Mononuclear trigonal octahedral

Mononuclear octahedral/trigonal bipyramidal iron centers are found in either the ferric or the ferrous oxidation state (Whittaker etal., 1984 Arciero et ai, 1983). Because the iron may participate directly in catalysis as either a Lewis acid or base, only one state is the active form for a given enzyme. Transient redox changes may occur during turnover, but the enzyme returns to its initial condition. In contrast the tetrahedral mononuclear iron proteins appear to function primarily as electron transfer agents and therefore change oxidation state with a single turnover. 

The mononuclear metal carbonyls contain only one metal atom, and they have comparatively simple structures. For example, nickel tetracarbonyl is tetrahedral. The pentacarbonyls of iron, ruthenium, and osmium are trigonal bipyramidal, whereas the hexacarbonyls of vanadium, chromium, molybdenum, and tungsten are octahedral. These structures are shown in Figure 21.1. 

In general, it is not possible to predict the stereochemistry about the separate copper(II) ions in most cases they are the same and may or may not be related by a centre of symmetry. The actual stereochemistries produced are recognizably the same as those occurring in mononuclear copper(II) complexes (Figure 19.1). The most common is that of square-based pyramidal with rhombic coplanar, compressed tetrahedral, trigonal bipyramidal and elongated rhombic octahedral stereochemistries all occurring. 

Mononuclear Complexes. They show at least 10 different geometries that are represented in Table 20. They range from tricapped trigonal prismatic (e.g. [ReH9] ), pentagonal bipyramidal (e.g. [OsH7] ), octahedral (e.g. 

Cw-dioxomolybdenum(Vl) complexes of sterically bulky (thiolato)pyridine (41) and bi(thiolato)pyridine (42) ligands were designed to prevent comproportionation and maintain mononuclearity during oxygen-atom transfer reactions. The complex with (41) had a distorted octahedral structure and achieved its designed objective, whereas that with (42) did not, having trigonal bipyramidal structure with axial thiolate donors. 

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