did not reveal the presence of ionization bands arising from a purely M–M bonding manifold. ,,, …”

“…The dimeric cyclopentadienyl complexes [(μ‑EO) 2 ­[CpM] 2 ] n (E = C, N; M = Fe, Co, Ni; n = 0, 1−) are well known species that provided early tests for metal–metal multiple bonding and mixed valency in organometallic systems. − However, the degree of M–M bond multiplicity in these dimers, as well as the fundamental question as to whether M–M bonds are present at all, has remained contentious. − The central issue in this debate has stemmed from the discrepancy between classical electron-counting formalisms and the crystallographic and/or theoretically determined structural features of the (μ‑EO) 2 M 2 core. , These structural features (Scheme ), which include the M–M separations ( d ), (μ‑EO) 2 M 2 core puckering (θ), and Cp-ring slippage ( s ), do not track straightforwardly with the formal d n combination of the two metal centers (i.e., d 8 –d 8 , d 9 –d 8 , d 9 –d 9 ). In addition, the well-recognized and substantial contribution that the bridging ligands make to the electronic structure has also compounded the difficulty in determining the extent of M–M bonding interactions of these dimers. , …”

“…While significant synthetic, spectroscopic, and theoretical attention has been given to [(μ‑EO) 2 ­[CpM] 2 ] n dimers, − it has been noted previously that synthetic access to a homologous series of d 8 –d 8 , d 8 –d 9 , and d 9 –d 9 partners has been the main limitation of a full, systematic investigation into this class of complexes. , For example, the neutral bridging carbonyl complex (μ‑CO) 2 ­[CpCo] 2 and its radical anion [(μ‑CO) 2 ­[CpCo] 2 ] − , which function as d 8 –d 8 and d 8 –d 9 pairs, respectively, have been well studied. − However, the corresponding dianion [(μ‑CO) 2 ­[CpCo] 2 ] 2– , which represents the d 9 –d 9 combination, has remained elusive toward isolation. , To combat this synthetic limitation, the neutral dinitrosyl dimer [(μ‑NO) 2 ­[CpCo] 2 ], has served as a surrogate for the d 9 –d 9 system. ,,,, However, the differing orbital energies attendant between CO and NO can dramatically affect the energetics of the d-orbital manifold, , thus rendering this comparison imprecise for the goal of elucidating M–M bond multiplicity. ,, …”

Reaction of ruthenium and iron alkynyl complexes with [Mo2(CO)4(η-C5H5)2] ‡

“…Our interest, in particular, is the reactivity of the dimetalloalkynes or ethyne-1,2-diyl complexes. 12- 24 The binuclear complex [Mo 2 (CO) 4 (η-C 5 H 5 ) 2 ] has a rich alkyne chemistry 25, 26 and we have recently observed the unexpected course of the reaction of [{Ru(CO) 2 (η-C 5 H 5 )} 2 -(µ-C᎐ ᎐ ᎐ C)] with [Mo 2 (CO) 4 (η-C 5 H 5 ) 2 ] 27 and decided to gauge the effect of removing one metal centre from the dimetalloalkyne on the course of the reaction.…”

Terminal vs. bridging coordination of CO and NO ligands after decarbonylation of [W2Cp2(μ-PR2)(CO)3(NO)] complexes (R = Ph, Cy). An experimental and computational study

“…These species are of general interest because of their high reactivity under mild conditions towards a great variety of organic and inorganic molecules, whereby a very wide range of new products can be prepared in a selective way, many of them being not accessible through more conventional synthetic routes involving higher energy inputs, because of their decomposition under harsh conditions. [1][2][3][4][5] The ejection of a CO ligand from a binuclear complex generates a two-electron deficiency which, according to the 18-electron rule, will tend to be balanced by an increase in the intermetallic bond order by one unit. This is, however, usually accompanied by some rearrangement in the molecule to partially block the vacancy left after decarbonylation, particularly if other carbonyl ligands remain at the dimetal centre.…”