AbstractThe diprotic, tridentate O,S,Oâligands LH2 {[RC(=O)CH2]2S; R=tBu (3 a) or Ph (3 b)}, comprising hard (O) and soft (S) donor atoms, have been employed for the first time in zincâmediated hydrosilylation of various ketones, giving, after protolytic workup, the corresponding alcohols. The respective precatalysts used are novel thiobis(enolato) zinc complexes, [LZn(tmeda)] [L=3 aâ2H+ (4 a) or 3 bâ2H+ (4 b); tmeda= N,N,Nâ˛,Nâ˛âtetramethylethylenediamine], [LZn(bipy)] [L=3 aâ2H+ (5 a); bipy=2,2â˛âbipyridine], [LZn(phen)] [L=3 aâ2H+ (6 a); phen=1,10âphenanthroline], and [LZn(dabco)] [L=[3 aâ2H+] (7 a); dabco=1,4âdiazabicyclo[2.2.2]octane]. These complexes are accessible by simple Brønsted acidâbase reaction of 3 a or 3 b with dimethylzinc in a 1:1 molar ratio in the presence of tmeda, bipy, phen, or dabco as auxiliary ligands. The first four complexes are isolated as yellow or colorless crystals in 76 % (4 a), 53 % (4 b), 58 % (5 a), and 61 % (6 a) yields, whereas 7 a (74 % yield) is isolated as colorless powder. The zinc center in 4 a, 4 b, 5 a, and 6 a has trigonal bipyramidal coordination, as proven by singleâcrystal Xâray diffraction analysis. Remarkably, 4 a shows the highest catalytic activity hitherto reported for zincâcatalyzed hydrosilylation over a wide range of substrates with a turnover frequency of 970 hâ1. Furthermore, a catalytic mechanism is proposed.