Photochemistry of η5 cylclopentadienylcobalt) tricarbonyl, tris(η5-cyclopentadienylcobalt monocarbonyl) and tetra(η5-cyclopentadienylcobalt) dicarbonyl*

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

“…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. ,, …”

“…14,21 While significant synthetic, spectroscopic, and theoretical attention has been given to [(μ-EO) 2 [CpM] 2 ] n dimers, 1−24 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. 11,13 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. 9−22 However, the corresponding dianion [(μ-CO) 2 [CpCo] 2 ] 2− , which represents the d 9 −d 9 combination, has remained elusive toward isolation.…”

“…Density Functional Theory (DFT) calculations of these bridgingisocyanide dimers are also presented and suggest that, like their carbonyl and nitrosyl counterparts, 14,21 terphenyl isocyanide CNAr Mes2 in a benzene solution provides the monoisocyanide complex, CpCoI 2 (CNAr Mes2 ) (1) in 94% yield as a purple solid. Reduction of 1 with 2.0 equiv of KC 8 in a 1:2 DME/Et 2 O mixture afforded the neutral, bridging isocyanide dimer (μ-CNAr Mes2 ) 2 [CpCo] 2 (2) as determined by X-ray diffraction (Figure 1). Dimer 2 was isolated in 87% yield as a green crystalline solid by this synthetic route.…”

Magnetic Nanoparticles as MRI Contrast Agents

“…They concluded that the polymers added during the reaction not only coated the dispersions forming stable particles, but also acted as catalysts for the decomposition [ 33 , 34 ]. They suggested that the decomposition takes place at 140–160 ℃ in the presence of butadiene polymers while gathering support from the mechanistic studies conducted by Bergman and coworkers [ 35 ]. Later, their hypothesis was verified experimentally, showing the presence of an intermediate carbonyl complex formed after decomposition of Fe(CO) 5 [ 36 ].…”