Coordination Chemistry of Transitional Metals, Periods

[Horseb0x]

For example cobalt, rhodium and iridium. I know that Co(II) and the Co(III) ions have a coordination number of 6 and that Co(II) is more common because cobalt is usually more stable in the 2+ oxidation state but when it coordinates NH3 ligands the 3+ oxidation state becomes more stable. Is this a trend among the group 9 elements or are things different for rhodium and iridium? Also I read that the smaller the ligand, the greater the coordination number of the metal since there's more ligands can fit in the coordination sphere. Does that mean the larger the metal, the more ligands it can coordinate? For example the atomic radius of rhodium should be a fair bit greater than that of cobalt. Does that mean rhodium can coordinate more than 6 NH3 ligands?

 

[jbsteele]

The trend of the group 9 elements is that they are all capable of forming coordination complexes with 6-coordinate geometries. This is due to the fact that their oxidation states range from +2 to +3, which results in a larger number of electrons available for coordination. In general, as the size of the metal ion increases, the coordination number increases, although this is not always the case and depends on the particular ligands involved. For example, rhodium is larger than cobalt and may coordinate more than 6 NH3 ligands if the coordination sphere is large enough to accommodate them. However, this is not always the case and depends on the particular ligands involved.