Carbon distance, metal

An interesting example of t 3 coordination of a boratabenzene to a transition metal has been observed for a Zr(IV)-boratanaphthalene complex (Scheme 20).35 Thus, treatment of the illustrated 1-boratanaphthalene with Cp+ZrCl3 furnishes an adduct in which the metal-carbon distance for the 3, 4, and 5 positions of the heterocycle (2.53-2.56 A) is significantly shorter than for the 2 and 6 positions (2.77-2.81 A). It is suggested that this distorted bonding mode is the consequence of the high electron demand of Zr(IV), which prefers coordination to the most electron-rich carbon atoms. ... 

V 0.48), Ammeter and Swalen also calculated the adiabatic distortion parameter for the Co(Cp)2/Fe(Cp)2 system, finding A = 528 cm 1. In both cases however calculations were carried out to determine the value of the purely static distortion which would reproduce, via the vibronic coupling mechanism, the results for c and 45- For the Ru(Cp)2 host the corrected value of A proved to be 200 cm 1 and for the Fe(Cp)2 host 840 cm-1. Thus in the Ru(Cp)2 host, with a rather long metal to carbon distance, the vibronic effect... 

In addition, mention may be made of more recent electrostatic calculations due to Voitlander and his coworkers 110,177). Qualitatively the results are very similar to those of Robertson and McConnell 30), although the latter contain erroneous numerical values (177), but the choice of R as the metal to ring centroid distance, rather than the metal to carbon distance, leads to much better agreement with experiment. Thus, for Fe(Cp)2, the latter choice predicts 6 < n < o, but the former yields 5 o < ir, with a total splitting of some 18000 cm-1, in quite good agreement with experiment. 

The crystal structures of a number of s-trans (diene)metal complexes (3) have been determined10-14. The diene ligand in all s-trans complexes is distinctly non-planar the torsional angle between the two olefin groups is between 114° and 127°. In general, the terminal carbon to metal distance is greater than for the internal carbon to metal distance, and the Cl—C2/C3—C4 bonds are shorter than the C2—C3 bond. 

Crystal structure data15 indicate that in the vast majority of (cyclobutadiene)metal complexes (4) the cyclobutadiene ligand is approximately square-planar with nearly equal C—C bond distances (ca 1.46 A) and bond angles of ca 90°. Within a given complex the cyclobutadiene carbon-to-metal distances are roughly equal. 

In agreement with the presence of an additional electron in the bonding orbital, compared to vanadocene (chromocene 16 valence electrons, terminal electronic configuration e a ), the metal-carbon distance is shorter (2.15 A compared to 2.27 A of vanadocene). 

A decrease in metal-carbon distances when going down a group has also been pointed out. However, if this observation can roughly be applied to Zr and Hf (owing to lanthanide contraction), it does not fit the data obtained for Ti (Table 11.1). In contrast, for tetracoordinated complexes of the third transition row, replacement of just one neopentyl ligand is reported only when silica was pretreated at 700°C [18, 19, 23, 24]. For the tantalum complex [Ta(=CHCMe3)Np3, its... 

At this stage, sulfoximines had been shown to function as chiral ligands for various palladium and copper catalysts which led to enantioselectivities of >95% ee in various reactions. Furthermore, most of those catalyzed reactions were C-C bond formations. Obvious questions were, therefore, whether sulfoximines could also be applied in combination with other metals and whether reductions and oxidations could be catalyzed as well. A structural comparison of the sulfoximines leading to high ee values such as 55, 60, 81, and 85 revealed that all of them had a two-carbon distance between the two coordinating atoms (which were all nitrogen in these cases). 

Unfortunately it is difficult to resolve the X-ray diffraction of the carbon atoms in these compounds from that of the heavy elements present, and in no case are the metal-carbon or carbon-carbon distances known with sufficient accuracy to enable pronouncements about the carbon-carbon bond order to be made. In this respect the infrared spectra ought to be helpful, but there is considerable controversy over the assignment of absorption... 

Both the long C-C bond distance (1.50 A) and the very short Pt—C distances (2.0 A) indicate the strong interaction between the adsorbed molecule and the three platinum surface atoms. The covalent Pt—C distance would be 2.2 A. The shorter metal-carbon distances indicate multiple metal-carbon bonding that may be carbene or carbyne-like. Compounds with these types of bonds exhibit high reactivity in metathesis and in other addition reactions The carbon-carbon single bond distance indicates that the molecule is stretched as much as possible without breaking of this chemical bond. 

Among these compounds, transition metal alkyls are the most widespread. For simple metal alkyls, the M — R bond distance is typically 190 to 220 pm. This is approximately the sum of the covalent radii of carbon and metal, rc = 77 pm and rM 120pm [184h]. Simple alkyls are simple a-donors (Fig. 2.5). 

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