2 Homometallic Alkoxides

346 Alkoxo and Aryloxo Derivatives of Metals
Figure 4.61 Structure of [Sr2Cd2Hf4⊲ 3-OPr i ⊳4⊲ -OPr t ⊳12⊲OPr i ⊳8]
( D Cd; D Sr, D Hf; H atoms omitted).
a linear central core of [Sr⊲ -OPr i ⊳2Cd⊲ -OPr i ⊳2Cd⊲ -OPr i ⊳2Sr] capped at each
end by octahedrally coordinated confacially bridged Hf2⊲OPr i ⊳9 moieties acting
as 4 -chelating units. The M2⊲OR⊳9 moiety is also featured in the structure of
[M⊲ 3-OEt⊳4⊲ -OEt⊳4fTi2⊲ -OEt⊳⊲OEt⊳4g2] (MD Ca, Sr, Ba) in which the central
bivalent metal ion is eight-coordinated (distorted square antiprism). 490,491 With the
more bulky isopropoxo ligands the coordination of strontium is limited to six in
the trinuclear complex [⊲Pr i OH⊳3⊲Pr i O⊳Sr2⊲ 3-OPr i ⊳2⊲ -OPr i ⊳3Ti⊲OPr i ⊳2] which has
an isosceles triangle of metal atoms capped above and below the triangle by 3-OPr i
ligands and bound by three bridging isopropoxides in the plane. This molecule has
the familiar M3O11 framework found in other metal alkoxide structures (Figs 4.5
and 4.25). 490
3.2.3 Barium
In the trinuclear complex [Baf -OCH⊲CF3⊳2g4Y2⊲thd⊳4]fthd D Bu t C(O)CHC(O)Bu t g
the three metal atoms are in a non-linear chain (Y�BaY, 129.3 Ž ) and the central Ba ion
is joined to the Y ions by double fluoroalkoxo bridges and interacts with eight fluorine
atoms from neighbouring CF3 groups. The yttrium ions are six-coordinated (trigonal
prismatic). 492
With the isopropoxo ligand a novel tetranuclear species [⊲Pr i O⊳3Ti⊲ -OPr i ⊳2Ba⊲ 3-
OPr i ⊳2⊲ -OPr i ⊳2fTi2⊲ 3-OPr i ⊳2⊲ -OPr i ⊳⊲OPr i ⊳4g] is formed, with the central sixcoordinated
(trigonal prismatic) Ba ion being bridged by bidentate Ti⊲OPr i ⊳5 and
quadridentate Ti2⊲OPr i ⊳9 units. 493
With zirconium the hexanuclear molecule [⊲Pr i O⊳4⊲ 3-OPr i ⊳2⊲ -OPr i ⊳Zr2⊲ -
OPr i ⊳2Ba⊲ -OPr i ⊳2Ba⊲ -OPr i ⊳2Zr2⊲ 3-OPr i ⊳2⊲ -OPr i ⊳⊲OPr i ⊳4] was obtained, with
six-coordinated metals. The confacial bi-octahedral Zr2⊲OPr i ⊳9 units act as tetradentate
bridging groups to each barium. 494 The pentanuclear species [BaZr4⊲OPr i ⊳18] hasa
similar structure to that of [BaTi4⊲OEt⊳18].
The novel trimetallic octanuclear homoleptic complexes [Ba2Cd2M4⊲ 3-OPr i ⊳4⊲ -
OPr i ⊳12⊲OPr i ⊳8] (M D Ti, Zr, Hf) were the first of this class of compound to
be structurally characterized. 495 These structures, which are analogous to that
of [Sr2Cd2Hf4⊲OPr i ⊳24] (Fig. 4.61) already mentioned, 200 contain a linear array