Nido-structures

Although the polyhedral structure of 3b seems to point to a bonding situation as described by Wade for a precloso or nido structure,13 the calculations... 

Related to the coordination chemistry of P4S3 is that of a-P4S3l2, which also displays multiple potential coordination sites in a semi-open (nido) structure, but with fewer steric constraints and no P3-unit. The coordination abilities of a-P4S3l2, however, have also proven rather limited. Baudler et al reported a series of molecular low-valent metal adducts of type M(a-P4S3l2)jj (CO)n-ni(M = Ni, Fe m = 1-3) (98). It is remarkable - and frus-... 

The same polyhedra serve as the basis for the structures of nido and arachno compounds, too, although for these boranes and carboranes the polyhedra are incomplete. Nido structures are adopted by neutral boranes BjjHw j.4, carboranes CBy 4Hn.. 3, CsBji—... 

Compounds with a skeletal atoms and b skeletal bond pairs adopt closo structures if 6 = (o + 1), nido structures if 6 = (a + 2), and arachno structures if 6 = (o + 3). 

The relationship between boranes and metal-carbonyl clusters can be extended by considering the compound Fe5(CO)i5C, which has the square-based pyramidal structure shown in Fig. 13, with the carbide carbon atom just below the center of the Fe square, clearly contributing all its valence shell electrons to the cluster 24). The metal-carbonyl residue FeB(CO)i4 formally left by removal of this carbon as has the nido structure appropriate for a cluster with 5 skeletal atoms and seven skeletal bond pairs. 

Fig. 24. The nido structures of some metal-hydrocarbon tt complexes containing CnHfi ring systems.

Fig. 28. (a) The arachno structure of benzvalene and nido structures of C5H5+ and CsMee . (b) The boron heterocycles Ph4C4BPh and H4C4B2F2. 

A number of studies published since 1971 confirm several of the initial predictions. Two groups at Indiana University [Siedle et al. 128) and Streib et al. (62)] have revealed that the nine-vertex nido structures B9Hi2 (II-N9) and the isoelectronic C, C-dimethyl derivative of C2B7H11 (IX-N9) do have nonicosahedrcd fragment strnctures (see Figs. 1 and 9 ... 

If the bonding in BgHig and in C2BgHxo is described in terms of 2- and 3-center bonds, then four 3-center bonds should reside within the skeletal framework and, in spite of the more open arachno structure, the required four 3-center bonds are successfully accommodated, at least in the crystalline phase. The possibility that the true nido structures (I-N8 for BgHi2 and X-N8 for CgBgHio) may exist in the fluid phases is under investigation. 

Those species related to C2B9HH (II-C4) 2,135,145), i.e., CBioHu 62, 72, 155) and BuHn 70), due to the presence of the seven-coordinate boron (violation of rule 4p) tend to scramble their borons on an NMR time scale via a valence bond tautomerism mechanism 142, 155) which possibly involves the transient and reversible opening of the eleven-vertex closo structure to the eleven-vertex nido structure 55, 96). 

Since a rather unstable C2BaHio ido-carborane had been known for several years to exist, this isomer of C2BeHio was structurally identified (41) as having (probably) the arachno structure of V-N8 instead of the nido structure X-N8, but this is not yet sure. 

XI-, XII-, and/or IX-N8 the most probable candidate for a true nido structure is VIII-N8 (CgBgHg""). 

Since n = 5. we see that there are 2/1 + 4 framework electrons and we predict a nido structure which is found experimentally. The square pyramidal structure (Fig. 16.54) can be thought of as resulting from substitution of two BH units with two FefCO units in B5H9 (Fig. I6.49a). 

A Pd4 species has been isolated from the reaction of CO with [Pd(N02)2L2] (L = PPh3, PPh2Me, PPhMe2). The complex has been shown to have the nido structure (21) with Pd—Pd (bonded) 2.750 A and Pd—Pd (non-bonded) 3.365 A.111... 

On similar considerations [Sn9]4- which has 22 skeletal bonding electrons should possess nido structures. 

The main geometries encountered in this review are shown in Fig. 1. In the closed (closo) structure (Fig. lb) the skeletal atoms occupy all corners of a polyhedron, while in the open (nido) structure (Fig. la) one corner of the polyhedron is vacant. This open face is usually the location of attachment when the carborane bonds to a metal group. In some cases, the metal can occupy common vertices of two polyhedra to give a commo structure (Fig. lc). 

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