Postsynthetic modi ﬁ cations of [2,2,2-(H)(PPh 3 ) 2 - closo -2,1-RhSB 8 H 8 ] with Lewis bases: cluster modular tuning †

It has been demonstrated that the reaction of [2,2,2-(H)(PPh 3 ) 2 - closo -2,1-RhSB 8 H 8 ] ( 1 ) with PPh 3 a ﬀ ords the boron substituted rhodathiaborane – PPh 3 adduct, [6,6-(PPh 3 ) 2 -9-(PPh 3 )- arachno -6,5-RhSB 8 H 9 ] ( 2 ). Building upon this reaction, we report herein that the 10-vertex hydridorhodathiaborane 1 reacts with the Lewis bases, PCy 3 , py, 2-Mepy, 2-Etpy, 3-Mepy and 4-Mepy to form the rhodathiaborane – ligand adducts, [6,6-(PPh 3 ) 2 -9-(L)- arachno -6,5-RhSB 8 H 9 ], where L = PCy 3 ( 3 ), 2-Mepy ( 4 ), 2-Etpy ( 5 ), py ( 6a ), 3-Mepy ( 7a ) or 4-Mepy ( 8a ), and [8,9-μ -(H)-9-(PPh 3 ) 2 -8-(L)- arachno -9,6-RhSB 8 H 8 ], where L = py ( 6b ), 3-Mepy ( 7b ) or 4-Mepy ( 8b ). The selectivity of the reactions depended on the nature of the entering Lewis bases, a ﬀ ording the 6,5-isomers, 2 , 3 , 4 and 5 as single products for PPh 3 , PCy 3 , 2-Mepy and 2-Etpy; and mixtures of the 6,5-/9,6-regioisomers, 6a / 6b , 7a / 7b and 8a / 8b for py, 3-Mepy and 4-Mepy, respectively. The molecular structures of both regioisomers were characterized by X-ray di ﬀ raction analysis for the 6,5-isomers, 3 and 4 , and for the 9,6-isomers, 7b and 8b . Variable temperature NMR studies of the reaction between 1 and PPh 3 or 2-Mepy demonstrated that at low temperatures there is formation of the 9,6-species that subsequently isomerizes to the 6,5-regioisomer, indicating that for the more sterically hindered Lewis bases, PPh 3 , 2-Mepy and PCy 3 , the latter isomer is more stable and accessible through an intramolecular {Rh(PPh 3 ) 2 } vertex ﬂ ip. The formation of both isomers with py, 3-Mepy and 4-Mepy indicates that the kinetic and thermodynamic energies of the 6,5 and 9,6 rhodathiaborane – ligand adducts are similar for these Lewis bases. Lewis base bonding to exo -polyhedral boron vertices results in a change of the metal coordination from pseudo-octahedral Rh( III ) in 1 to pseudo-square planar Rh( I ) in the adducts. The chemistry described here highlights the remarkable structural ﬂ exibility of these polyhedral boron-containing compounds, their modular architecture and their easy postsynthetic modi ﬁ cation.


Introduction
The possibility to alter the properties of metal complexes by ligand change is without doubt an important driving force for the synthesis of new transition element complexes that may find application in, for example, the activation of small unreactive molecules. 1In particular, the design of molecular catalysts based on non-innocent ligands with potential to cooperate with the metal centre has witnessed an increasing interest. 2Some of these complexes promote chemical transformations in which the metal acts, for example, as a Lewis acid centre in cooperation with the surrounding ligand that performs as a base. 3These metal-ligand cooperations are often referred to as bifunctional mechanisms.
5][6] In these molecules, the borane/heteroborane moieties act as polyhapto non-innocent ligands. 7The vertices in these polyhedral compounds are potentially different reactive centres that allow the modular tuning of the polyhapto borane/heteroborane ligand, leading to rapid and simple assembly of a range of complexes from a common precursor. 8his postsynthetic modification circumvents the inconvenience of multistep ligand syntheses.
