Prueba de contrastación de hipótesis

The simplest polynitrosyl species is M (NO)2- In a linear geometry it has the symmetry D^h, in a bent geometry it has C2V symmetry. The correlation diagram given by Enemark and Feltham [9] fits very well with the results of these calculations. The ordering of the orbitals found here fits well with that predicted by these workers.The one electron orbital scheme put forward by Enemark and Feltham predicts a C2V symmetry for the {M (N0)2}® species, however in the {M(NO}}^° species the electrons would be placed in a non-bonding orbital (of b-j symmetry) with respect to the metal, and antibonding with respect to N and O. They predict that the N-M-N angle should increase from the 90° value. The [Fe(N0)2]^ fragment from Roussin's Red dianion is an {M (N0)2}^ species and the N-M-N angle is 116°, this is in accordance with the above prediction. As the angle increases from 90° the non-bonding orbital will become bonding as the nitrosyl ligands achieve a net bonding interaction with the metal as shown in figure 2-3d,(orbital 3b^). Thus further addition of electrons to this orbital will cause the N-M-N angle to increase more. However these workers also predict that the oxygens of the hitrosyls should bend away from each other as the angle widens, but the crystal structure [10] of the Red dianlon shows the nitrosyls bending towards each other .

The bending of supposedly linear M-N-O in an M(NO>2 fragment has been examined In detail by Enemark [9] and also by Hoffmann[11]. According to Enemark if the electrons populate an a^ or b2 orbital then the bending of the oxygens towards each other will stabilise the orbital; in an a2 or b^ orbital a decrease in stability is proposed if the nitrosyls bend towards each other. Examination of the relevant orbitals in figure 2-3d show this idea. The polymeric complex {Co(NO)2l)n » for example, has a N-Co-N angle of 118° and a Co-N -0 angle of 170° bending towards each other [12], In the complex [(PhgP)2R h (N0 )2]^ a Rh-N-O angle of 159° bending away from each other is observed [13]. The actual structure of [^ °2 ^ 2 (^ ^ )4 ^ ^ shows an N-Fe-N angle of 113° and an F e-N -0 angle of 165° bending towards each other[10].

Hoffmann puts forward the view that the N-M-N angle is a consequence of the number of electrons on the metal and the n accepting properties of the ligand. However the explanation put forward by Hoffmann to explain the M-N-O bending contradicts that of Enemark. From Hoffmann's work an orbital of b^ symmetry will be stabilised if the nitrosyls bend towards each other, and an a^ orbital will be stabilised if the nitrosyls bend away from one another. This explanation is based on the overlap of a lone pair on the nitrogen increasing or decreasing

the antibonding overlap with the central metal atom. From his calculations which way the nitrosyls will bend is strongly dependent on the N-M-N angle.

The structure of [Fe2(S2 0g)2(N0)^]^" shows that the Fe-N-O fragments bend towards each other, the F e-N -0 angle is 1 7 0 °, with a N-Fe-N angle of 118° (see chapter-3). Calculations on [CoH2(N O )2]" at a N-M-N angle of 120° predict an M-N-O angle of 174°

bending towards each other[11]. At greater N-M-N angles the nitrosyls begin to bend away I from each other. The similarity of the geometry of the M-NO for [Fe2(S2 0g)2(N0 )^]^" to the geometry predicted by Hoffmann for a similar N-M-N angle suggests the validity of Hoffmann's ideas, however since in this case the fragments are very different it would be difficult to separate the effect predicted by Hoffmann to that of Enemark.

The real effect may be a mix of the two ideas. At a low N-M-N angle there is greater possibility of overlap between the nitrosyls and a stabilisation of the a-j and b2 orbitals wouid occur, destabilising a2 and b^. At large N-M-N angles there will be little overlap between the nitrosyls and the effect of the lone pair on nitrogen would be the dominant effect again stabilising the a^ and b2 orbitals. This does suggest that the a2 and b-j orbitals will only be at greatest stability at intermediate N-M-N angles.

2 - 3

Bonding in di-iron(-l), [Fe2]^ “

The Fe-Fe distance of the parent complex is 2.72Â. When two irons each with a negative charge are brought together at this distance the d-orbitals are split from their parameterlsed value of -12.700 eV, and range from -13.3 to -12.2 eV, (fig 2-4). From an examination of the overlap only 0.18 of an electron Is shared between the two irons. [Fe2]^' is however more stable than two isolated iron anions by 108 KJmoF^. An Fe-Fe bond strength of 156+/- 25 KJmol"^ is reported for the neutral [14]. The dz^ orbitals Interact the most via a direct (if somewhat weak) 0 bond (orbital la'^g) followed by the dxz and dyz which are involved in a 7t bonding arrangement (orbitals 1 b ' a n d 1b'2u respectively). The dx^-y^

and the dxy are 9 bonded (orbitals 2a'^g and 1a*2g respectively), but these 'interactions' are W insignificant and best regarded as non-bonding.

unfilled), and a weak bond Is shown to exist by virtue of a dz^ cf -bond. The theoretical reduction of this system would lead to the loss of iron-iron bonding, by populating the last unfilled orbital, leading to the instability of the couple. In the same vein it can be appreciated that oxidation would increase the bond order and stabilise the molecule. The HOMO/LUMO gap is so small that if the species was ever produced it would probably exist as a triplet. From this simple examination of the interaction between the two irons at this distance it appears that there is little significant overlap between the two.

Figure 2-4

Molecular Orbital Energy Diagram of [Fe2l^"