= Solution
\b[Step 1 is <oxidative addition> of Si–H to Co(I), giving Co(III)–H and Co(III)–SiR$_3$.] It cleaves the Si–H bond and forms two <metal>–ligand bonds, increasing the formal <metal> <oxidation state> by two. The phosphine dissociation shown before this numbered step makes room for the reaction; <alkene> binding and phosphine exchange are ligand-substitution events rather than <oxidative additions>.
\b[Step 2 is <migratory insertion> of the coordinated <alkene> into Co–H], equivalently intramolecular <hydride> migration to the <alkene>. One <alkene> <carbon> gains H and the other gains the Co–C bond. Co remains formally Co(III); a two-electron <alkene> ligand plus a <hydride> is replaced by one alkyl ligand, reducing the <electron> count by two before phosphine reassociation.
\b[Step 3 is C–Si <reductive elimination>] between the alkyl and <silyl ligands>. It releases the hydrosilylated alkane and regenerates Co(I). The <electron> count for the drawn sequence is consistent: the one-PMe$_3$ <alkene> adduct is 16-electron; <oxidative addition> gives 18; insertion gives 16; PMe$_3$ association gives 18; <reductive elimination> returns the 16-electron <catalyst>. Because insertion occurs into Co–H, this is the conventional <Chalk-Harrod mechanism>.
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