Transition metal complexes coordination

In summary, many attempts have been made at achieving enantioselective reduction of ketones. Modified lithium aluminum hydride as well as the ox-azaborolidine approach have proved to be very successful. Asymmetric hydrogenation catalyzed by a chiral ligand-coordinated transition metal complex also gives good results. Figure 6-7 lists some of the most useful chiral compounds relevant to the enantioselective reduction of prochiral ketones, and interested readers may find the corresponding applications in a number of review articles.77,96,97... 

The preceding perturbation theory analysis is supported by extended Hiickel calculations by Cusachs and his co-workers (166, 167, 237) on model platinum(II)- and platinum(0)-olefin and -acetylene complexes and Hoffmann and Rossi s extensive analysis of five-coordinate transition metal complexes (194). By using similar arguments, Hoffmann and Rosch (190) predicted that the planar conformation would be energetically preferred for d10 M(C2H4)3 complexes. This geometry has now been established by Stone (214) and his co-workers for the platinum-olefin complex shown in Fig. 12. 

In another very insightful application, Bertrand et al. employed ligand lib for the isolation of low-coordinate transition metal complexes. In these compounds 16 and 17 (Fig. 7), the cyclohexyl ring shields one coordination site of the metal and stabilizes it by means of agostic interactions [65]. 

Among 4-coordinate transition metal complexes fluxional behavior based on planar/tetrahedral interconversions is of considerable importance. This is especially true of nickel(II) complexes, where planar complexes of the type Ni(R3P)2X2 have been shown to undergo planar tetrahedral rearrangements with activation energies of about 45 kJ mol 1 and rates of —105 s 1 at about room temperature. 

S.8.4.2.2. by Oxidative Addition to Low-Coordinate Transition-Metal Complexes... 

Thermodynamic aspects of 1,3-diborolanes, 2,3-dihydro-l//-l,3-diboroles, 1,3-azaborolidines, 2,3-dihydro-l,3-thia-boroles 2,3-dihydro-l//-l,3-stannaboroles, or 2,3-dihydro-l//-l,3-silaboroles are only sparsely mentioned. It has been found that the 127t-electron antiaromatic heterocycle 23 is stabilized by electron delocalization via the boron atom (cf. compound 9) <2002ZN1125>. Noteworthy is the comparison between the 8jt-electron antiaromatic 2,3-dihydro-l,3-benzothiaborole 24 or 4jt-electron antiaromatic 2,3-dihydro-l,3-thiaborole 26 and the corresponding lOtt-electron 25 or 67t-electron 27 aromatic lithium compounds, the latter forming stable Jt-coordinated transition metal complexes. 

To illustrate this definition, we will find it useful to compare fragments of methane with fragments of an octahedrally coordinated transition metal complex, MLg, For simplicity, we will consider only o bonding between the metal and the ligands in this complex. The fragments to be discussed are shown in Figure 15-2. 

Raymond, K. N. (1970) Changes in geometry of five-coordinate transition metal complexes structure and bonding. Proceedings of the Xlllth. International Conference on Coordination Chemistry, Vol. II, p. 94 ed. W. Kakolowicz, Polish Academy of Sciences. 

Sulfur-coordinated transition metal complexes have attracted considerable interest due to their electronic and structural properties, which are useful in both industrial catalysis and biological systems [141, 142]. Dithioate ions act as bidentate ligands in Group 4-12 transition metal complexes, such those with Ti (227 and 228) [141], V (229) [141], Mo and W (230, 231) [143,144], Mn (232)... 

An alternate way to examine these preferences is to look for trends within the vast collection of known four-coordinate metal complexes. Alvarez and coworkers analyzed the structures of more than 13,000 four-coordinate transition-metal complexes and reported these trends (1) (f, d, (f, (f, and d configurations prefer the tetrahedral geometry, (2) d and (f complexes show a strong preference for the square planar geometry, (3) d, ct, cfi, and metals appear in either tetrahedral or square planar structures, (4) a significant fraction of ions have structures intermediate between square planar and tetrahedral, and (5) a large number of structures that cannot be adequately described as tetrahedral, square planar, or intermediate are found for d, cfi, and d complexes. These trends build on the angular overlap-derived preferences. 

A systematic DFT (density functional theory) computational study examining the correlation between stereochemistry and spin state in four-coordinate transition-metal complexes has been reported. " One outcome was the development of a magic cube for the prediction of the preferred spin state of tetrahedral complexes with electron configurations between d and cfi (those for which high and low spin are possible within a tetrahedral ligand field). The factors predicted to predispose tetrahedral complexes to a low spin-state include (1) no ir-donor ligands, (2) a metal oxidation state +4, and (3) a metal from the second or third transition series. This DFT study provides an excellent discussion of the factors that dictate the geometry of four-coordinate complexes. 

Carbon-shielding Tensors in Tj -Coordinated Transition Metal Complexes of Alkenes and Alkynes... 

A study of the conformations of amylose and cellulose oligomers using vibrational spectroscopies has been reported. At the same conference, the use of FTIR (near, mid and far IR), and EXAFS, for spectroscopic characterization of mono- and disaccharides and coordination transition metal complexes was described. IR has been applied to characterize l,2 3,4-di-0-isopropylidene-6-0-triphenylstannylmethyl-a-D-galactopyranose. ... 

Haymore BL, Ibers JA (1975) Linear vs. bent nitrosyl ligands in four-coordinate transition metal complexes. Structure of dinitrosylbis (triphenylphosphme)osimum (—II)hemibenzene, Os (N0)2 (P (C6H5)3)2 l/2CsH6. Inorg Chem 14 2610-2617... 

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