Advanced Inorganic Chemistry/Dissociative Mechanism

Dissociative substitution mechanism describes one of the common pathways through which a ligand substitution reaction takes place. Found often in octahedral complexes, dissociative mechanisms are distinguished by having an ion X^- dissociate from a metal complex, resulting in an intermediate compound with a lower coordination number. This lost ion is then replaced by an ion Y^-, as opposed to associative complexes which form a complex of X^-, Y^-, and the metal before the metal to X^- ion bond is broken. This process is analogous to an S_N1 reaction. In a dissociative mechanism, the complex is usually has fully saturated coordination, with 18 or more electrons. While most octahedral dissociate, not all do, with key examples being [Cr(NH₃)₅H₂0]³⁺ which follows I_d substitution and [Cr(H₂O)₆]³⁺ following I_a.

Mechanism

The mechanism is below as follows with Y^- substituting in for X^- on a general octahedral complex.

${\ce {L_{\mathit {5}}M-X<=>[-\mathrm {X} ,k_{1}][+\mathrm {X} ,k_{-1}]L_{\mathit {5}}M-\Box ->[+\mathrm {Y} ,k_{2}]L_{\mathit {5}}M-Y}}$

Usually the rate determining step (RDS) of the mechanism is the dissociation of X from the complex, which is dependent upon the strength of the metal to X^- bond as well as other factors such as steric hinderance of the species which helps to speed the mechanism along as crowding favors dissociation. [Y] does not affect the rate of reaction, leading to the simple rate equation:

${\ce {Rate={{\mathit {k}}_{1}[L_{\mathit {5}}M-X]}}}$

When the metal to X^- is not the rate determining step, the rate of reaction is what is seen below:

${\ce {Rate}}={\frac {d[{\ce {L_{\mathit {5}}M-Y}}]}{dt}}$ = ${\ce {{\mathit {k}}_{2}[L_{\mathit {5}}M]}}$

Lability

A metal complex is either labile or inert depending on how easily the reaction proceeds. A labile compound undergoes reactions with a relatively high rate of substitution. The opposite to labile is inert, a term describing metal complexes whose reactions are slow. There are three main factors that affect the whether a complex is labile or inert.

1. Size: Smaller metal ions tend to be more inert as ligands are held more tightly.
2. Charge on Metal: The greater the charge on a metal ion in a complex, the greater the tendency towards the complex being inert.
3. d Electron Configuration: in octahedral geometries, d electrons have t_2g and e_g orbitals meaning the 5 d orbitals are not at the same energy level. The number of d electrons can predict if the metal complex behaves as inert or labile as according to the table below. Each electronic configuration is either labile or inert as a result of partial of full occupancy of the corresponding orbitals.

Number of D-Electrons and Configuration	Reactivity	Notes
d1	Labile	N/A
d2	Labile	N/A
d3	Inert	N/A
d4 Low Spin	Inert	N/A
d4 High Spin	Labile	Especially labile as it is structurally distorted by the Jahn-Teller effect.
d5 Low Spin	Inert	N/A
d5 High Spin	Labile	N/A
d6 Low Spin	Inert	N/A
d6 High Spin	Labile	N/A
d7 High Spin	Labile	N/A
d8 Square Planar	Inert	For d8 and above low spin is the same as high spin.
d8	Intermediate	This configuration is intermediate, especially with weak field ligands.
d9	Labile	Like d4 H.S. this configuration is especially labile as it is distorted by Jahn-Teller effect.
d10	Labile	N/A

Traits of Dissociative Mechanisms

• The rate of substitution varies little with the incoming Y^-.
• The rate of substitution varies over five orders of magnitude depending on the nature of the leaving group. The weaker the bond between the metal and the leaving X^-, the faster the reaction runs, as the loss of X^- occurs in the rate determining step.
• Steric crowding around the metal has the ability to increase the rate of substitution because steric crowding favors dissociation. This runs completely contrary to rate of substitution for associative mechanisms as steric crowding is harder to penetrate.
• Dissociative substitutions have a positive entropy (ΔS^±) and usually have a positive volume of activation (ΔV^±).

The Shift Mechanism

Another way in which a substitution reaction can take place is through use of the shift mechanism, which is most notably present in ring slippage. The shift mechanism consists of the center metal putting some charge back on a ligand either through breaking a double bond and having the ligand hold the charge or through changing the hapacity of the bond. This lowers the overall coordination number on the metal, allowing it to associate ligands for substitution. This phenomena is most commonly found in associative coordinations rather than dissociative mechanism as it lowers the coordination number to one that can accommodate incoming ligands, An example of this is when a η⁵-Cp ring undergoes a ring slippage, becoming a η³-Cp bond, thus allowing an incoming ligand while maintaining the 18-electron rule.