“…Moreover, the M−C−C angle in the Ir complex is 2.7° greater than in the Rh analogue. The data are all within the ranges shown by other metal complexes featuring diphenylketene bound in an η 2 -(C,O) fashion. ,− Finally, we observe that the structure of 2c is similar to that of 2a , with the notable difference that the phosphine ligands are oriented to place the sterically unassuming methyl groups closer to the ketene ligand. 1 Molecular structure of 2a .…”
Using a series of Ir(I) and Rh(I) ketene complexes, conclusions about the structure and bonding of complexes of the fundamentally important ketene ligand class are reached. In a unique comparison of X-ray structures of the same metal fragment to ketenes in both the eta(2)-(C,C) and the eta(2)-(C,O) binding mode, the Ir-Cl bond distances in complexes of trans-Cl(Ir)[P(i-Pr)(3)](2) to phenylketene [4, eta(2)-(C,C)] and diphenylketene [2a, eta(2)-(C,O)] are 2.371(3) and 2.285(2) A, respectively. This would be consistent with greater trans influence of a ketene ligand bound to a metal through its C=C bond than one connected by its C=O bond. Back-bonding of Ir(I) and Rh(I) to diphenylketene was assessed using trans-Cl(M)[P(i-Pr)(3)](2)[eta(2)-(C,O)-diphenylketene] (2a and 2d). Most bond lengths and angles are identical, but slightly greater back-bonding by Ir(I) is suggested by the somewhat greater deformation of the ketene C=C=O system [C-C-O angles are 136.6(4) and 138.9(4) in the Ir and Rh cases 2a and 2d, respectively]. Syntheses of new labeled ketenes Ph(2)C=(13)C=O and Ph(2)C=C=(18)O and their Ir(I) and Rh(I) complexes are reported, along with the generation of an Ir(I) complex of PhCH=(13)C=O. The effects of isotopic substitution on infrared absorption data for ketene complexes are presented for the first time. Preliminary normal coordinate mode analysis allowed definitive assignment of absorptions ascribed to the C-O stretching frequencies of coordinated ketenes, which are near the absorptions for aromatic ring systems commonly found as substituents on ketenes. For free diphenylketene and four of its complexes and a phenylketene complex characterized by X-ray diffraction, the magnitude of the (13)C-(13)C coupling between the two ketene carbons is correlated to carbon-carbon bond distance.
“…Moreover, the M−C−C angle in the Ir complex is 2.7° greater than in the Rh analogue. The data are all within the ranges shown by other metal complexes featuring diphenylketene bound in an η 2 -(C,O) fashion. ,− Finally, we observe that the structure of 2c is similar to that of 2a , with the notable difference that the phosphine ligands are oriented to place the sterically unassuming methyl groups closer to the ketene ligand. 1 Molecular structure of 2a .…”
Using a series of Ir(I) and Rh(I) ketene complexes, conclusions about the structure and bonding of complexes of the fundamentally important ketene ligand class are reached. In a unique comparison of X-ray structures of the same metal fragment to ketenes in both the eta(2)-(C,C) and the eta(2)-(C,O) binding mode, the Ir-Cl bond distances in complexes of trans-Cl(Ir)[P(i-Pr)(3)](2) to phenylketene [4, eta(2)-(C,C)] and diphenylketene [2a, eta(2)-(C,O)] are 2.371(3) and 2.285(2) A, respectively. This would be consistent with greater trans influence of a ketene ligand bound to a metal through its C=C bond than one connected by its C=O bond. Back-bonding of Ir(I) and Rh(I) to diphenylketene was assessed using trans-Cl(M)[P(i-Pr)(3)](2)[eta(2)-(C,O)-diphenylketene] (2a and 2d). Most bond lengths and angles are identical, but slightly greater back-bonding by Ir(I) is suggested by the somewhat greater deformation of the ketene C=C=O system [C-C-O angles are 136.6(4) and 138.9(4) in the Ir and Rh cases 2a and 2d, respectively]. Syntheses of new labeled ketenes Ph(2)C=(13)C=O and Ph(2)C=C=(18)O and their Ir(I) and Rh(I) complexes are reported, along with the generation of an Ir(I) complex of PhCH=(13)C=O. The effects of isotopic substitution on infrared absorption data for ketene complexes are presented for the first time. Preliminary normal coordinate mode analysis allowed definitive assignment of absorptions ascribed to the C-O stretching frequencies of coordinated ketenes, which are near the absorptions for aromatic ring systems commonly found as substituents on ketenes. For free diphenylketene and four of its complexes and a phenylketene complex characterized by X-ray diffraction, the magnitude of the (13)C-(13)C coupling between the two ketene carbons is correlated to carbon-carbon bond distance.
