Ion Pair Charge-Transfer Complexes between Anionic and Cationic Metal-Dithiolenes [M(II) = Pd, Pt]

New [M(R2pipdt)2](BF4)2 salts [R2pipdt = N,N‘-dialkyl-piperazine-2,3-dithione; M = Pd(II), R = Me and M = Pt(II),
R = Me, Et, Pri] bearing redox-active cationic dithiolene complexes have been prepared and characterized. These
cations react with the redox-active [M(mnt)2]2- [M = Pd(II), Pt(II); mnt = maleonitrile-2,3-dithiolate] anionic dithiolenes
to form salts describable as ion pair charge-transfer complexes. X-ray crystallographic studies have shown that
[M(Me2pipdt)2][M(mnt)2] complexes, with M = Pd(II) and Pt(II), are isomorphous. Crystal data of the Pt salt (3a): 
triclinic, P1̄ (No. 2); Z = 1; T = 293(2) K; a = 6.784(7) Å, b = 8.460(6) Å, c = 13.510(5) Å, α = 100.63(2)°,
β = 104.04(2)°, γ = 96.90(2)°; R1 = 0.0691 [wR2 = 0.2187 (all data)]. Structural data show that approximately
square-planar [Pt(Me2pipdt)2] dications and regular square-planar [Pt(mnt)2] dianions form an infinite anion−cation
one-dimensional stack along axis a with a Pt···Pt a/2 distance of 3.392 Å and a Pt···Pt···Pt angle of 180°. Anions
and cations arrange themselves face-to-face so as to take on a staggered arrangement. These salts exhibit strong
absorptions in the visible−near-infrared region assigned to ion pair charge-transfer transitions. A relation between
the optical and thermal electron transfer in the solid state is obtained using a “Marcus−Hush model”, and a solid-state electrical conductivity in agreement with expectations is observed. Vibrational spectroscopy is in agreement
with the existence of charge-transfer interactions between the cationic and anionic components of the salts.