In recent years, we have demonstrated the benefits of this modular approach in the synthesis of a significant range of 11-vertex polyhedral metallathiaboranes that bear {SB 9 H 9 } moieties acting as polyhapto ligands bound to a transition metal fragment (Scheme 1).][11] This reaction chemistry highlights the redox structural flexibility of these clusters, which can be altered by the linkage of different Lewis bases to the vertices of the cage.In consequence, we have developed a rich organometallic chemistry based on a family of 11-vertex metallathiaboranes.Overall, we have discovered unprecedented stoichiometric cycles that involve structural redox transformations that are promoted by dehydrogenation/hydrogenation processes in which the dihydrogen is heterolytically cleaved on the clusters.][13] Cleavage of dihydrogen on the cluster follows a mechanism in which the metal vertex and the thiaborane fragment simultaneously participate in the bond-breaking and -forming processes, resembling, therefore, the above-mentioned metalligand bifunctional systems.This feature has been recently studied by performing DFT calculations dealing with the carbene-ligated rhodathiaborane, [1,1-(IMe)(PPh 3 )-3-( py)-1,2-RhSB 9 H 8 ] (IMe = 1,3-dimethylimidazol-2-ylidene), which undergoes a cluster rearrangement to accommodate the entering H 2 molecule that is subsequently heterolytically cleaved.This mechanism highlights the involvement of the entire rhodathiaborane cluster in the activation process.
In an attempt to find new types of reactivities promoted by metallathiaboranes, we altered the composition of the polyhapto thiaborane ligand by using a smaller nine vertex cluster based on the {SB 8 H 8 } fragment.And, after systematic reactivity studies, we were able to accomplish the high yield synthesis of [2,2,2-(H)(PPh 3 ) 2 -closo-2,1-RhSB 8 H 8 ] (1), providing the opportunity for the development of the reaction chemistry of this 10-vertex hydridorhodathiaborane. 14 Thus, we found that, similarly to the 11-vertex rhodathiaborane above, compound 1 exhibits reactive B-H centers capable of forming exopolyhedral Ph 3 P-B bonds that lead to the formation of the rhodathiaborane adduct, [6,6-(PPh 3 ) 2 -9-(PPh 3 )-arachno-6,5-RhSB 8 H 9 ] (compound 2, see Schemes 1 and 2).This reaction reveals an interesting structural redox switchability that combines the {RhH(PPh 3 ) 2 } group and the {SB 8 H 8 } polyhapto ligand in 1, which has yielded new stoichiometric and catalytic cycles driven by dihydrogen and ethylene. 15imilar to the 11-vertex rhodathiaborane introduced above, this reactivity opens the door for a postsynthetic modification of the 10-vertex clusters by the systematic change of the exopolyhedral ligands bound to a boron vertex of the {SB 8 H 8 }-fragment.We hypothesized that varying the boron-ligated Lewis base could be a powerful way to tune the reactivity of the 10-vertex rhodathiaboranes.We report herein the reactions of the parent hydridorhodathiaborane 1 with the Lewis bases PCy 3 , 2-Mepy, 3-Mepy and 4-Mepy.These reactions have led to the isolation and characterization of a set of new 10-vertex rhodathiaborane-ligand adducts.The compounds were structurally and spectroscopically analyzed in order to understand the metal-thiaborane bonding interaction.A new isomerisation process is also reported, which further illustrates the remarkable chemical and structural flexibility of these types of polyhedral boron-containing compounds.
From a descriptive point of view, the formation of the Lewis base-rhodathiaborane adducts can be envisioned as the addition of the metal centre, {Rh(PPh 3 ) 2 }, to the neutral 9-vertex thiaborane adducts exo-6-L-arachno-SB 8 H 10 (Chart 1, schematics Ia and Ib) .In the arachno-6,5-isomers the linkage between the metal fragment and the thiaborane cage occurs at the {S(4)B( 9)B(8)} vertices of the hexagonal, chair-like face; whereas in the arachno-9,6-isomers the metal group is accommodated on the {B( 6)B( 7)B(8)} section of the same chair-like face of the thiaborane-ligand adducts (Chart 1).This description raises the hypothesis that, alternatively to the procedure described herein, the 10-vertex L-substituted rhodathiaboranes could be synthesized from reaction of exo-6-L-arachno-SB 8 H 10 with [RhCl(PPh 3 ) 3 ]: a hypothesis we hope to experimentally test in the future.
