Square-based pyramidal/octahedral

Indium clusters have also recently been characterized, notably in intermetallic compounds. Thus, the Zintl phase, Rbzinj, (prepared by direct reaction between the two metals at I530°C) has layers of octahedral closo-lnf, clusters joined into sheets through exo bonds at four coplanar vertices. These four In atoms are therefore each bonded to five neighbouring In atoms at the comers of a square-based pyramid, whereas the remaining two (Irans) In atoms in the Ine cluster... 

Although the majority of complexes have structures that are linear, tetrahedral, square planar, or octahedral, a few compounds have a trigonal bipyramid structure. Most notable of these are Fe(CO)5, Ni(CN)5ji, and [Co(CN)5]3T Some complexes having a coordination number of 5 have the square-base pyramid structure, including [Ni(CN)5 3. Although it is not particularly common, the coordination number 8 is found in the complex [Mn(CN)8]4-, which has a cubic structure with CIST ions on the corners. 

It is made by reaction of RuClj with PPh in methanol, and forms shiny black crystals [881, 882], The X-ray crystal structure shows the molecule to have a distorted square-based pyramidal structure, a phosphorus atom forming the axial bond (Ru-P 2.230(8) A) while the basal plane has two trans chloro ligands (Ru-C1 2.387(7) and 2.388(7) A) and two trans phosphine ligands (Ru-P 2.374(4) and 2.412(6) A). It can be regarded as octahedral, with the sixth position blocked by a phenyl ring (Fig. 1.39) [883]. The catalytic efficacy of the complex may well depend on the availability of this vacant coordination site. 

On standing in the above-mentioned polar solvents, the octahedral Os6H2(CO)18 rearranges to give the square base pyramidal isomer (in dichloromethane, 30 min), which exhibits the following IR absorptions (dichloromethane solution, cm-1) 2112(w), 2081 (m), 2073(vs), 2045(m), 2018 (w), 2002 (w). Additional spectroscopic data on Os6H2(CO)18 have been reported.11... 

ERO — elongated rhombic octahedral TB = trigonal bipyramidal SBP = square-based pyramidal L = linear Td = tetrahedral Tr = trigonal. ... 

Five-coordination is as abundant in copper(II) complexes as the six-coordinate elongated rhombic octahedral stereochemistry (Figure 19.1). The regular square-based pyramidal geometry with five equivalent ligands is only of limited occurrence, but does arise in... 

In general, it is not possible to predict the stereochemistry about the separate copper(II) ions in most cases they are the same and may or may not be related by a centre of symmetry. The actual stereochemistries produced are recognizably the same as those occurring in mononuclear copper(II) complexes (Figure 19.1). The most common is that of square-based pyramidal with rhombic coplanar, compressed tetrahedral, trigonal bipyramidal and elongated rhombic octahedral stereochemistries all occurring. 

For a discussion of oxyanion coordination numbers see Chapter 15.5, structures I-IV. b tsglyo — JV-tosylglycinate. cCTd = compressed tetrahedral SBP - square-based pyramidal ERO = elongated rhombic octahedral TB = trigonal bipyramidal LB = long bipyramidal SP - square planar. 

Elongated tetragonal octahedral Elongated rhombic octahedral Square coplanar Square-based pyramidal Compressed tetragonal octahedral Square coplanar Linear... 

Compiled from Table 9.2 ref. 1065. bSP = square planar CTd = compressed tetrahedral SBP = square-based pyramidal TP = trigonal pyramidal ETO = elongated tetragonal octahedral. 

ETO = elongated tetragonal octahedral SBP = square-based pyramidal TB = trigonal bipyramidal ERO = elongated rhombic octahedral CRO = composed rhombic octahedral SP - square planar O = octahedral CTO = compressed tetragonal octahedral 7C = seven coordinate cis-O = cis-octahedral RC = rhombic coptanar TO = tetragonal octahedral CTd — compressed tetrahedral. 

The coordination number of the square-planar complexes of Cu, Ni, or Pt is 4. Higher coordination numbers of 5 or 6 with one or two additional ligands such as water or ammonia result in square-based pyramidal, tetrahedral, or octahedral structures (3) [19-22],... 

A less obvious conclusion, in terms of the experimental evidence available at the time, was that d6 pentacarbonyls such as Cr(CO)5 should prefer a square-based pyramidal geometry that is barely distorted away from the truncated octahedral structure. In other words, the CO ligands lying in the basal plane are at an angle of ca. 90° with respect to the axial CO. This geometry, rather than the one in which the basal CO ligands bend away from or toward the other CO, was predicted to be favored for reasons of orbital overlap. At the time of this publication, there was only limited experimental evidence from matrix isolation studies that this type of fragment had such a square-based pyramidal structure. 

HOs6(CO)18] and H2Os6(CO)18 marks one of the highlights of the early period of osmium carbonyl cluster chemistry [162]. While both the dianionic cluster and the monoanionic system have the expected octahedral metal core, a capped square based pyramidal structure was found for H2Os6(CO)18. This turned out to be the archetypal example for the capping rule , a concept which proved to be very successful in the analysis of the structures of metal carbonyl clusters of the iron and cobalt triads [166],... 

Figure 1.10 Molecular orbitals for octahedral and square-based pyramidal complexes...

The condensation reactions of carbonyl metallates with neutral species, which are either coordinatively unsaturated or which will readily generate coordinatively unsaturated fragments, have also yielded a variety of mixed-metal clusters. The square-based pyramidal dianion [Fe5C(CO)i4] , for example, has been shown to react with a number of such species to yield octahedral FcjMC cluster compounds (Scheme 9) 269, 381). In some cases. 

Addition of protons to anionic clusters to generate hydrides leaves the cluster electron count unaffected, yet the process is sometimes accompanied by structural changes. For example, both [Os6(CO)i8] (87) and [Os6(CO)igH] (88) have octahedral arrangements of metal atoms as predicted by Wade s rules. In the dihydride Osg(CO)i8H2, however, the metal atoms describe a capped square-based pyramid (87). 

Recent molecular orbital (MO) calculations by Wade et al. (408) using the series [B Hg] , [BgH,]", and BgHg as models for the protonation of hexanuclear metal carbonyls have attempted to rationalize these findings. The charge distribution is symmetrical in an octahedral [BgHg] but asymmetrical in the capped square-based pyramidal isomer. It was found that upon protonation, significant charge redistribution occurs. This results in a substantial decrease in the symmetry of the octahedral cluster framework, which is disfavored in comparison with the capped square-based pyramidal structure much less affected by the protonation process. 

Further evidence for the stabilization of this structure in the Osg system comes from the fact that the octahedral isomer of Osg(CO)igH2, isolated by varying the conditions for protonation of [Osg(CO)ig] (433), slowly rearranges in solution to the more thermodynamically stable capped square-based pyramidal structure. The dihydride Rug(CO) gH2, however, does not show preference for this structure and retains the octahedral metal atom geometry observed for [Rug(CO),g] " and [Rug(CO)igH] (30,31,35) even on warming. 

Related synthetic polyaminoacids, which do not feature the iminodiacetate unit but rather are composed of molecules with multiple -NH-CH2-COOH functionalities, are also common. An example is (160), which stabilizes the unusual square-based pyramidal geometry in Co in conjunction with an axial S03 ion. The macrocycle (148) is another example, which binds octahedral metal ions with the pendants in axial sites. ... 

SiFs as a molecule with its threefold axis tilted with respect to Cq or as a square based pyramid (an octahedral MFg less one F ligand) would not pack very differently from MFg species. The latter fit with least separation of the carbon-atom sheets when a threefold axis is perpendicular to the sheets. 

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