Closo structure

The octahedron is classified into the c/o o-structure by Wade [3,4]. Closo-structures with n skeletal atoms are stable when they have 4n-i- 2 valence electrons. Wade s rules predict that the 26 (= 4 x 6 + 2) valence electrons could stabilize the regular octahedrons since n is 6 for the octahedron. This prediction is contained in our 6N + 14 (N= 2) valence electron rule. Our rule also predicts the stability of octahedral metal clusters with the other numbers (14 and 20) of valence electrons. 

For a B H X borane, the preferred structure is a closo structure with a preferred charge of 2 (there are (n + 1) pairs of skeleton electrons). 

Noth and Thorn reported the preparation of P4(NH)5[M(CO)5l4 with M = Cr and Mo, by thermal decomposition of P(NH2)3M(C0)5, and of P4(NH)5S4, through the reaction of Sg on the chromium adduct the mixed P4(NH)g[Cr(CO)5jjjS4 jj were identified during the process (62). Since the parent compound P4(NH)5 itself is still unknown, this exemplifies the possibility of obtaining new closo-structures, or less stable isomers of known ones, in a complexed form. 

There are three main structural types, closo, nido, and arachno. Closo structures are adopted by borane anions ( = 6 -> 12),... 

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). 

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). 

If we remove two vertex boron atoms, the resulting framework is an arachno (Gr., spider s web ) structure. With two vertices missing, the structure is even more open than is the nido case and the resemblance to the parent closo structure is less appurent. Arachno structures obey the electronic formula 2n + 6 (or n + 3 electron pairs). Pentaborane(l I), BjB,. must therefore have an arachno structure. In the arachno series the extra hydrogen atoms form ertdo B—H bonds (lying dose to (he framework) as well as bridges. 

Chlorophyll, and photosynthenc reaction center. 917-919 Chromium carbonyl complexes, bond lengths in, 427 Circular dichrotsm ICD). 496-499 Claasen, H. H., 70 Clathrate compounds, 304-306 Claihro-chelates, 530 Clays. 750 Clementi, E., 31, 32 Closo structures, 798-800. 807 Clostridium pasieurianttm, 934 Clusters, 738, 807-819 Coenzyme, 919 Coenzymes, vitamin B,-,... 

Two different types of gallacarboranes with a closo structure have been reported in the literature. They can also be regarded as n complexes of a nido-carborane dianion and a RGa2+ center. 

Boranes of the general formula B Hn2 and isoelectronic carboranes such as CB iHk- and C2B 2H have closo structures, in which n skeletal B and C atoms are located at the vertices of a polyhedron bounded by trianglular faces (deltahedron). For the BMH 2 cfo.vo-boranes... 

For example, B5H9 (nidoborane) has 14 skeletal bonding electrons. Wade s rules are applicable to "naked" clusters formed by / -block elements and so in case of [Pb5]2- having 22 valence electrons we expected 10 used lane pairs and 12 skeletal bonding electrons, i.e. a closo structure. 

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). 

H12 and [C2B9Hn]2- (or C2B9H13) both have 13 electron pairs, C2B1()H12 will have a closo structure (13 =12 + 1) and C2B9HJ3 will have a nido structure (13 = 11 + 2). 

---