In the text, we refer to arachno compounds 2-5 and 6a-8a as 6,5-isomers, and to arachno 6b-8b as 9,6-isomers, where the numerals 6,5 and 9,6 indicate the position of the rhodium and sulphur vertices in the cluster framework (see Chart 1 and Fig. 1 for numbering).

X-Ray diffraction analysis
The molecular structures of 3 and 4 (6,5-isomers), and of 7b and 8b (9,6-isomers) were determined by X-ray diffraction analysis.Tables 1 and 2 list selected distances and angles for the new 10-vertex arachno-rhodathiaborane together with data for the previously reported PPh 3 -ligated derivative, 2. Fig. 1 shows an ORTEP-type picture for the 2-Mepy-and 3-Mepy-ligated isomers, 4 and 7b.Both types of isomers exhibit an open, hexagonal boat-shaped face.In the arachno-6,5-species, 3 and 4, there is an {Rh(PPh 3 ) 2 } fragment and an {L-B} group, where L = PCy 3 or 2-Mepy, present on opposite sides of the open face and occupying the vertices of connectivity three at positions 9 and 6, respectively.Other structural features of note on the 6,5-isomers, 3 and 4, also found in previously reported 2, 14 are the presence of an endo-hydrogen atom on the ligandsubstituted vertex, L-B(9)-H, and the presence of a bridging hydrogen atom along the B(7)-B(8) edge.
In the arachno-9,6-species, 7b and 8b, the Lewis base-substituted boron vertex occupies cluster position 8 of connectivity four, adjacent to the metal vertex, on the hexagonal boat-like open face.A hydrogen atom bridges the B(8)-Rh(9) edge, and a second hydrogen atom lies along a B-B edge that flanks the positions 6 and 9 of the 10-vertex clusters: the numbering changes from B(7)-B(8) in the 6,5 isomers to B(5)-B(10) in the 9,6 isomers but the edge is formally the same (Fig. 1).
It is of interest to point out that the Rh-P distances for the phosphine ligands that lie trans to boron vertices [P(2) in Fig. 2] are significantly longer than the phosphines trans to the sulfur vertex.This structural trend is a manifestation of the stronger trans influence of the cage boron atoms relative to the cage sulfur atoms.A similar situation has been recognized in metallacarboranes, where the cage boron atoms have a larger trans influence than the cage carbon atoms. 17It is also interesting to note that the Rh-P length for the PPh 3 ligand trans to the hydride ligand in compound 9 is the longest in this family of polyhedral clusters, demonstrating that the structural trans effect of the cage B atoms is not as strong as the effect of a hydride ligand.
From the discussion above, it is clear that the metal-to-thiaborane configuration is directed by the relative trans influences of the exo-polyhedral ligands with respect to the metal-bound cage atoms.Thus, in compound 9, the metal hydride, having the choice, avoids lying trans to cage boron vertices.The same tendency has been identified in a significant number of hydridometallathiaboranes 9,14,15,18 where the exo-polyhedral ligand orientation is mainly controlled by cage S atoms that, in the metal-thiaborane linkage, force the hydride ligands to occupy positions trans to the heteroatom: the strong transinfluence hydride ligand avoids the cage B atoms that exhibit a stronger structural trans effect than the cage sulfur atoms.
An important characteristic of these types of polyhedral boron-containing compounds that we want to highlight in Fig. 2 is their modular architecture, which a priori makes possible the interchange of their constituents while maintaining a basic chemical structure.This is an interesting feature since, as pointed out in the Introduction, it permits the modification of the reactivity of a parent cluster simply by changing the units in the "molecular model" (a concept that is well appreciated in the construction of car), saving, therefore, the time that is usually involved in the preparation of different ligands to change the electronic properties of classical coordination complexes.In compounds 4 and 7b, the sulfur vertex and the Lewis base boron-substituted group, L-B-H, are exchangeable, indicating that both units are isolobal and exhibit a similar bonding interaction within the cluster framework.In addition, from the point of view of the electron counting rules, these two units, S and L-B-H, are also isoelectronic, formally contributing four electrons to the cluster skeletal bonding.It should be noted that the hydrogen atom of the LBH group occupies an endo-position in 4, pointing towards the hexagonal boatlike face; whereas in the 9,6-isomer, 7b, the hydrogen atom lies bridging along the Rh(9)-B(8) edge.
