From a historical point of view “From Silicon to Titanium and Back to Silicon”, the genesis of the titanocene complex of bis(trimethylsilyl)acetylene (1) is explained. Similarities to the known 1-silacyclopropenes led us to describe the bonding in 1 in terms of a 1-titanacyclopropene. Complex 1 is a useful source of the highly reactive bis(cyclopentadienyl)titanium(II), which undergoes both stoichiometric and catalytic reactions that bear a striking similarity to some of the reactions of the isolobal silicon analogues.

As early as 1894 Michaelis and Schroeter isolated the first phosphorus(III)-nitrogen heterocycle from the reaction of aniline hydrochloride with an excess of PCl 3 (Scheme 1). [1] Interestingly, the authors assumed to have isolated the monomeric species, C 6 H 5 ÀN=PÀCl, which they called "phosphazobenzene chloride" but already speculated on the existence of the dimer. Today we know that the dimer is the stable form, and such four-membered rings of the type [XP(m-NR)] 2 , which contain alternating phosphorus(III) and nitrogen centers, are called cyclo-1,3-diphospha(III)-2,4-diazanes (X = halogen, R = organic group; old name: 1,3-diaza-2,4diphosphetidines). [2] They play a major role in preparative phosphorus-nitrogen chemistry because such species are good starting materials for polycyclic inorganic and organometallic compounds. [3, 4] Cyclo-1,3-diphospha(III)-2,4-diazanes (1) can exist as cis or trans isomers with trigonal-pyramidal P and trigonal-planar N atoms. [3] Both the N and the P atoms have one localized lone pair. Thus, formally eight electrons are found for this type of electron-rich heterocycles (Scheme 2). To the best of our knowledge, four-membered P 2 N 2 rings bearing 6 p electrons are unknown. As illustrated in Scheme 2, three possible target molecules (2, 3, and 4) with electronic structures that are related to those of aromatic hydrocarbons ([4 n + 2] p electrons), [5] can be considered. The most promising candidate for synthesis seemed to be the neutral cyclo-1,3-diphospha-2,4-diazane biradicaloid, [P(m-NR)] 2 (3; R= bulky group) which should formally be generated in a twoelectron reduction process upon chloride ion abstraction. Different reducing reagents, such as lithium metal, [Cp 2 Ti-(btmsa)] [6] (btmsa = bis(trimethylsilyl)acetylene, Me 3 SiÀC CÀSiMe 3 ) with the reactive {Cp 2 Ti II } fragment concealed in the h 2 -bounded btmsa complex, and [{Cp 2 Ti III Cl} 2 ] were utilized. Furthermore, it was of interest to study the effect of the bulky group on the reduction process. Thus, the terphenyl (Ter = 2,6-Mes 2 À C 6 H 3 , Mes = 2,4,6-Me 3 C 6 H 2 ) [7] and hypersilyl group (Hyp = (Me 3 Si) 3 Si) [8] were used for kinetic stabilization.Following our interest in Group 15 heterocycle chemistry, [9] we describe herein the synthesis, isolation and full characterization of a formal aromatic P 2 N 2 heterocycle of the type [P(m-NR)] 2 (R = Hyp, Ter) with an unusual biradicaloid bond situation.

A thorough investigation of the reactions of diynes, R(C&tbd1;C)(2)R, and polyynes, R(C&tbd1;C)(n)()R, with titanocene and zirconocene was performed using the metallocene sources Cp(2)M(L)(eta(2)-Me(3)SiC&tbd1;CSiMe(3)) (M = Ti, L = -; M = Zr, L = THF, pyridine). The conversions show an array of different products generated in complexation, coupling, and cleavage reactions. These results include remarkable structures (e.g., five-membered metallacyclocumulenes) and reactions (e.g., C-C single-bond cleavage). In addition, the first C-C single-bond metathesis in homogeneous solution was discovered. The presented findings have been supported by theoretical studies.

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