Capping atom

Note The capping atom s are on ly supported in th e semi-em pirical quantum m ech an ics m eth oils in flyperChem. If yon want to use themixed model in th c a/z i/rilir) m cth od in HyperCh ein, yon must select an en tire molecn le or molecii les with on t any bonn dary atom between the selected and unselectcd regions and then carry ont the calculation. ... 

Note The capping atoms are only supported in the semi-empirical quantum mechanics methods in HyperChem. If you want to use the mixed model in the ab mi/io method in HyperChem, you must select an entire molecule or molecules without any boundary atom between the selected and unselected regions and then carry out the calculation. 

The second type of trinuclear compounds containing (M302(02CR)(,(H20)3] and obtained by the reaction of M(CO)6 (M = Mo, W) with carboxylic acids, features a similar triangle of M-M bonded metal atoms but this time capped on both sides by hj-O atoms (Fig, 23.8d). Complexes m which either one or both of these capping atoms arc replaced by /iy-CR, alkylidene. 

NiRh (CO) ] (29) clusters respectively, (Fig case, three terminal carbonyls in [Rh.(CO)-c] become edgebridging to the capping atom with little other change of the carbonyl polyhedron. 

The initial complex HOs3(CO)nX cannot be isolated for X = I, is readily transformed to the decacarbonyl for X = Br, but is more stable if X = Cl. The iodide initially gives the decacarbonyl and on warming yields the nonacarbonyl, HOs3(CO)9I, in which the iodine becomes a capping atom to the three metal centers. The corresponding "capped bromide and chloride complexes are not formed. 

The Jensen symbols are very important in helping to unravel the relationship between the different structure types in neighbouring domains. For example, it is not fortuitous that the NaCl and NiAs domains adjoin each other. Their Jensen symbols 6/6 and 8IV/6 tell us immediately that in NaCl the Na and Cl sites are octahedrally coordinated, whereas in NiAs the Ni site is octahedrally coordinated (but with two extra capping atoms), and the As site is trigonally coordinated. It is also not surprising that at the boundary between cF8 (NaCl) 6/6 and hP4(NiAs) IV/6 we find the two much smaller domains of hP8(TiAs) 7/6, 6 and tI8(NbAs) 6 /6. Nor is it unexpected to find the two islands of oP8(MnP) 10 78 " stability in the hP4(NiAs) 8rv/6 domain. A distorted NiAs structure type, MnP leads to the bicapping of the trigonal prismatic coordination about the As site, that is 6 - 8W (cf Fig. 1.9). Further, we see that the cP8(FeSi) 13713 domain adjoins a cP2(CsCl) 14/14 domain they are related structure types as mentioned earlier. 

Analytic derivatives have been reported for both the LSCF and GHO models, making them attractive options for MD simulations (Amara et al. 2000). Their generalization to ab initio levels of theory through the use of core pseudopotentials (along the lines of the pseudohalogen capping atoms described above) ensures that they will see continued development. 

Below a normalized bite of 1.1 there is a shallow potential energy trough that incorporates the capped trigonal prism with the bidentate ligand spanning the prism edge trans to the capping atom... 

The repulsion energy calculations also predict that each bidentate ligand will be unsymmetrically bonded, the capping atom experiencing more repulsion than the prismatic atom. This is experimentally observed (M—D)/(M—G) 1.05 for the above trinitrato complexes. 

Fig. 27. Osl0C(CO)24I, 22, as in its (Ph P)2N+ salt (55). The cluster is related to 21 by the addition of an iodine atom across one of the Os-Os bonds from a capping metal atom to an Os6C core metal atom. The octahedral Os-Os bonds average 2.90(5) A the bonds from the capping atoms to the core metal atoms average 2.80(3) A.

---