Reflections collected 54219 Independent reflections 12015 [R int = 0.0669] Absorption coefficient 1.026 mm-1 Absorption correction None Max. and min. transmission 0.8614 and 0.6785 Structure solution program SHELXS-97 (Sheldrick, 1990) Primary solution method Direct methods Secondary solution method Difference Fourier map Hydrogen placement Difference Fourier map Structure refinement program SHELXL-97 (Sheldrick, 1997) Refinement method Full matrix least-squares on F 2 Data / restraints / parameters 12015 / 0 / 491 Treatment of hydrogen atoms Unrestrained, disordered riding Goodness-of-fit on F 2 1.596 Final R indices [I>2σ(I), 7974 reflections] R1 = 0.0458, wR2 = 0.0749 R indices (all data) R1 = 0.0776, wR2 = 0.0783 Type of weighting scheme used Sigma Weighting scheme used w=1/σ 2 (Fo 2) Max shift/error 0.002 Average shift/error 0.000 Largest diff. peak and hole 1.183 and-0.759 e.Å-3

A variety of C-H and C-C bond forming reactions of group 7 carbonyl complexes have been studied as potential steps in a homogeneously catalyzed conversion of syngas to C 2þ compounds. The metal formyl complexes M(CO) 3 (PPh 3 ) 2 (CHO) (M=Mn, Re) are substantially stabilized by coordination of boranes BX 3 (X = F, C 6 F 5 ) in the form of novel boroxycarbene complexes M(CO) 3 -(PPh 3 ) 2 (CHOBX 3 ), but these boron-stabilized carbenes do not react with hydride sources to undergo further reduction to metal alkyls. The related manganese methoxycarbene cations [Mn(CO) 5-x (PPh 3 ) x (CHOMe)] þ (x=1 or 2), obtained by methylation of the formyls, do react with hydrides to form methoxymethyl complexes, which undergo further migratory insertion under an atmosphere of CO. The resulting acyls, cis-and trans-Mn(PPh 3 )(CO) 4 (C(O)CH 2 OMe), can be alkylated to form the cationic carbene complex [Mn(PPh 3 )(CO) 4 (C(OR)CH 2 OMe)] þ , which undergoes a 1,2 hydride shift to form 1,2-dialkoxyethylene, which is displaced from the metal, releasing triflate or diethyl ether adducts of [Mn(PPh 3 )(CO) 4 ] þ . The acyl can also be protonated with HOTf to form a hydroxycarbene complex, which rearranges to Mn(PPh 3 )(CO) 4 (CH 2 COOMe) and is protonolyzed to yield methyl acetate and [Mn(PPh 3 )(CO) 4 ] þ ; addition of L (L = PPh 3 , CO) to the manganese cation regenerates [Mn(PPh 3 )(CO) 4 (L)] þ . Since the original formyl complex can be obtained by the reaction of [Mn(PPh 3 )(CO) 5 ] þ with [PtH(dmpe) 2 ] þ , which in turn can be generated from H 2 , this set of transformations amounts to a stoichiometric cycle for selectively converting H 2 and CO into a C 2 compound under mild conditions.