“…We note the bright blue color in both solution and in the solid state (including the crystal used for structural determination). Fe(0) complexes are typically yellow, − and none to our knowledge have been reported as blue. The color of 3 arises from an electronic absorption band peaking at λ max = 627 nm in acetonitrile solvent.…”
We report a structural, spectroscopic, and computational study of two tBu PNP (2,6-bis(di-tert-butylphosphinomethyl)pyridine) complexes of iron, ( tBu PNP)FeCl 2 (1) and ( tBu PNP)Fe(CO) 2 (3). Complex 1, ( tBu PNP)FeCl 2 , also independently synthesized by Milstein, has unusually long iron-ligand bond distances. DFT calculations show that these are clearly attributable to its high-spin electronic structure, and in particular to occupancy of the strongly antibonding d x 2 -y 2 orbital. The crystal structure of 3 reveals two unusual aspects. (1) The geometry around the iron atom in 3 is much closer to square pyramidal (SQP; apical CO) than to trigonal bipyramidal (TBP), although five-coordinate Fe(0) complexes are generally expected to be TBP; moreover, Chirik et al. have reported that ( iPr PNP)Fe(CO) 2 has essentially a perfect TBP structure ( iPr PNP ) 2,6-bis(di-isopropyl-phosphinomethyl)pyridine). ( 2) The apical carbonyl ligand in 3 deviates significantly from linearity (Fe-C-O ) 171.9°). Additionally, complex 3 is intensely blue in color, which is unusual for an Fe(0) complex and also significantly different from the red color of Chirik's ( iPr PNP)Fe(CO) 2 species. Results from DFT calculations reproduce and explain these structural and spectroscopic aspects as well as the contrast between 3 and its iPr PNP analogue.
“…Some time oc" I "O ago we could synthesize a PEt~ Fe(CO)2L2(Ph2C=C=O) complex, also by a substitution route. 49 The relationship of both types of complexes is obvious and needs no further comment. Recently we were able to extend the iron ketene chemistry in a synthetically very useful way.…”
This work explores M. E. Vol'pin's early notion that TiCp 2 possesses carbenoid character, and transfers this concept to the related dicarbonyl bis(phosphorus donor)iron fragment. Similarities and differences in electronic and molecular structure of the titanium and iron fragment are elucidated based on density functional calculations, lsolobal analogies to the carbene unit are presented to rationalize the computational results. Experimental verification of analogies between TiCP2 and Fe(CO)2L 2 chemistries is given.One of the early "bridge building" ideas of M. E. Vol'pin arose in the area of early transition metal metallocene chemistry, t-z~ In 1963 he published an article zt together with the coauthors in which the following curious statements were put forward: "It has previously been shown that derivatives of divalent silicon and germanium possess "carbenoid" properties, i.e., the ability to enter into the same types of reactions as compounds of divalent c',.rbon (the carbenes). We presumed that compounds of the transition elements might also possess analogous carbenoid properties, provided the following fundamental conditions were observed..." "From this point of view, a "carbenoid" character should be possessed by dicyclopentadienyititanium, in particular, since the electronic configuration of titanium in this compound satisfied all of the conditions enumerated above..." and "It would be expected that this compound, like carbenes, would be capable of reacting with unsaturated compounds with the formation of two or four new titanium-carbon bonds. On the basis of these considerations, we undertook a study of the reaction of dicyclopentadienyititanium (nascent) with diphenylacetylene..."z~ Later on this thoughtful idea that titanocene TiCp 2 is in its chemical and electronic properties comparable to carbenes CR 2 was supported further by quantum mechanical EHT calculations by Brintzinger and Bartell in 1970 2z and by Hoffmann and Lauher in 1976. z3 Furthermore, in 1982 R. Hoffmann z4 announced in a unifying approach his concept of isolobal analogy, which provided a general base for Vol'pin's original thought. Meanwhile this model of the isolobal analogy is extensively used to especially build bridges between inorganic or organometallic and organic chemistry relating these different areas of chemistry mostly by a fragmental view of molecules. As a quite general model the isolobal analogy is bound to abstract only a piece of reality and, as R. Hoffmann stated in his article, z4 it has to be seen "how far the model can be pushed" and "where it breaks down."Based on this analogy this personal review attempts to trace the relation of Vol'pin's and others" early transition metal metallocene chemistry with the carbenoid Fe(CO)2(PR3) 2 chemistry of our laboratory. Indeed, the group IV metallocenes are isolobal to carbenes and these in turn are isolobal to Fe(CO)2(PR3)2 fragments:MCp;z -~c.)~ R2C ~.'-Fe(CO)2(PR3) 2, M = Ti, Zr, Hf.When we apply this concept in the further context, it will be intriguing to see that ma...
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