Transition metal monoboryl (M-BR 2 ) complexes 1 have received considerable attention due to their role in the metalcatalyzed hydroboration 2 of unsaturated organic substrates. Recently, the first well-characterized polyboryl complexes M(BR 2 ) n (n ) 2,3) have been isolated, [3][4][5] particularly from the oxidative addition 4,5 of B 2 (cat) 2 (cat ) 1,2-O 2 C 6 H 4 ) (1) and its analogues to low-valent unsaturated metal fragments. This is of importance with regard to the metal-catalyzed diboration of alkynes 5 and alkenes 6 and the related formation of ArBpin (Ar ) aryl, pin ) OCMe 2 CMe 2 O) compounds 7 from ArX + B 2 (pin) 2 catalyzed by a Pd system.In order to study the reactivity of the M-B bond in lowvalent, electron-rich, late metal boryl complexes, and to examine the reversibility of the B-B bond oxidative addition process, we sought to prepare a rhodium(I) boryl complex; the only previous example 8 being the poorly characterized species [(PPh 3 ) 3 Rh(BBr 2 )]. Our previous attempts to prepare Rh(I)-B(cat) compounds via reactions of [(PMe 3 ) 4 RhH] or [( i Pr 2 PCH 2 -CH 2 P i Pr 2 )Rh(η 3 -2-Me-allyl)] with HB(cat) 1c,9 resulted in the formation of compounds such as [(PMe 3 ) 4 Rh] + , [(PMe 3 ) 4 Rh-(H) 2 ] + and the zwitterion [(dippe)Rh(η 6 -(cat)B(cat))]. We reasoned that reaction of 1 with [(PMe 3 ) 4 RhMe] (2), however, would likely lead to [(PMe 3 ) 4 Rh(B(cat))] ( 3) via oxidative addition of the B-B bond followed by rapid reductive elimination of MeB(cat) (4) (eq 1).Indeed, when this reaction was conducted using a 1:1 molar ratio of 1:2 in heptane, 3 and 4 were formed rapidly in essentially quantitative yield 10 as evidenced by 1 H, 11 B{ 1 H}, 13 C{ 1 H}, and 31 P{ 1 H} NMR spectroscopy. The presence of 4 in solution was confirmed additionally by ambient temperature vacuum transfer of all volatiles to a separate vessel and subsequent examination of these by GC/MS.Compound 3 is fluxional in solution as evidenced by the appearance of one doublet (-21.9 ppm, J Rh-P ) 137 Hz) in the room temperature 31 P{ 1 H} NMR spectrum which displays a sharp doublet of doublets (-22.0 ppm, J Rh-P ) 157, 2 J P-P ) 48 Hz, 3P) and a broader overlapped doublet of quartets (-13.2 ppm, J Rh-P ) 91, 2 J P-P ) 48 Hz, 1P) at 193 K. The lowtemperature limiting spectrum indicates a trigonal bipyramidal geometry with the B(cat) ligand occupying an axial site. This structure was confirmed by single-crystal X-ray diffraction 11 (Figure 1, top). That the B(cat) group prefers an axial rather
