This study describes the first use of a bis(phosphoranyl)methanido aluminum hydride, [ClC(PPh 2 NMes) 2 AlH 2 ] (2, Mes = Me 3 C 6 H 2 ) for the catalytic hydroboration of CO 2 . Complex 2 was synthesized by the reaction of a lithium carbenoid [Li(Cl)C(PPh 2 NMes) 2 ] with two equivalents of AlH 3 •NEtMe 2 in toluene at -78 o C. 10 mol % of 2 was able to catalyze the reduction of CO 2 with HBpin in C 6 D 6 at 110 o C for 2 days to afford a mixture of methoxyborane [MeOBpin] (3a; yield: 78 %, TOF: 0.16 h -1 ) and bis(boryl)oxide [pinBOBpin] (3b). When more potent [BH 3 •SMe 2 ] was used instead of HBpin, the catalytic reaction was extremely pure, resulting in the formation of trimethyl borate [B(OMe) 3 ] (3e) [catalytic loading: 1 mol % (10 mol %); reaction time: 60 min (5 min); yield: 97.6 % (>99 %); TOF: 292.8 h -1 (356.4 h -1 )] and B 2 O 3 (3f). Mechanistic studies show that the Al-H bond in complex 2 activated CO 2 to form [ClC(PPh 2 NMes) 2 Al(H){OC(O)H}] (4), which was subsequently reacted with BH 3 •SMe 2 to form 3e and 3f, along with the regeneration of complex 2. Complex 2 also shows good catalytic activity towards hydroboration of carbonyl, nitrile and alkyne derivatives. ASSOCIATED CONTENTSupporting Information. The Supporting Information is available free of charge on the ACS Publications website.Experimental procedures (PDF), theoretical studies (XYZ) X-ray crystallographic data (CIF)

So* [a] 0.5 mol% of the amidinato disilyne [LSi:] 2 (1, L=PhC(NtBu) 2 ) and 1 mol% of B(C 6 F 5 ) 3 cooperatively polymerized methyl methacrylate (MMA) to form poly(MMA) with the H and CH 2 C-(=CH 2 )COMe end groups (P1, M n = 3.1 × 10 3 gmol À 1 ; repeating unit, n = 31; polydispersity index, Đ: 1.55). The catalytic mechanism is proposed, where compound 1 could react with MMA and B(C 6 F 5 ) 3 to form a zwitterionic active species [LSi{CH 2 C-(Me)=C(OMe)O}B(C 6 F 5 ) 3 ] 2 . The latter could activate MMA molecules affording poly(MMA) chains on the silicon centers. The methyl group of the last enolate of a poly(MMA) chain on a silicon center could then react with the SiÀ C bond of the adjacent poly(MMA) chain to form P1, as well as regenerate compound 1 and B(C 6 F 5 ) 3 . When compound 1 was replaced by the amidinato amidosilylene [LSiN(SiMe 3 ) 2 ] (2), 1 mol% of compound 2 and 1 mol% of B(C 6 F 5 ) 3 mediated living MMA polymerization in toluene to form the poly(MMA) P2 (M n = 1.04 × 10 4 gmol À 1 ; Đ: 1.97).