# Why are palladium and platinum carbonyls unstable at room temperature?

By palladium and platinum carbonyls, I mean the mononuclear, homoleptic, charge-neutral, binary carbonyl complexes of the form $$\ce{M(CO)_n}$$ and not any heteroleptic complexes like $$\ce{Pd(CO)(PPh3)3}$$ and similar compounds of platinum which are abundant in chemical industry and used as catalysts.

So, I was reviewing carbonyls of transition metals when I encountered this statement:

Group 10 elements with 10 valence electrons form metal carbonyls $$\ce{Ni(CO)4}$$ ($$10 + 4 \times 2 = 18$$ electrons). Curiously $$\mathbf{Pd(CO)_4}$$ and $$\mathbf{Pt(CO)_4}$$ are not stable.

I am proposing that heavy metal carbonyls tend to be unstable(?) and decarbonylate to form polynuclear carbonyls. For example: $$\ce{Ru(CO)5}$$ and $$\ce{Os(CO)5}$$ being unstable form $$\ce{Ru3(CO)12}$$, and $$\ce{Os3(CO)12}$$. But in the case of palladium and platinum, no carbonyl complex seems to exist in stable condition at room temperature. I reviewed two papers which described the Group 10 carbonyl complexes (links below):

1. Complexes where the carbonyl group is the only ligand present have been reported for all the platinum group metals except palladium.[...] No substantiated reports have been published on platinum or palladium compounds similar to the nickel carbonyl $$\ce{Ni(CO)4}$$. Thus the polymeric platinum dicarbonyl is the only known carbonyl of these two metals and even this compound is unstable in air.[...]

The unstable polymeric platinum dicarbonyl $$\ce{Pt_n(CO)_{2n}}$$ has been obtained as a cherry red precipitate by the action of carbon monoxide on aqueous or ethanolic solutions of Pt(II) compounds (e.g. $$\ce{K2PtCl4}$$) at temperatures up to 80 °C; or as a purple colloidal precipitate by the action of water on a benzene solution of $$\ce{Pt(CO)2Cl2}$$ under an atmosphere of carbon monoxide.

2. It was not until 1972 that a definite existence of the congeners $$\ce{Pd(CO)4}$$ and $$\ce{Pt(CO)4}$$ was reported. Using matrix isolation and Raman techniques, data for the products of Pd atoms CO and CO/Ar cocondensation reaction were able to characterize the series of binary complexes $$\ce{Pd(CO)_n}$$ analogous to $$\ce{Ni(CO)4}$$, the most stable being $$\ce{Pd(CO)4}$$ but still unstable than $$\ce{Ni(CO)4}$$ which explains the failed attempts in its synthesis. Similar technique were used in making $$\ce{Pt(CO)4}$$ at 4.2-10 K which is stable up to 30 K. (slightly abridged)

The question is: why they are unstable/don't exist at room temperature?

1. Platinum Metals Rev., 1972, 16, (2), 50 "The Carbonyls of the Platinum Group Metals" By C. W. Bradford, Research Laboratories, Johnson Matthey & Co Limited (link)
2. J. Am. Chem. Soc., 1973, 95 (22), pp 7234–7241 "Binary carbonyls of platinum, Pt(CO)n (where n = 1-4). Comparative study of the chemical and physical properties of M(CO)n (where M = nickel, palladium, or platinum; n = 1-4) DOI: 10.1021/ja00803a009