Mononuclear compounds coordinate numbers

Mononuclear silver(I) complexes with nitrogen ligands can have several coordination numbers, the simplest is dicoordinated and compounds of the type [AgL2]+ occur for a variety of ligands... 

Mononuclear ER4 and simple four-coordinate compounds of E(IV) states are the baseline for viewing the other coordination numbers, the effect of bulky ligands, bonds to other E or metals, E(II) compounds, multiple bonds and other phenomena discussed in later sections. Basic parameters for some simple compounds are presented in Table 1, taken from the gas-phase data summarized by Molloy and Zuckerman5 and Haaland6. These data show the unperturbed molecules in the gas phase and provide the base for... 

We have previously seen examples of the carbon-like formulas of mononuclear and dinuclear osmium compounds, namely the methane-like tetrahydride (4.50c), ethylene-like H20s=CH2 (4.51c) and H2Os = OsH2 (Table 4.15), acetylenelike HOs = CH (4.54c) and HOs = OsH (Table 4.15), allene-like H2C = Os = CH2 (4.55a), and so forth. While the coordination numbers and Lewis-like formulas are formally analogous, the actual structures of Os and C species may be quite similar (e.g., the Td structures of OsfL and CH4) or dissimilar (e.g the strongly bent Cs structure of H20s = CH2 [Fig. 4.13(c)] versus the planar D2h structure of H2C = CH2). 

In this section we summarise the most common coordination numbers and the associated geometries in mononuclear complexes, and also in bridged polynuclear complexes where there is no direct M-M bond the situation in cluster compounds (dealt with in Section 8.5) is rather more involved. 

The use of unsubstituted or 4-methyl phenols resulted in the formation of cluster compounds [58]. However, 2,6-di(fcrt-butyl) substituted aryloxide ligands allowed the isolation of mononuclear 3-coordinate homoleptie complexes of the lanthanide elements, the coordination mode of which was first demonstrated with the N(SiMe3)2 ligand [59], The 2,6-substitution pattern is very effective because the alkyl groups are directed towards the metal center and impose a steric coordination number onto the metal which is comparable to the Cp ligand (Cp 2.49 OC6H3rBu2-2,6 2.41) [60],... 

NO binds covalently to transition metals M forming a diversity of structural types comprising mononuclear, NO bridging, and cluster compounds.8-10 Within the first class, main coordination numbers are 6,5, and 4, as sketched in Table 7.1, revealing an important structural result, namely, the adoption of linear or bent geometries of the MNO fragments. 

The monoanionic, bidentate four-electron-donor amidinates are also suitable for stabilizing low oxidation states and/or high coordination numbers at germanium centers. The monosubstituted and mononuclear compound 4, as well as compound 5, have been isolated and were characterized spectroscopically and structurally. 

With bulky carboxylate anions, the mononuclear compound formed can be expressed [REL3(sol) ], where sol is a coordinating solvent, such as H2O, EtOH, MeOH, DMF, or DMSO. In most cases, the three carboxylates are in the chelating rf-) mode, and n is 2 or 3, CN = 8 or 9. However, when the ligand is too bulky, there would be not enough room for all of the three carboxylates to be in the chelating mode, and one or two of them have to be in the monodentate( 7 ) mode, and the number of the solvent molecules (n) becomes 3 or 4. Figure 3 shows the structures... 

Fixation of carbon dioxide and formaldehyde in their intact form on a metal centre is a primary goal in metal-promoted transformations of a C molecule, provided it forms metal-carbon bonds. Formation of formaldehyde and carbon dioxide complexes is, however, a quite rare reaction, in spite of the various strategies applied so far. On the contrary, coordination of dinitrogen has been found in a number of complexes all the mononuclear compounds so far identified prefer the end-on bonding mode. Activation of dinitrogen, however, can be much more pronounced in ca.se N2 binds the metal in a side-on fashion. 

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