From a single atom to an infinite solid the example of

1 From a single atom to an infinite solid the example of C 

The variety of transition-metal carbonyl carbide clusters is mirrored in the large number of solid-state carbide compounds known. A difference is that the C atom in solid-state systems is nearly always six coordinate whereas the cluster systems can 

Note that in both interstitial transition metal carbides and the molecular clusters with interstitial atoms, the octahedral metal arrangement changes to trigonal prismatic as the d-electron count increases. That is, octahedral carbides are found for d4 or d5 metals as exemplified by ZrC and NbC and trigonal prismatic carbides are found for d6 metals as in WC. 

C2 units are also found in solid-state compounds with C-C separations that depend on formal electron count. These are viewed as deprotonated ethyne, ethylene or ethane using a popular solid-state idea the Zintl-Klemm concept. This concept is based on the simple idea that the metals transfer their valence electrons to the non-metal atoms thereby generating filled anion-centered bands at low energy, well separated from empty cation-based bands. Of course, this concept fails when the electronegativities of the metal and non-metal are not very different, 

The principal bonding interaction occurs between metallic u and cr frontier orbitals and the a C2 orbitals. You expect this because the [Mn(CO)s]+ fragment is isolobal with [CH3]+ and the product is isolobal with 2-butyne. However, the metal fragment provides the opportunity for weaker interactions between the tt-symmetry orbitals. These are of two types. A backbonding interaction (occupied metallic 77 orbitals with unoccupied C2 tt ) transfers charge from the metal to the C2 unit, thereby reducing the C-C bond order. For this compound, the observed C-C distance is consistent with a bond order of three hence, the interac- 

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