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 theres 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?
In order to determine the stability of chemical complexes you have to take into account both steric and electronic factors. A lot of chemistry research studies these "rules" that govern the stability of chemical complexes. Sterics has to deal with whether atoms in the complex "bump into" each other. This would include the observation that metal ions with larger ionic radii can accommodate more ligands. In addition, the size of the ligand can play a major role in the allowed coordination geometries. Also there electronic effects to take into account, which include the total number of electrons that are shared between the metal and ligands (see 18-electron rule) as well as the stability of the bonds between the ligand and the metal (electron donation from the ligand to the metal as well as bonding from the metal to the ligand). This is kind of a simple answer to a complicated question, but there is not really a periodic trend of coordination geometries although late transition metals tend to have tetrahedral geometries while mid-transition emtals tend to be octahedral. Higher weight transition metals are polarizable (i.e. have a more diffuse electron cloud because electrons in higher energy shells are shielded by electron in inner shells, thereby, decreasing the nuclei's abililty to "pull" the electrons toward the center) and therefore can accommodate some different geometries than their lower energy cousins can.