The modular character of these clusters is further appreciated in the mutual interchangeability of the metal fragments, {Rh(PPh 3 ) 2 } and {Rh(CO)(H)(PPh 3 ) 2 }, in 4 and 9, respectively.Thus, although these two fragments, according to Wade's  rules, 19 contribute a different number of electrons to the cluster framework [Rh(L) 2 one electron vs. Rh(H)(L) 3 four electrons], they behave as two isolobal fragments that bind the thiaborane framework in a trihapto-fashion: the metal-to-thiaborane interaction is fundamentally the same in both species.Therefore, even though, compounds 4 and 7b have 10 + 2 skeletal electron pairs (steps) that are typical of 10-vertex nidometallaheteroboranes and 9 has an additional pair (10 + 3) that gives the cage an arachno-electron counting, these clusters are nevertheless all conveniently described as 10-vertex arachno-species.
Metallaboranes and metallaheteroboranes with 10-vertex nido-/arachno-structures are well represented in polyhedral boron chemistry. 6,20The metal fragments that form part of these polyhedral boat-like cages are diverse, and although there are many examples that incorporate the {Pt(L) 2 } group in the framework, 21,22 10-vertex nido-/arachno-rhodadecaboranes and rhodaheterodecaboranes with pseudo-square planar {Rh-(L) 2 } centers in their structures are very uncommon.Previous to this work, the {Rh(PPh 3 ) 2 } moiety has only been crystallographically characterized as a component of a 10-vertex nido-/ arachno-boat-like cluster in compound 2. 14 It should be noted that polyhedral molecules incorporating C 2v fragments such as {M(L) 2 }, where M = Rh, Ir, Pd or Pt, usually do not fulfill the requirements of the electron-counting rules, leading to metallaboranes and metallaheteroboranes that can be one or even two steps short of those formally required by Wade's rules. 23The discrepancy arises because the metal fragments, {M(L) 2 }, differ significantly from the isolobal schemes exhibited by the boron vertices and they do not bear additional ligands [i.e.Rh(L) 2 vs. Rh(H)(L) 3 ] that formally contribute the electrons needed to fulfill the counting rules.

Comparative NMR analysis: bonding considerations
All compounds reported in this work were characterized by multielement NMR spectroscopy.The measured data for the PCy 3 (3), 2-Mepy (4) and 2-Etpy (5) adducts are gathered in Table 3, and the data for the py (6a-6b), 3-Mepy (7a-7b) and 4-Mepy (8a-8b) counterparts are listed in Table 4.The latter data were measured for samples that contained both isomers, arachno-6,5 (6a-8a) and arachno-9,6 (6b-8b).The assignments of the 1 H and 11 B resonances for all the new ten-vertex rhodathiaboranes were made using 1 H-{ 11 B(selective)} experiments and GIAO NMR nuclear shielding predictions on optimized cluster geometries.It should be noted that a reasonable measure of the validity of the calculated structures of these rhodathiaboranes is given by a comparison of the measured 11 B NMR chemical shift values and the calculated boron nuclear shielding properties (GIAO approach).The 11 B chemical shifts calculated for all the new species reported in this paper reproduce well the experimental trend, and are sufficiently in agreement to support the assignments.
The new Lewis base substituted clusters, reported in this work, exhibit a similar pattern in their 11 3 and 4).Similarly, the arachno-9,6 clusters, 6b-8b exhibit broad overlapping peaks close to δ B 0 ppm that correspond to the B(5) and B (7) vertices.
Similarly, the 11 B NMR shielding patterns of the isomeric arachno-9,6 and arachno-6,5 adducts can be conveniently compared to the parent neutral nine-vertex pyridine-ligated thiaborane, exo-6-py-arachno-4-SB 8 H 10 (Fig. 3). 25Overall, there is a small high frequency shift in the 11 B NMR spectra of the L-ligated clusters 4, 3 and 7b, compared with that of the thiaborane pyridine adduct.In the arachno-6,5-isomer, the most significant change corresponds to B(8) in the thiaborane [B(7)  in 4] that shifts ∼20 ppm toward a high frequency.Considering that the {Rh(PPh 3 ) 2 } fragment subrogates the B(8)-H-B(9) hydrogen atom in the L-ligated thiaboranes (Chart 1, schematics Ia and Ib), forming a bond with B( 7), the mentioned large shift is most likely due to changes in the electronic distribution upon metallation and to deshielding effects of the adjacent PPh 3 ligands.
