Heterobimetallic hydride complexes are desirable synthetic targets as they are highly reactive species and exhibit unique chemical reactivity properties distinct from their monometallic counterparts. However, their high reactivity makes the synthesis and isolation of heterobimetallic hydrides challenging. This is particularly true in the case of Ln aluminium hydrides, and as such require careful ligand design to stabilise the highly electrostatic Ln and aluminium cations which are bridged by small hydrides. In the bis substituted cyclopentadienyl (Cp R ) ligand environment heterobimetallic Ln/Al hydrides have been synthesised predominantly by salt metathesis for example by reacting [{Ln(Cp R )2(X)}n] (X = halide) with LiAlH4. The hydride and heterobimetallic Ln/Al hydride chemistry in Trofimenko's scorpionate N-donor tris(pyrazolyl)borate (Tp R ) ligand environment is much less well established. Homometallic lanthanide hydrides [Ln(Tp R )(H)2]n supported by a mono (Tp R ) ligand, have been isolated in which the size of the substituents in the 3,5-position of the Tp ligand dictates the nuclearity of the complex or cluster (R = i Pr, n = 3; R = Me, n = 4; R = H, n = 6). Notably only for [Ln(Tp Me2 )(H)2]4 were Ln larger than Y stable, and when Tp R = Tp 3-tBu,5-Me was used no Ln(III) complexes were isolable. However, using Tp 3-tBu,5-Me did enable the isolation of the first molecular Yb(II) hydride ([I] in Figure ). The Tp R supported Ln hydrides are reactive, complex [I] undergoes facile activation chemistry at room temperature, whilst [II-Y] mediated hydrogenation and coupling of carbon monoxide (Figure ). The Tp 3-tBu,5-Me ligand environment can also support monomeric heterobimetallic Ln/Al dihydrides [Ln(Tp 3-tBu,5-Me )(-HAlMe3)2] where Ln = Y and Lu ([III] in Figure ). Complex [III] was synthesised by reaction of [Ln(Tp 3-tBu,5-Me )(Me)2] with 2 equivalents of HAlMe2, and moreover was shown to deprotonate amines to access a rare imide-bridged Ln-Al hydride ([IV] in Figure ). The insertion of substituted alanes into Ln-Me bonds has also been shown to be a viable route to heterobimetallic Ln/Al dihydrides in the bis-Cp* (Cp* = C5Me5) ligand environment. The reaction of HAl(Nʹʹ)2 ) and Al(Nʹʹ)3. Complex [V] was the first example of a Ln catalyst in the hydroalumination of 1-octene. Using Tp R the Ln scope is limited to Y and Lu, and there are no bis-Tp stabilised heterobimetallic Ln/Al complexes, directly analogous to the isolable bis-Cp R complexes. Recently, monomeric bis-Tp complexes [Ln(Tp)2(X)] (Ln = Y, Dy, Yb; X = triflate) have shown themselves to be a versatile tool for accessing new Ln-element bonds (E = O , N ) as the anion can be readily exchanged via metathesis or protonolysis. Here we show that aminoalane 2 metathesis between the heteroleptic Sm(III) triflates [Sm(Tp)2(OTf)] (OTf = CF3SO3) with K(Nʹʹ) in toluene. To target Ln-aluminium hydride species we opted to use the Lewis base stabilised parent alanes of the type L•AlH3 (L = NMe3, N-Heterocyclic Carbene (NHC)). As these should undergo insertion chemistry into the Ln-Nʹʹ bond akin to formation of [III] and [V], with the latter providing additional steric and electronic stabilisation from the NHC ligand. Firstly, the reaction of IDippAlH3 (IDipp = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) with 1-Ln (Ln = Y, Dy, Yb) was slow and resulted in a mixture of products. Major products were identified as [Ln(Tp)3], The diamagnetic 2-Y and paramagnetic 2-Ln (Ln = Sm, Dy, Yb) complexes exhibit multinuclear NMR data, which can be readily assigned by relative integration (see ESI Table ), and are comparable to literature. Variable temperature NMR studies