Transition element complexes five-coordinate

In the last few years there has been a prodigious amount of activity in the field of five-coordinate complexes of the first row transition elements... 

The recent interest in five coordination1 has led to an intensive study of a number of transition-metal complexes which appear from their stoichiometry to contain a five-coordinate metal atom. Whereas most of this effort has been focused on the later transition elements, certain key complexes of titanium, vanadium,... 

The coordination about a transition metal and the chemistry of transition metal complexes is often dictated by the electronic needs of the metal center. The stabilization of compounds when the elements present acquire a closed-shell configuration is well known. For a transition metal, with its nine valence orbitals (one s, five d, and three p orbitals), the stabilizing closed-shell configuration is obtained when the electron count about the metal center reaches 18. When a metal center has fewer than 18 electrons in its valence shell it is said to be electronically unsaturated. One step toward alleviating this electronic unsaturation is the formation of metal-metai bonds. Because of their covalent nature, metal-metal single bonds are considered to add one electron to the valence shell of each metal atom. The electron donation from one metal atom to the other mirrors the bond order so a quadruple bond increases the valence count of each metal by four electrons. 

Transition metals are like the elements that we have studied earlier in that they are most stable when they have the electronic configuration of a noble gas. In addition to s and p orbitals, transition metals have five d orbitals (which can hold a total of 10 electrons). Therefore, the noble gas configuration for a transition metal is 18 electrons, not 8 as with carbon, nitrogen, oxygen, and so on. When the metal of a transition metal complex has 18 valence electrons, it is said to be coordinatively saturated. ... 

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