Problem 2

Square-planar Pt(II) complexes substitute with retention of stereochemistry, and the rate of substitution of X by Y depends on the ligand trans to X (trans effect):

\ce{CN-} > \ce{H-} > \ce{NO2-}, \ce{I-} > \ce{Br-}, \ce{Cl-} > \text{pyridine}, \ce{NH3}, \ce{OH-}, \ce{H2O}

. (a) Draw all stereoisomers of square-planar

\ce{Pt(py)(NH3)BrCl}

(py = pyridine). (b) Write reaction schemes, via intermediates, to prepare each stereoisomer of

\ce{[Pt(NH3)(NO2)Cl2]-}

in water starting from

\ce{PtCl4^{2-}}

\ce{NH3}

\ce{NO2-}

, exploiting the trans effect. (c) Substitution follows

\mathrm{Rate} = k_S[\ce{ML3X}] + k_Y[\ce{Y}][\ce{ML3X}]

; for

[\ce{py}] \gg [\ce{Pt}]

\mathrm{Rate} = k_{obs}[\ce{Pt}]

. For the displacement of

\ce{Cl-}

by pyridine in methanol at 25 °C:

[\text{py}]

= 0.122, 0.061, 0.030 mol dm

^{-3}

give

k_{obs}

7.20 \times 10^{-4}

3.45 \times 10^{-4}

1.75 \times 10^{-4}

^{-1}

. Find

k_S

and

k_Y

and decide which pathway dominates at

[\text{py}] = 0.10

mol dm

^{-3}

. (d) A gold nanoparticle of diameter 13 nm carries 90 oligonucleotide groups, 98 % bound to a Pt(IV) complex; a 1.0 cm

^3

vessel contains

1.0 \times 10^{-6}

mol dm

^{-3}

in Pt (density of Au = 19.3 g cm

^{-3}

). Calculate the masses of Pt and Au used.

Step 1 of 3: Isomers and trans-effect routes

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\text{3 isomers of } \ce{Pt(py)(NH3)BrCl};\quad \text{cis}: \ce{PtCl4^{2-} ->[NH3] [Pt(NH3)Cl3]- ->[NO2-] cis\text{-}[Pt(NH3)(NO2)Cl2]-};\quad \text{trans}: \ce{PtCl4^{2-} ->[NO2-] [Pt(NO2)Cl3]^{2-} ->[NH3] trans\text{-}[Pt(NH3)(NO2)Cl2]-}

Analysis

$\ce{Pt(py)(NH3)BrCl}$ has 3 isomers (each ligand can be trans to one of the three others). For $\ce{[Pt(NH3)(NO2)Cl2]-}$ : adding $\ce{NH3}$ first to $\ce{PtCl4^{2-}}$ gives $\ce{[Pt(NH3)Cl3]-}$ ; the incoming $\ce{NO2-}$ substitutes the Cl trans to Cl (Cl labilises trans position weakly, $\ce{NO2}$ enters cis to $\ce{NH3}$ ). Conversely $\ce{NO2-}$ first gives $\ce{[Pt(NO2)Cl3]^{2-}}$ ; the strong trans-effector $\ce{NO2}$ directs $\ce{NH3}$ trans to itself, giving the trans isomer.