Could you please explain the theory behind this please?

Mechanism of Substitution Reactions in Metal Carbonyls

Even electron count: Dissociative Process $($Sn$1)(16$ or $18$ or $20$ ve$\text{'})$

$LnM-CO{?}^{CO}LnM-{}^{+L}LnM-L^{\text{'}}$

vacant coord. site

CO replaced by L

$2^{nd}$ row reacts faster than $3^{rd}$ row...

Therefore $2^{nd}$ row are better catalysts.

Mechanism of Substitution Reactions in Metal Carbonyls

Odd electron count: Associative Process $($Sn$2)$

$(17$ or $($:$19v{e}^{-}\}$

CO replaced by $L$

$(CO{)}_{n}M+L(CO{)}_{n}M-L(CO{)}_{(n-1)}ML$

$V(CO{)}_{6}L-V(CO{)}_{6}L-V(CO{)}_5$

$17v{e}^{-}($RDS$)19v{e}^{-}($fast$)17v{e}^{-}$

$V(CO{)}_6$ reacts ${10}^{10}$ faster than $Cr(CO{)}_6$