The main changes in the 11 B NMR spectra of the arachno-9,6 isomers with respect to the spectrum of exo-6-py-arachno-4-SB 8 H 10 correspond to the B(2) and B(4) vertices that in the pyligated adduct are B(1) and B (7), respectively (see Chart 1, schematics Ia and III).Both signals shift to high frequencies in the rhodathiaborane adducts, perhaps (i) as an enhancement of the antipodal effect of the S(6) vertex on B(4) that may occur upon metallation of the 9-vertex thiaborane-ligand cages, {SB 8 H 9 (L)}, at the B(6)-B(7)-B(8) interface, combined with PPh 3 deshielding effects and (ii) as a manifestation of an antipodal effect that the new {Rh(PPh 3 ) 2 } vertex may exert on B(2).
It is also worth noting that the resonance of the Lewis basesubstituted boron atom suffers a marked shift toward a low frequency when the ligand is PPh 3 or PCy 3 instead of a pyridinic substituent.In this regard, it has been reported that Lewis baseborane adducts such as LBH 3 , arachno-[6-L-B 10 H 13 ] − and arachno-6,9-L 2 B 10 H 12 , where L = NR 3 , NHMe 2 , NH 2 Me, PPh 3 , PMe 3 , etc., exhibit a high frequency shift of 20-30 ppm in the 11 B NMR signal of the L-substituted vertex with ammines and pyridine with respect to the values of the phosphine-ligated adducts. 24he changes in the 11 B NMR pattern of the thiaborane fragments {SB 8 H 10 } and {SB 8 H 9 L} upon bonding to the metal fragments {Rh(CO)(H)(PPh 3 ) 2 } and {Rh(PPh 3 ) 2 }, respectively, have been rationalized mainly based on long-range antipodal effects of the metal and the sulphur vertices and on the empirical "μH rule". 24It is reasonable to assume that the {Rh(CO)(H)(PPh 3 ) 2 } fragment in compound 9 acts, to a large extent, as a surrogate of the two μH bridges along the B(6)-B (7) and B(7)-B(8) edges in the parent nine-vertex thiaborane arachno-4-SB 8 H 12 .

Kinetic studies and mechanistic considerations
The reaction between the hydridorhodathiaborane, 1, and 2-Mepy was studied by 1 H NMR spectroscopy, monitoring the decrease of the Rh-H hydride resonance versus time.The concentration of 2-Mepy was kept in excess to hold the pseudofirst-order conditions.The values of k obs for different concentrations of 2-picoline are summarized in Table 5, together with the data previously reported by us for the reaction with PPh 3 . 14he analysis of the variation of the concentration of 1 with the time follows a first-order kinetic with respect to this reactant and the plot of k obs versus [2-Mepy] reveals a reaction order of one.Therefore, the reaction of 1 with 2-Mepy obeys an overall second-order rate law in agreement with the results found with PPh 3 as the reactant.
In the reaction of the 10-vertex hydridorhodathiaborane, 1, with PPh 3 to give the arachno-adduct, 2, we detected the formation of an intermediate that, in the 1 H-{ 11 B} NMR spectrum, exhibited a pseudo-quartet at δ H −8.41 ppm, diagnostic of the presence of a Rh-H-B bridging hydrogen atom.This intermediate evolved to give the arachno-cluster, 2, following a zero-order kinetic.Without X-ray diffraction data, we proposed that the intermediate was an arachno-6,5-species with a bridging hydrogen atom along the Rh(6)-B( 7) edge and the trans- formation into 2 was described, therefore, as a simple migration of the Rh( 6)-H-B (7) bridging hydrogen atom to the B(7)-B(8) edge of the hexagonal boat-like face of the 10-vertex arachno-cluster (Chart 1, schematic IV). 14 The reactions reported herein with a range of Lewis bases have revealed unambiguously that there is a formation of two regioisomers: arachno-6,5 and arachno-9,6.Therefore, it is clear that the intermediate detected at low temperatures in the reaction between 1 and PPh 3 is the arachno-9,6-isomer that subsequently undergoes cluster rearrangement to form the arachno-6,5-isomer.The PPh 3 -ligated arachno-9,6-species is sufficiently stable to permit the study of its transformation into the arachno-6,5-isomer.
