Dear Shafikul,

many thanks for posting this important technical question on RG which is certainly of broad general interest to many other RG members as well. First of all we need to define organometallic compounds in general. In order to speak of an organometallic compound there must be at least one direct metal carbon bond. For example, tetracarbonyl nickel, Ni(CO)4, is a typical organometallic compound (four direct Ni-C bonds), while nickel acetate, Ni(O2CCH3)2, is not, although the compound contains an organic part (but no direct Ni-C bond).

The prerequisite for organometallic clusters is that we must have three or more direct metal-metal bonds. Organometallic clusters became very popular in the 1980's and continue to be quite fascinating. One of the simplest cluster molecules is Ru3(CO)12 with a metal-metal bonded triangle of ruthenium atoms. In addition to triangular clusters, tetrahedral and octahedral clusters are also quite common, and much larger metal arrays have also been synthesized. A special case are clusters containing so-called interstitial atoms. These can be for example octahedral clusters with a nitrogen or carbon atom located in the center of the octahedron.

Good luck with your research work and best wishes, Frank Edelmann

Dear Dr. Edelmann

Thank you very much for your informative answer. However, I think the necessary condition for organometallic clusters is that, they should have three or more metal atoms along with at least one direct metal-metal bond. For example, trinuclear clusters with TVEC 50 and 52 have two and one metal-metal bonds respectively, but to the best of my knowledge, they are regarded as cluster compounds. There are some 54-electron trinuclear clusters/complexes having no direct metal-metal bonding interaction as expected, and hence often these are not considered as clusters.Please share your opinion with me in this regard.

Dear Shafikul,

many thanks for your kind response and explanation. Unfortunately I have absolutely no idea what "TVEC 50 and 52" are....😳

According to the famous organometallic chemist Albert Cotton, a metal cluster is defined as "a finite group of metal atoms that are held together mainly, or at least to a significant extent, by bonds directly between metal atoms, even though some non-metal atoms may also be intimately associated with the cluster" (see attached presentation). Moreover "molecular clusters are composed of 3 or more metal atomsstabilized by small molecules acting as ligands, and characterized by metal-metal bonds, either localized or delocalized over the metal framework." I know that the term "metal cluster" is often used in a broader sense. Personally I prefer to adhere to the definition in the strict sense. Thus if you have a metal complex wich comprises three metal atoms but only one or two M-M bonds or even no M-M bond, I would better call this a "trinuclear complex" instead of a cluster. This indicates that real metal clusters are somehow intermediates between molecular complexes and metals.