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The $\ce{[PbCl4]^{2-}}$ complex ion is formed when $\ce{PbCl2}$ is exposed to excess $Cl^-$ ions in solution as explained here and here through the following reversible reaction:

$$\ce{PbCl2(s) + 2Cl^-(aq) <=> [PbCl4]^{2-}(aq)}$$ What I wish to understand is the mechanism behind this reaction. I managed to find the mechanism to a similar complex ion formation of $\ce{[CuCl4]^{2-}}$. It is explained here that $\ce{[CuCl4]^{2-}}$ is formed by the hybridisation of the $Cu^{2+}$ 4s and 4p orbitals: $$sp^3:\boxed{\uparrow \ }\boxed{\uparrow \ }\boxed{\uparrow \ }\boxed{\uparrow \ }$$

Would the $\ce{[PbCl4]^{2-}}$ complex ion be obtained in a similar way by $Pb^{2+}$ forming the following hypridisation orbitals? $$sp^3:\boxed{\uparrow \downarrow }\boxed{\uparrow \downarrow }\boxed{\uparrow \ }\boxed{\uparrow \ }$$ or no need for the above hybridisation since the 6s orbital was already filled and there is space in the empty 6p orbital so the configuration for the complex ion would be as follows? $$6p:\boxed{\uparrow \downarrow}\boxed{\uparrow \ }\boxed{\uparrow \ }$$

JulianS
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    There's no hybridisation in there. – Mithoron Jan 20 '21 at 21:39
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    https://chemistry.stackexchange.com/questions/8717/what-is-the-inert-pair-effect – Mithoron Jan 20 '21 at 21:49
  • Thank you for your input, Mithoron. The inert pair effect would explain why PbCl2 is more stable than PbCl4 and also why no hybridisation takes place in the above (the outer s orbital acting as an inner orbital rather than a valence orbital). Does this mean that when forming the complex ion, the empty p orbital is simply filled like my second suggestion in my question? – JulianS Jan 21 '21 at 21:55
  • Could the inert pair effect also explain why the total valence and shared electrons on the Pb element in the [PbCl4]2- ion add up to 10 thus overshooting the octet rule? because the 2 valence electrons in the s orbital of Pb do not really count as valence electrons? – JulianS Jan 21 '21 at 23:39
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    It's more like octet rule hardly works here. Such cases can be fitted to it like SF6, though https://chemistry.stackexchange.com/a/5242/9961 https://chemistry.stackexchange.com/a/49844/9961 – Mithoron Jan 22 '21 at 00:20
  • Mithoron, I think it would make more sense that the 6p and 6d orbitals of the Pb hybridise since co-ordinate covalent bonds will form with 4 pairs of electrons from 4 Cl- ions. what do you think? – JulianS Jan 23 '21 at 22:36
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    sight best don't mention this hypervalent menace here. There was a reason I put these links specifically. – Mithoron Jan 23 '21 at 22:59

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