2 Homometallic Alkoxides

100 Alkoxo and Aryloxo Derivatives of Metals
The d 7 cobalt(II) methoxide shows variation of magnetic moment with temperature
involving an octahedral CoO6 model ( eff D 5.46 B.M., D 15 Ž ⊳. 219 The halide methoxides
also follow the Curie–Weiss law and thus chloride methoxide exhibits a room
temperature magnetic moment of 4.89 š 0.01 B.M. 595 The expected spin only magnetic
moment for a high-spin d 7 system is 3.88 B.M. and spin–orbit coupling depends
on the symmetry (octahedral or tetrahedral). The magnetic moments were calculated
by Figgis and Lewis 600 using a spin–orbit coupling constant D 170 cm 1 which
gave the values 4.70 and 5.2 B.M., for tetrahedral and octahedral Co(II) and in actual
practice these values vary in the range 4.10–4.90 B.M. and 4.70–5.20 B.M., respectively.
Applying the magnetic moment and ligand field parameter relationship 560,561
and using D 170 cm 1 for the tetrahedral chloride methoxide, the magnetic moment
would be expected to be of the order of 4.50 B.M. which is less than the observed
value of 4.90 B.M. and, therefore, it may be assumed to involve octahedral geometry.
On the other hand, the observed 595 magnetic moment for cobalt bromide methoxide,
eff D 4.69 š 0.01 B.M., is comparable with that needed for the tetrahedral Co(II)
ion, and thus a tetrahedral structure is expected in the latter compound. However,
the magnetic moment increases abruptly from 4.69 to 5.10 B.M. in methanol, which
indicates that a change in geometry from tetrahedral to octahedal occurs in solution,
probably due to solvation. The room temperature magnetic moment of cobalt iodide
methoxide was found to be 4.82 š 0.01 B.M. consistent with a tetrahedral structure for
the product. However, the observed value is slightly higher than the 4.69 B.M. observed
for the bromide methoxide which may be attributed to the relative positions of bromide
and iodide in the spectrochemical series.
The d 8 nickel(II) methoxide follows the Curie–Weiss law as indicated by its magnetic
susceptibility measurements in the range 193 to 77 Ž C with a magnetic moment of 3.38
B.M. It appears to involve a structure containing NiO6 octahedra with weak antiferromagnetic
interactions. 219 The nickel chloride alkoxides NiCl(OMe), Ni3Cl2⊲OMe⊳4,
and Ni3Cl⊲OMe⊳5 exhibit 541 magnetic moments eff ¾ 3.2–3.7 B.M. which are higher
than the spin only value of 2.84 B.M. Furthermore, the magnetic susceptibility is
temperature dependent and the magnetic moment increases with decreasing temperature.
The magnetic moment is not affected by dissolving the chloride methoxide in
methanol. On the basis of this anomalous magnetic behaviour shown by nickel chloride
methoxide, it appears to involve the cubane structure, having Ni4⊲OMe⊳4 clusters
in which the nickel atom is octahedrally surrounded by methoxo bridges and terminal
methoxide and chloride groups involving ferromagnetic interactions. On the other hand
NiCl⊲OPr i ⊳ and Ni3Cl⊲OMe⊳5 show temperature-independent magnetic moments which
is expected for magnetically dilute nickel(II) ion. Nickel bromide methoxide exhibits
a low magnetic moment (3.00 š 0.03 B.M.) and this anomaly may be ascribed to
weak antiferromagnetic interactions. NiCl⊲OPr i ⊳ and Ni3Br⊲OMe⊳5 on the other hand
increase magnetic moments to 3.21 and 3.64 B.M. in methanol and acetone, respectively.
The significantly high value in acetone implies a tetrahedral nickel(II) ion, which
requires magnetic moments of the order of 3.54–4.20 B.M.
Similar to the d 4 chromium dimethoxide, the d 9 copper(II) alkoxides also exhibit
temperature-dependent magnetic susceptibility. 219,542 The magnetic moments of copper
dimethoxide and diethoxide at room temperature were reported 221 to be in the range
1.07–1.23 B.M. which were less than the spin only value of 1.73 B.M. This abnormal
behaviour was attributed to be due to the highly polymeric structure of copper(II)