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?