2 Homometallic Alkoxides

X-Ray Crystal Structures of Alkoxo Metal Compounds 337
In the Cr(II) complex [Na4Cr2⊲ 3-OPr i ⊳8⊲thf⊳4] the two chromiums are in axial
positions and the four sodiums in the equatorial positions of an octahedron with triplebridging
Pr i O ligands capping the eight faces. Each Cr atom is above the plane of
its four bridging Pr i O ligands whilst each Na achieves five-coordination by virtue of
the terminally bound THF ligand. 475 In the sodium chromium(II) bis-disiloxanediolate
[⊲thf⊳4Na2f -O⊲SiPh2O⊳2g2Cr] the Na ions are four-coordinated and the chromiums
square planar. 476
In the hydrido complex [⊲diglyme⊳Na⊲ -H⊳⊲ -OPr i ⊳4W2⊲OPr i ⊳4] (W–W, 2.431 ˚A)
there is a confacial bi-octahedral W2⊲ -H⊳⊲ -OPr i ⊳2 core with the five-coordinated Na
bridging two apices of this unit. 313
A cubane-like NaTc3O4 core is present in [Na⊲ 3-OMe⊳4fTc⊲CO⊳3g3] wherethe
octahedral coordination of each Tc atom is completed by three terminal carbonyl
ligands. 477
In the Cu(II) complex [Na2f -OCH⊲CF3⊳2g4Cu] the sodium ions are edge bridged
with a near square planar CuO4 unit but five fluorines are involved in Na....F agostic
interactions. 478 The octanuclear compound [Na4Cu4⊲ 3-OCEt3⊳4⊲ -OCEt3⊳4] has a
structure similar to that of [Li4Cu4⊲OBu t ⊳8] (Fig. 4.59) with linear two-coordinated
Cu(I). 465
The sodium tin(II) tert-butoxide [Na2⊲ 3-OBu t ⊳2⊲ -OBu t ⊳4Sn], like its lithium
analogue, has the Sn2O6M2 cage comprising two seco-norcubane Sn2M2O3 units
sharing a Na2O2 four-membered ring. 468 By contrast the bridging-chelating ligand
⊲Bu t O⊳3SiO gave rise to the dinuclear species [Naf , 2 -OSi⊲OBu t ⊳3g3Pb] and
[Kf , 2 -OSi⊲OBu t ⊳3g3Sn] in which the alkali metal is six-coordinated and the
heterometal is three-coordinated (pyramidal). 484
3.1.3 Potassium
Various potassium metal alkoxides have been structurally characterized. One of the first
examples reported was the homoleptic complex [K⊲ 3-OBu t ⊳2⊲ -OBu t ⊳3U2⊲OBu t ⊳4]
which has the confacial bioctahedral U2O9 framework with the K ion occupying a
pocket between the two uranium(IV) atoms(U–U, 3.631 ˚A). 151
In [K⊲ 3-OPr i ⊳⊲ -OPr i ⊳3Ti⊲OPr i ⊳]n both metals are five-coordinated and alternate
in infinite zig-zag chains significantly different in structure from the analogous
[NaTi⊲OPr i ⊳5]n. 473
A novel structure is exhibited by [⊲py⊳2K2⊲ -OCH2But⊳8M2] (MDMo, W). The
bimetallic core (Mo–Mo, 2.257 ˚A; W–W, 2.335 ˚A) is encapsulated in a pseudo-O8 cube
of alkoxo oxygens which are bridged to the two potassiums on opposite faces. 259 By
contrast in [M2⊲ 3-OBu t ⊳2⊲ -OBu t ⊳4Zn2] (MDNa, K) the three-coordinated alkali
metals are located above and below the Zn2O2 plane. 479
In potassium aluminium isopropoxide isopropanolate [⊲PriOH⊳2K⊲ -OPr i ⊳4Al]n the
alternating K and Al atoms are joined by double isopropoxo bridges in a zig-zag linear
chain. The two coordinated isopropanols give the potassium a distorted octahedral
configuration whilst the Al is distorted tetrahedral. 480
In the Sn(II) complex [K⊲ 3-OBu t ⊳2⊲ -OBu t ⊳Sn]n both metals are in distorted
five-coordinated configurations in a polymeric structure of alternating potassium
and tin atoms linked by bridging alkoxo groups in a structure different from the
Li and Na analogues. 468 The binuclear potassium tin(II) triphenylsiloxide exists in
two forms both containing the K⊲ -OSiPh3⊳3Sn unit with three-coordinated tin. In