2 Homometallic Alkoxides

218 Alkoxo and Aryloxo Derivatives of Metals
The product [KZr⊲OBu t ⊳5]n was actually isolated 41,182 in 1:1 molar reaction of
KOBu t and Zr⊲OBu t ⊳4 and characterized 182 by X-ray crystallography.
In view of the larger size of the central metal atoms, the stability of [KU2⊲OBu t ⊳9], 217
[NaCe2⊲OBu t ⊳9], 218 and [NaTh2⊲OBu t ⊳9] 219 can be easily understood.
In contrast to the stability of KZr2⊲OPr i ⊳9, 41 the comparative instability of similarly
synthesized KTi2⊲OPr i ⊳9 was explained on the basis of the difficulty of the
smaller titanium (0.64 ˚A) atom to accommodate six OPr i groups around itself like
zirconium (0.80 ˚A). However, Veith 164 has been able to characterize X-ray crystallographically
the identity of products such as KTi2⊲OPr i ⊳9 (low temperature technique)
and of BaTi3⊲OPr i ⊳14 as fTi⊲OPr i ⊳5gBafTi2⊲OPr i ⊳9g; 140 the latter finding is of special
interest as it might finally throw light on the nature of products like K2Zr3⊲OPr i ⊳14 41
and may lead to the isolation of similar derivatives, MTi3⊲OPr i ⊳14, of other divalent
metals like Ca, Sr, Zn, Cd, Ni, Co, and Cu.
The stability of monomeric [ClCufZr2⊲OPr i ⊳9g] 126 in contrast to the dimeric
[CdfZr2⊲OPr i ⊳9⊲ -Cl⊳g]2 135 again illustrates the effect of the size of central metal atom.
Further, the monomeric nature of isostructural CdfM2⊲OPr i ⊳9gI derivatives (M D Zr,
Hf, Ti, Sn(IV)) can also be ascribed to the large size of I 160–163 compared to that
of Cl. 135 Although [ClSnfZr2⊲OPr i ⊳9g]2 is also dimeric like [ClCdfZr2⊲OPr i ⊳9g]2, the
fZr2⊲OPr i ⊳9g ligand binds Sn(II) in a bidentate manner 141,142 in place of the usual
tetradentate ligating mode 188 in the cadmium analogue; this difference has been
ascribed 141 to the presence of a lone pair of electrons on tin(II). The replacement
of Cl in [ClSnM2⊲OPr i ⊳9]2 by C5H5 (cyclopentadienyl) by interaction with NaC5H5
led 142 to monomeric derivatives [⊲C5H5⊳SnM2⊲OPr i ⊳9], as confirmed by 119 Sn NMR
spectra and X-ray crystallography.
An eight-coordinated barium derivative, [BafZr2⊲OPr i ⊳9g2] 131 has been crystallographically
characterized; the structure consists of a “bow-tie” or “spiro” Zr2BaZr2
unit wherein barium is eight-coordinated by two face-shared bi-octahedral fZr2⊲OPr i ⊳9g
units.
3.4.2.2 Tri- and bidentate modes of coordination
The small lithium ion (0.78 ˚A) interacts with only three isopropoxo groups 131 of a
fZr2⊲OPr i ⊳9g unit in contrast to the four utilized by its larger congeners like Na C
(0.98 ˚A) and K C (1.33 ˚A). Lithium appears to be too small to span the distance between
isopropoxo groups on both zirconium centres (see Chapter 4). The fourth coordination
site on lithium is occupied by an isopropanol molecule, which is hydrogen bonded to
an oxygen of a terminal isopropoxo group on zirconium.
The structure of [SnfZr2⊲OPr i ⊳9⊲ -I⊳g2] 161 also shows a fZr2⊲OPr i ⊳9g unit interacting
with Sn(II) in a tridentate fashion, which appears to result from the presence of
the stereochemically active lone pair of electrons on tin(II).
The electronic spectra of [CofZr2⊲OPr i ⊳9g2] 115 and [CufZr2⊲OPr i ⊳9g2] 122 show the
central transition metals Co and Cu to be hexacoordinate, indicating a tridentate
bonding mode of the fZr2⊲OPr i ⊳9g ligand.
Although the size of lead(II) is larger (1.32 ˚A) than that of tin(II) (0.93 ˚A),
fZr2⊲OPr i ⊳9g appears to bind the former in a bidentate ( 2 ) manner, as revealed
by the X-ray structure of [PbfZr2⊲OPr i ⊳9g⊲ -OPr i ⊳]2 with a “serpentine” rather than
“close” pattern 181 as exhibited by the [Sn⊲ -OPr i ⊳3Zr⊲OPr i ⊳3] derivative in its crystal
structure. In fact, Caulton et al. 82 have shown that the reactions of Zr⊲OBu t ⊳4 and
M⊲OBu t ⊳2 (M D Sn, Pb) yielded