Dedicated to Professor William J. Evans on the occasion of his 60th birthdayThe continuing quest for molecular complexes of lowoxidation-state lanthanides (other than the well-known samarium(II), europium(II), and ytterbium(II)) has produced significant achievements.[1] The majority of such compounds are derivatives of neodymium, dysprosium, and thulium, [2] obtained from the corresponding molecular iodides (often prepared in situ): [NdI 2 (thf) [2c,d] or by the reduction of an appropriate Ln III derivative. [2e,f] The present state of lanthanum, cerium, praseodymium, and gadolinium redox chemistry was described by S. A. Cotton: "None of these metals exhibits a stable + 2 state in any of its compounds and … they are unlikely to form stable compounds in this state." [3] Thus, the isolation of room-temperature-stable solid La II compounds is a very unexpected and significant breakthrough in the chemistry of lanthanides.Related to the subject of this study is a large group of inorganic materials: reduced or metal-rich rare-earth-metal halides, [4] including LaI 2 (featuring an extended structure with La 5d1 electrons delocalized on a conduction band), in which a lanthanum atom can be considered as being in the + 2 oxidation state.[4b] These compounds are better treated in a materials science context, where the metal valence [5,6] rather than the oxidation state is of primary importance.Previously [8] The latter featured redox ambiguity, which was also found in cerocene [9] and bis(pentalene)-cerium, [10] in which a metal-bound ligand had readily accessible adjacent oxidation states. For the binuclear arenebridged complexes, three different metal-ligand formal charge distributions are possible: Ln 2+

Simple two-coordinate acyclic silylenes, SiR(2), have hitherto been identified only as transient intermediates or thermally labile species. By making use of the strong σ-donor properties and high steric loading of the B(NDippCH)(2) substituent (Dipp = 2,6-(i)Pr(2)C(6)H(3)), an isolable monomeric species, Si{B(NDippCH)(2)}{N(SiMe(3))Dipp}, can be synthesized which is stable in the solid state up to 130 °C. This silylene species undergoes facile oxidative addition reactions with dihydrogen (at sub-ambient temperatures) and with alkyl C-H bonds, consistent with a low singlet-triplet gap (103.9 kJ mol(-1)), thus demonstrating fundamental modes of reactivity more characteristic of transition metal systems.

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By employing strongly σ-donating boryl ancillary ligands, the oxidative addition of H2 to a single site Sn(II) system has been achieved for the first time, generating (boryl)2SnH2. Similar chemistry can also be achieved for protic and hydridic E-H bonds (N-H/O-H, Si-H/B-H, respectively). In the case of ammonia (and water, albeit more slowly), E-H oxidative addition can be shown to be followed by reductive elimination to give an N- (or O-)borylated product. Thus, in stoichiometric fashion, redox-based bond cleavage/formation is demonstrated for a single main group metal center at room temperature. From a mechanistic viewpoint, a two-step coordination/proton transfer process for N-H activation is shown to be viable through the isolation of species of the types Sn(boryl)2·NH3 and [Sn(boryl)2(NH2)](-) and their onward conversion to the formal oxidative addition product Sn(boryl)2(H)(NH2).

Si in sight: a one-pot, single-step synthesis of an acyclic silylsilylene, Si{Si(SiMe(3))(3)}{N(SiMe(3))Dipp} (Dipp=2,6-iPr(2)C(6)H(3)), from a silicon(IV) starting material is reported, together with evidence for a mechanism involving alkali metal silylenoid intermediates.