Spin-paired complex

Formylthiophene thiosemicarbazone, 26, forms [Co(26)2A2]A and [Co(26)A3(H20)] (A = Cl, Br, I, NO3, and OAc), by varying the preparative hgand to cobalt(II) salt mole ratio in hot ethanol [201]. Addition of sodium hydroxide allows isolation of [Co(29-H)3]. All complexes are weakly paramagnetic indicating spin paired complexes. Spectroscopic studies showed bonding via the azomethine nitrogen and thiol sulfur, but not the thiophene sulfur. 

The variation of aEq with temperature varies with the state of the iron. For example, in many spin-paired complexes and Fe compounds the temperature coeflScient is small (0 to 0.006 cm./sec.) but larger for Fe salts (0.015 to 0.048 cm./sec.) (J3). Bums (8) has suggested that the small temperature coeflScient for Fe salts arises from differential homogeneous contraction of the crystal in two axis directions. 

The reduction of Mel by Co(CN)53-, which occurs by the mechanism represented by reaction (10),19 is offered as a second example of a facile inner-sphere process. The outer sphere alternative, reaction (11), has a very high energy barrier because of the formation of the five-coordinate intermediate Coin(CN)52. Spin-paired complexes of Co111 have a strong preference to be six-... 

Like ihe neighboring elements of group 8, iron forms large numbers of complexes. This is due to ihe availability of two d orbitals in Fe2+ and Fe3+ to form hybrid orbitals with the 4s and 4p orbitals to yield spin-paired complexes (the so called "covalent or inner-- complexes). 

SPIN.FREE vj. SPIN.PAIRED COMPLEXES OF THE TRANSITION ELEMENTS 411... 

It is emphasized that in octahedral fields, the distinction between spin-free and spin-paired complexes is meaningful only if more than three but less than eight d electrons are present in the valence shell (p. 360). 

This suggests that simple carbonyls are invariably spin-paired complexes (p. 134). These metals have even atomic numbers, those with Z odd do not form simple carbonyls. In the carbonyl nitrosyls and carbonyl hydrides which are mononuclear, a metal atom with an odd atomic number can similarly complete its d shell. Thus in Co(CO)aNO and Co(CO)4H, the —NO group and —COH group can be considered to contribute the single electron necessary to complete the 3d shell of the metal atom. The three CO molecules and the isoelectronic NO or COH are co-ordinated tetrahedrally, as are the four CO molecules in Ni(CO)4. 

Mn also occurs in both spin-free complexes such as [Mn(acac)3] , and in spin-paired complexes with two unpaired electrons such as Mn(CN) The Re complexes Uke Re(NH3) + ReCl4 are, however, diamagnetic. 

The stable coordination numbers of spin-paired complexes of transition metals range from 8 to 2 and exhibit a systematic inverse dependence on the number of d electrons of the metal atom, which is illustrated by Table II. 

For spin-paired complex ions, i.e. in strong ligand fields, X loses its usual significance. Such configurations may be characterized by an effective spin angular momentum S assuming the values 1, ], 0 for tfj,... 

These considerations cannot be applied to spin-paired complexes of Fe(II) and Fe(III) with vacant tt-orbital ligands. Fur such cases the metal-to-ligand tt -bonding delocalizes the d-electrons with the consequent decreases of the shielding of s-electrons and strong raise of the s-density at the nucleus. This becomes apparent when the comparison is made between the s-electron densities of complexes with iso-electronic ligands ... 

The central met ll atom or ion may use inner (n-l)d orbitals or outer nd orbitals for hybridization. On this basis, the complexes may be classified as follows (I) Inner orbital complexes The complexes which use inner d-orbitals, i.e., (n - 1) d orbiteds of the central metal atom or ion for bond formation are called as inner orbital complexes. These complexes formed by strong ligands i.e., CN, NO2, CO, NH3 etc. In these complexes the impaired electrons of the central metal atom or ion are forced to pair up by strong ligands against Hund s rule. These complexes are, therefore, either diamagnetic or contain lesser number of unpaired electrons. They are also called low spin or spin paired complexes. These complexes... 

For the same reason the absorption spectrum of the ligand is altered. The shift is related to the electronegativity of the cation. Of course, for several of them the electronegativity is not that of their ground state. Several cations are engaged indeed in spin paired complexes (states of lower spin multiplicity). Measuring absorption is a valuable experimental... 

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