Tracing an organometallic early—late transition element relationship

Abstract

This work explores M. E. Vol'pin's early notion that TiCp 2 possesses carbenoid character, and transfers this concept to the related dicarbonyl bis(phosphorus donor)iron fragment. Similarities and differences in electronic and molecular structure of the titanium and iron fragment are elucidated based on density functional calculations, lsolobal analogies to the carbene unit are presented to rationalize the computational results. Experimental verification of analogies between TiCP2 and Fe(CO)2L 2 chemistries is given.One of the early "bridge building" ideas of M. E. Vol'pin arose in the area of early transition metal metallocene chemistry, t-z~ In 1963 he published an article zt together with the coauthors in which the following curious statements were put forward: "It has previously been shown that derivatives of divalent silicon and germanium possess "carbenoid" properties, i.e., the ability to enter into the same types of reactions as compounds of divalent c',.rbon (the carbenes). We presumed that compounds of the transition elements might also possess analogous carbenoid properties, provided the following fundamental conditions were observed..." "From this point of view, a "carbenoid" character should be possessed by dicyclopentadienyititanium, in particular, since the electronic configuration of titanium in this compound satisfied all of the conditions enumerated above..." and "It would be expected that this compound, like carbenes, would be capable of reacting with unsaturated compounds with the formation of two or four new titanium-carbon bonds. On the basis of these considerations, we undertook a study of the reaction of dicyclopentadienyititanium (nascent) with diphenylacetylene..."z~ Later on this thoughtful idea that titanocene TiCp 2 is in its chemical and electronic properties comparable to carbenes CR 2 was supported further by quantum mechanical EHT calculations by Brintzinger and Bartell in 1970 2z and by Hoffmann and Lauher in 1976. z3 Furthermore, in 1982 R. Hoffmann z4 announced in a unifying approach his concept of isolobal analogy, which provided a general base for Vol'pin's original thought. Meanwhile this model of the isolobal analogy is extensively used to especially build bridges between inorganic or organometallic and organic chemistry relating these different areas of chemistry mostly by a fragmental view of molecules. As a quite general model the isolobal analogy is bound to abstract only a piece of reality and, as R. Hoffmann stated in his article, z4 it has to be seen "how far the model can be pushed" and "where it breaks down."Based on this analogy this personal review attempts to trace the relation of Vol'pin's and others" early transition metal metallocene chemistry with the carbenoid Fe(CO)2(PR3) 2 chemistry of our laboratory. Indeed, the group IV metallocenes are isolobal to carbenes and these in turn are isolobal to Fe(CO)2(PR3)2 fragments:MCp;z -~c.)~ R2C ~.'-Fe(CO)2(PR3) 2, M = Ti, Zr, Hf.When we apply this concept in the further context, it will be intriguing to see that ma...