The spectroscopic data for the 2-Mepy-ligated 9,6-isomer are consequently limited due to the low concentration of this species and the fact that some signals in the 1 H-{ 11 B} and 11 B NMR spectra overlap with those of the major components, 1 and 4.However, the 1 H-{ 11 B} NMR spectrum shows a broad doublet of doublets at δ H −6.56 that can be assigned with confidence to the Rh( 9)-H(8,9)-B(8) bridging hydrogen atom, being, as mentioned above, diagnostic of the presence of the arachno-9,6-species versus its arachno-6,5-isomer (Fig. S1 †).The low intensity doublet of doublets at δ P +26.7 and +49.3 in the 31 P-{ 1 H} NMR spectrum can also be assigned with confidence to the 2-Mepy-ligated kinetic intermediate (Fig. S3 †).
The mechanism of conversion of the 9,6 isomer to the 6,5 isomer may seem to take place through a Rh(9)-vertex transfer (or flip) and a concomitant movement of the B( 5)-H-B(10) bridging hydrogen atom to the B(7)-B( 8) edge (Scheme 3).Cluster rearrangements by vertex flips have been invoked, for example, to rationalize the fluxional enantiomerization of [(PMe 2 Ph) 2 PtS 2 B 7 H 7 ] and also for the conversion of nido-6-iridadecaboranes to nido-5-iridadecaboranes. 22,27 These results have revealed that the 9,6 isomers are the kinetic products that subsequently rearrange to give the 6,5 species.Thus, under typical work-up conditions, the reactions with PPh 3 , PCy 3 , 2-Mepy and 2-Etpy afford selectively the 6,5 isomers, and it was only during our experiments at low temperatures that we could detect in solution the 9,6 species for the reactions with PPh 3 and 2-Mepy.The selectivity of the reaction is lower for the reactants, py, 3-Mepy and 4-Mepy, which affords mixtures of both isomers.The presence of the 9,6 isomers in higher ratios than the 6,5 isomers suggests that the formers are slightly more stable although it is clear that for the less sterically hindered ligands, py, 3-Mepy and 4-Mepy, the kinetic and thermodynamic stability of both regioisomers is very similar.
In other words, the transformation of the arachno-9,6products to give the arachno-6,5-regioisomers is driven, to an important degree, by steric interactions between the exo-polyhedral boron-bound ligands, L, and the PPh 3 ligands at the metal centre.The kinetic studies reported previously for the reaction of 1 with PPh 3 and herein with 2-Mepy demonstrate a bimolecular process.However, the intimate mechanism of interaction between the closo-hydridorhodathiaborane and the incoming Lewis base to form the transition state through which the system evolves to give the rhodathiaborane-ligated Scheme 3 Arachno-9,6 → arachno-6,5 isomerization via a metal vertex flip.adducts (6,5 and 9,6 regioisomers) is unknown at this time.
We can speculate that the entering ligand attacks the closocluster at the metal centre, an adjacent boron vertex or an Rh-B edge.Regardless of the mechanism, it is important to keep in mind that the hydridorhodathiaborane reagent, 1, exhibits an inherent stereochemical non-rigidity that facilitates, first the interaction of the Lewis base with the reactive vertices of the 10-vertex closo-cage, and second, the reorganization of the system to form a transition state that finally allows the formation of the arachno-regioisomers.As demonstrated in other examples, structural flexibility in metallaboranes and metallaheteroboranes is the key feature in the activation of unreactive molecules such as dihydrogen. 12n the reactions with PPh 3 and 2-Mepy, it have been demonstrated that the 6,5 isomer is the thermodynamic product that can be formed from the 9,6 isomer; however, we should consider that the more stable 6,5 isomer could be accessible directly from a bimolecular transition state without forming the 9,6 isomer.This idea arises from the perception that the formation of a 9,6 isomer bearing the very bulky PCy 3 ligand is very unlikely and, therefore, it is reasonable to believe that the pathway toward the 6,5 isomer may not require the formation of the 9,6 isomer, and that both isomers may be accessible from a common transition state.