of 2-Y in d8-toluene (ESI Figure ) maintained the magnetically equivalent hydride environment across the temperature range 220-353K, suggesting fast exchange of the hydrides in solution. In the FT-IR spectra of 2-Ln, weak absorptions centred around 2450-2470 cm -1 are assigned to the characteristic borohydride stretching frequencies (BH) for the Tp ligands, weak absorptions are also observed between 1800-1810 cm -1 and 1730-1740 cm -1 , consistent with terminal and bridging hydrides reported for Ln/Al heterobimetallic hydrides. Colourless single crystals of 2-Ln suitable for single crystal X-ray diffraction (SCXRD) were grown from either saturated hexane solutions (Ln = Y, Dy) or saturated toluene solutions in the presence of hexane antisolvent (Ln = Yb) at -35 °C. Important structural metrics are tabulated in the ESI (data comparison S2, and crystallographic information S3). The structures of 2-Ln are all monomeric with an 8-coordinate Ln(III) ion bound to two  3coordinated Tp ligands and two bridging hydrides, which are arranged around the Ln(III) ion in a distorted square antiprismatic geometry (see Figure for 2-Dy and ESI Figures ). The two hydrides bridge the Ln(III) to a tetrahedral Al(III) ion, which is also bound to a terminal hydride and the Nʹʹ anion (Figure ). In 2-Ln the bridging and terminal hydrides were found and refined freely, with Ln-H-Al bond distances comparable to previously reported Ln/Al heterobimetallic hydrides. The Ln•••Al separation is similar across the 2-Ln series (2-Dy 3.1826( ), 2-Y 3.1822(5), 2-Yb 3.1439(9) Å). The close Ln-Al separation (approximately the sum of Ln and Al covalent radii) is therefore indicative of strong Ln-(μ-H)-Al bonding, consistent with previous bridging hydride and 3c-2e bonds. The closest structural comparator of 2-Ln is [V], which exhibits the same  2 binding mode of the bridging hydrides. Notably, 2-Dy is the first crystallographically characterised example of the Dy-(-H)-Al unit. The Ln-N( 3 -Tp) bond distances in 2-Ln are consistent with 1-Ln and [Ln(Tp)2] + literature. The Al-N(Nʹʹ) bond distances of 1.8430( )-1.844(3) Å in 2-Ln are similar to those in [V] (1.837(2) Å), transition metal TM/Al heterobimetallic hydrides containing TM-(-H)2-Al(Nʹʹ)n, and the homoleptic complex [Al(Nʹʹ)3] (1.813(3) Å). The reactivity scope of 2-Ln was investigated to understand the nature of the heterobimetallic hydride. In a similar manner to the initial synthetic attempts, reaction of 2-Ln with 1 eq. of IDipp again led to the formation of Al-IDipp and multiple products. Importantly, no discrete homometallic Ln hydrides, putative '[{Ln(Tp)2(-H)}2]', were observed spectroscopically (see ESI Section B2.2). Previously, the ion pair reaction chemistry of Ln(L)2(μ-X)2AlX2 (L = Ligand, X = monoanionic substituent, e.g. H, CH3) has shown that these complexes should be considered as [Ln(L)2] + [AlX4] -. These Ln/Al ion pair complexes undergo facile metathesis reactions for example, metathesis of [Y(Cp*)2(-CH3)2Al(CH3)2] with KOAr (OAr = O2C-C6H2Me3-2,4,6) yielded [Y(Cp*)2(OAr)] and KAl(CH3)4. With this in mind, we examined the potential ion pair character of 2-Ln. The reaction of 2-Ln with K(Nʹʹ), should yield the starting material 1-Ln, however, this resulted in complicated reaction mixtures containing 1-Ln, [Ln(Tp)3], [{K(-Nʹʹ)(-H)Al(H)2}n], HNʹʹ, H2 gas, and in addition for 2-Yb, reduction to [Yb(Tp)2] (see ESI Sections B2.3-B2.7). Complexes 2-Ln do not undergo facile ion exchange, indicating that the strong Ln-(-H)-Al interaction that is observed in the solid state persists in solution (also consistent with the VT NMR data). Thus, 2-Ln should be considered as bimetallic hydrides.