Hi Mer Mercurate

Well there are a lot if things to compare, the first thing is the geometry, they have different geometries where Ir is octahedral while Pt is square planar 

secondly is the solubility, Ir complexes are more soluble and easier to prepare and the synthetic procedures are very well known while for Pt special conditions should be obeyed

third, the quantum yields where cyclomatalated Ir complexes have very high quantum yields probably more than 50% and can reach 100 % (contrary to Pt) and modifying the ligands or substituents you can fullfil all the emission spectra from the blue to green to red to NIR 

fourth, is the stability, Ir complexes can be air and water stable where most of Pt complexes are not and for cyclometalated Pt complexe, Pt(II) complexes can be easily oxidized to Pt(IV) mainly in chlorinated solvents such as DCM or chloroform and thus sometimes you quinch the emission 

and the last thing I want to comment is due to these properties of Ir, it is taking a great interest nowadays in NLO measurements (non linear optics) where very interesting results have been published 

I fully agree with Nidal Saleh. But one thing to add:  platinum complexes are square  planar and therefore   platinum orbitals are more opened for different quenching processes.

Thank you very much for your informative comments, Nidal Saleh and Mihail Vladimirovoch Puzyk.

I cannot say much about practical problems in the synthesis and the access to the different metal complexes. From a search in the Cambridge datafile, the Pt(II)-metallacycles (five membered ring with at least one M-C bond) are even more numerous than the Ir(III) ones. From the orbital viewpoint, the binding is not conceptually very different, nor the coordination type (octahedral vs square planar) can be highly discriminant. Perhaps, you may find pertinent the attached article.  

To Carlo Mealli, Thank you. I realized that cyclometallated platinum(II) was synthesized as early as those of iridium(III).  I was trying to understand the point why when it come to harvesting triplet excited state energy, then Ir(III) seems to be predominant.

Thank you!