Four coordination is much less common than six coordination in first row d–block elements, mainly du — Physical Chemistry — Electrochemistry Chemistry Question
Tetrahedral and square complexes
Four coordination is much less common than six coordination in first row d–block elements, mainly due to energy stabilization by the ligands. The most symmetric spatial location of ligands about a central ion is the tetrahedron, but square planar complexes are also well known. In tetrahedral geometry only one MA2B2 complex is possible, whereas two isomers are possible in square planar geometry. The hybridization of metal atomic orbitals for tetrahedral geometry is sp 3 or sd 3 while that for square planar geometry is dsp 2 . The electronic configuration of Ni 2+ is [Ar] 3d 8 .
With chloride ion a paramagnetic NiCl4 2– complex ion is formed, whereas with cyanide (CN – ) ion a diamagnetic Ni(CN)4 2– complex ion is formed. Show the distribution of metal ion valence electrons and ligand electrons in the 3d, 4s and 4p subshells for both complexes.
Model Answer
For [NiCl4]2- (sp3: tetrahedral / paramagnetic):
- 3d subshell contains the 8 d-electrons of Ni2+ distributed as three pairs and two unpaired electrons (paramagnetic: [⇅][⇅][⇅][↑][↑]).
- 4s and 4p subshells contain 4 pairs of electrons donated by the four Cl- ligands, corresponding to sp3 hybridization.
For [Ni(CN)4]2- (dsp2: square planar / diamagnetic):
- 3d subshell contains the 8 d-electrons of Ni2+ all paired in four d-orbitals ([⇅][⇅][⇅][⇅]), leaving the fifth d-orbital empty.
- dsp2 hybrid orbitals (formed by the empty 3d orbital, the 4s orbital, and two of the 4p orbitals) contain the 4 pairs of electrons donated by the four CN- ligands, leaving the last 4p orbital empty.