General procedures
The reactions were carried out under an argon atmosphere using standard Schlenk-line techniques.Dry solvents were obtained from a Solvent Purification System from Innovative Technology Inc.The thiaborane arachno-4-SB 8 H 12 , 28 the [PSH][arachno-4-SB 8 H 11 ] salt and Wilkinson's compound [RhCl(PPh 3 ) 3 ] were prepared according to the literature methods. 14,29All other reactants were used as received.
Compounds 1 and 2 were prepared using optimized procedures that are different from those published by us in ref. 14.The new synthesis of 1 and 2 is reported below.

Calculations
All calculations were performed using the Gaussian 09 package. 33Structures were initially optimized using standard methods with the STO-3G* basis-sets for C, B, P, S and H, and with the LANL2DZ basis-set for the rhodium atom.The final optimizations, including frequency analyses to confirm the true minima, were performed using the B3LYP methodology, with the 6-31G* and LANL2DZ basis-sets.The GIAO nuclear shielding calculations were performed on the final optimized geometries, and computed 11 B shielding values were related to chemical shifts by comparison with the computed value for B 2 H 6 , which was taken to be δ( 11 B) +16.6 ppm relative to the BF 3 (OEt 2 ) = 0.0 ppm standard.
Synthesis of [2,2,2-(PPh 3 ) 2 (H)-closo-2,1-RhSB 8 H 8 ] (1).0.2 g of [PSH][SB 8 H 11 ] (0.6 mmol) was placed in a Schlenk tube under an argon atmosphere, and the salt was dissolved in 18 mL of dry dichloromethane.The tube was immersed in an isopropanol bath at −35 °C, then an equimolar amount of [RhCl(PPh 3 ) 3 ] (0.56 g) was added to the solution and the mixture was stirred at −35 °C.After 15 minutes, the reaction mixture was filtered through silica gel.The filtrate was collected in a Schlenk tube and placed in an isopropanol bath at −15 °C.After low temperature solvent evaporation, addition of hexane gave 1 as a yellow precipitate that was washed with hexane (6 × 10 ml), isolated by filtration and dried in a vacuum.Yield 0.34 g (77%, 0.33 mmol).
Synthesis of [6,6-(PPh 3 ) 2 -9-(PPh 3 )-arachno-6,5-RhSB 8 H 9 ] (2).0.2 g of [PSH][SB 8 H 11 ] (0.6 mmol) was placed under an argon atmosphere in a Schlenk tube and dissolved in 18 mL of dry dichloromethane.0.56 g of [RhCl(PPh 3 ) 3 ] (0.6 mmol) was added at −35 °C and the solution was stirred for 15 minutes at a low temperature.Then the reaction mixture was filtered through silica gel and the solvent was reduced in volume until 10 mL.The remaining solvent was degassed and then 0.46 g of crystallized PPh 3 (1.18mmol) was added.The mixture was stirred for 5 hours at room temperature.After solvent evaporation and addition of hexane, compound 2 was isolated as a yellow precipitate.The solid was washed with hexane (6 × 10 ml), filtered and dried in a vacuum.Yield 0.4 g (67%, 0.39 mmol).

Conclusions
The hydridorhodathiaborane, 1, which exhibits a closo structure based on a 10-vertex bicapped square antiprism,
B NMR spectra with resonances in the interval between δ B +20 and −40 ppm.The signals corresponding to the cage B(4), B(7), B(8) and B(10) atoms in the arachno-6,5 isomers overlap in the region between 0 and −7 ppm, complicating the resolution of the spectra.And the 11 B NMR spectra were indirectly resolved by 1 H-{ 11 B(selective)} experiments that correlate the broad overlapping signals with four individual B-H terminal proton resonances (Tables ligated 9,6 isomer 2, which shows a pseudo-quartet at δ H −8.41, see ref.14), the proton resonance at the lowest frequency can be assigned with confidence to the Rh(9)-H-B(8) bridging hydrogen atom, being a proof of the presence of the arachno-9,6-species versus their arachno-6,5-isomers.
B chemical shift of these two signals, B(2) and B(3).