The synthesis and characterisation of a new anionic flexible scorpionate ligand, methyl(bis-7-azaindolyl)borohydride [MeBai]- is reported herein. The ligand was coordinated to a series of group nine transition metal centres forming the complexes, [Ir(MeBai)(COD)] (1), [Rh(MeBai)(COD)] (2), [Rh(MeBai)(CODMe)] (2-Me) and [Rh(MeBai)(NBD)] (3), where COD = 1,5-cyclooctadiene, CODMe = 3-methyl-1,5-cyclooctadiene and NBD = 2,5-norbornadiene. In all cases, the boron based ligand was found to bind to the metal centres via a κ3-N,N,H coordination mode. The ligand and complexes were fully characterised by spectroscopic and analytical methods. The structures of the ligand and three of the complexes were confirmed by X-ray crystallography. The potential for migration of the "hydride" or "methyl" units from boron to the metal centre was also explored. During these studies an unusual transformation, involving the oxidation of the rhodium centre, was observed in complex 2. In this case, the η4-COD unit transformed into a η1,η3-C8H12 unit where the ring was bound via one sigma bond and one allyl unit. This is the first time such a transformation has been observed at a rhodium centre.

The reaction of the syn-bimetallic bis(pentalene)dititanium complex Ti2(μ:η5,η5-Pn†)2 (Pn† = C8H4(1,4-SiiPr3)2) 1 with carbon suboxide (OCCCO, C3O2) results in trimerisation of the latter and formation of the structurally characterised complex [{Ti2(μ:η5,η5-Pn†)2}{μ-C9O6}]. The trimeric bridging C9O6 unit in the latter contains a 4-pyrone core, a key feature of both the hexamer and octamer of carbon suboxide which are formed in the body from trace amounts of C3O2 and are, for example, potent inhibitors of Na+/K+-ATP-ase. The mechanism of this reaction has been studied in detail by DFT computational studies, which also suggest that the reaction proceeds via the initial formation of a mono-adduct of 1 with C3O2. Indeed, the carefully controlled reaction of 1 with C3O2 affords [Ti2(μ:η5,η5-Pn†)2 (η2-C3O2)], as the first structurally authenticated complex of carbon suboxide.

A series of uranium(III) mixed sandwich complexes with sterically demanding CpR ligands, of the type [U(COTTIPS2)(CpR)] (CpR = CptBu (C5H4tBu), CptBu2 (C5H3tBu2-1,3), CptBu3 (C5H2tBu3-1,2,4), CpTIPS2 (C5H3(SiiPr3)2-1,3), CpMe4Bz (C5Me4CH2Ph), IndMe6 (C9HMe6) and IndMe7 (C9Me7), and COTTIPS2 = C8H6(SiiPr3)2-1,4), have been synthesised and their X-ray crystal structures determined. The reactivity of these complexes with CO and CO2 is reported, including the squarate complex [U(COTTIPS2)(IndMe6)]2(μ-C4O4), IR data on the long-lived carbonyl complex [U(COTTIPS2)(IndMe7)(CO)] and the carbonate complex [U(COTTIPS2)(CptBu)]2(μ-η1:η2-CO3). The Solid-G algorithm has been to assess the steric properties of these and previously reported mixed-sandwich complexes in the solid state and correlate these properties with the observed reactivity.

The mixed sandwich U(III) complex {U[η8-C8H6(1,4-Si(iPr)3)2](Cp*)(THF)} reacts with the organic azides RN3 (R = SiMe3, 1-Ad, BMes2) to afford the corresponding, structurally characterised U(V) imido complexes {U[η8-C8H6(1,4-Si(iPr)3)2](Cp*)(NR)}. In the case of R = SiMe3, the reducing power of the U(III) complex leads to reductive coupling as a parallel minor reaction pathway, forming R-R and the U(IV) azide-bridged complex{[U]}2(μ-N3)2, along with the expected [U] = NR complex. All three [U] = NR complexes show a quasi-reversible one electron reduction between −1.6 and −1.75 V, and for R = SiMe3, chemical reduction using K/Hg affords the anionic U(IV) complex K+{U[η8-C8H6(1,4-Si(iPr)3)2](Cp*) = NSiMe3}-. The molecular structure of the latter shows an extended structure in the solid state in which the K counter cations are successively sandwiched between the Cp* ligand of one [U] anion and the COTtips2 ligand of the next.

The activation of C3O2 by the U(iii) complex [U(η5-Cp′)3] (Cp′ = C5H4SiMe3) is described. The reaction results in the reductive coupling of three C3O2 units to form a tetranuclear complex with a central cyclobutane-1,3-dione ring, with concomitant loss of CO. Careful control of reaction conditions has allowed the trapping of an intermediate, a dimeric bridging ketene complex, which undergoes insertion of C3O2 to form the final product.

The reaction of the bis(pentalene)dititanium complex Ti2(μ:η5,η5-Pn†)2 (Pn† = C8H4(1,4-SiiPr3)2) (1) with the N-heterocyclic carbene 1,3,4,5-tetramethylimidazol-2-ylidene results in intramolecular C-H activation of an isopropyl substituent to form a tucked-in hydride (3). Whilst pyridine will also effect this cyclometallation reaction to form (5), the pyridine analogue of (3), the bases 1,2,4,5-tetramethyl-imidazole, 2,6-lutidine, DABCO or trimethylphosphine are ineffective. The reaction of (1) with 2,6-dichloro-pyridine affords crystallographically characterised (6) which is the product of oxidative addition of one of the C-Cl bonds in 2,6-dichloro-pyridine across the Ti-Ti double bond in (1). The tucked-in hydride (3) reacts with hydrogen to afford a dihydride complex (4) in which the tuck-in process has been reversed; detailed experimental and computational studies on this reaction using D2, HD or H2/D2 support a mechanism for the formation of (4) which does not involve σ-bond metathesis of H2 with the tucked-in C-H bond in (3). The reaction of (3) with tBuCCH yields the corresponding acetylide hydrido complex (7), where deuteration studies show that again the reaction does not proceed via σ-bond metathesis. Finally, treatment of (3) with HCl affords the chloro-derivative (9) [(NHC)Ti(μ-H)Ti{(μ,η5:η5)Pn†}2Cl], whereas protonation with [NEt3H]BPh4 yielded a cationic hydride (10) featuring an agostic interaction between a Ti centre and an iPr Me group.