Rh-catalyzed hydrosilylation of ethylene was theoretically investigated with the DFT, MP4-(SDQ), and CCSD(T) methods, where RhCl(PH 3 ) 3 was adopted as a model catalyst. The ratedetermining step in the Chalk-Harrod mechanism is Si-C reductive elimination, the activation barrier (E a ) of which is 27.4 (28.8) kcal/mol, where the values without parenthesis and in parenthesis are calculated with the DFT and MP4(SDQ) methods, respectively. The rate-determining step in the modified Chalk-Harrod mechanism is either ethylene insertion into the Rh-SiMe 3 bond (E a ) 13.5 (16.9) kcal/mol) at the MP4(SDQ) level or oxidative addition of HSiMe 3 (E a )15.7 (11.3) kcal/mol) at the DFT level. From these results, it should be clearly concluded that the Rh-catalyzed hydrosilylation of ethylene proceeds through the modified Chalk-Harrod mechanism, unlike Pt-catalyzed hydrosilylation of alkene, which takes place through the Chalk-Harrod mechanism. The difference between Rh and Pt catalysts arises from the facts that ethylene is more easily inserted into the Rh-SiMe 3 bond with a moderate E a value than that into the Pt-SiR 3 bond (E a ) 41-60 kcal/mol) and the Si-C reductive elimination of RhCl(CH 3 )(SiMe 3 )(PH 3 ) 2 (C 2 H 4 ) needs a very large E a value. This difference in the ethylene insertion between Pt and Rh catalysts is reasonably interpreted in terms that an alkyl group is formed at a position trans to hydride in the Pt catalyst but formed at a position trans to PH 3 in the Rh catalyst. This is because ethylene can take a position trans to PH 3 in the pseudo-octahedral six-coordinate Rh(III) complex, but ethylene must take a position trans to hydride in the four-coordinate planar Pt(II) complex (remember that Rh(III) and Pt(II) have d 6 and d 8 electron configurations, respectively). The large E a value of the Si-C reductive elimination results from the fact that both sp 3 valence orbitals of SiMe 3 and CH 3 must change their directions from the Rh center toward CH 3 and SiMe 3 , respectively, in the transition state. The present theoretical calculations also show that β-H abstraction by the Rh center easily occurs in RhClH-(CH 2 CH 2 SiMe 3 )(PH 3 ) 2 to yield a Rh(III) vinylsilane complex, RhCl(H) 2 (CH 2 dCHSiMe 3 )(PH 3 ) 2 , with a low activation barrier.

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A novel spin crossover Fe(III) complex, [Fe(H3L)](ClO4)3·(4-fim)·H2O (H3L = tris[2-(((imidazol-4-yl)methylene)amino)ethyl]amine, 4-fim = 4-formylimidazole), was synthesized and the crystal structure was determined at 293 K. Cryomagnetic measurements revealed a two-step spin conversion and a pressure induced spin crossover was observed.