Krypton-transition metal bonds

In 1934 Nevil Vincent Sidgwick published a paper in which he established the well-known Effective Atomic Number rule for the bonding and stoichiometry of transition metal carbonyl compounds (2), For the metals of even atomic number Z, he observed that if each carbonyl group bonded to the metal atom is considered to "donate two electrons to the metal atom (in its oxidation state of zero), then the sum of the atomic number, Z, of the metal plus two times the number of CO groups attached to it equals the atomic number of the next heavier inert gas. This worked beautifully for the 3d metals, i.e., Cr in Cr(CO)5, Fe in Fe(CO)5, and Ni in Ni(CO)4. All had an Effective Atomic Number of 36, Z of krypton. This idea was quicldy extended to deal with transition metals with odd atomic numbers and to groups that donated one or three electrons, e,g, Mn in Mn2(CO)iQ (to give a Mn—Mn covalent bond) and Co in Co(CO)3(NO) and Fe in Fe(CO)2(NO)2 in which the NO molecule contributed 3 electrons. 

The formation of complexes constitutes an especially important part of the chemistry of the transition metals. The special feature of the elec-, tronic structure of the transition metals that leads to their formation of stable complexes is the availability of d orbitals, as well as s and p orbitals, for bond formation, as discussed in the following section. The transition metals have in their outer shells electrons occupying d, s, and p orbitals. Thus for the elements from potassium to krypton the outer-shell electrons may occupy the five 2d orbitals, the As orbital, and the... 

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