Stereochemical Control of Valence and Its Application to the Reduction of Coordinated NO and N(2).

The electronic structures of transition metal complexes of NO are controlled by the stereochemistry about the metal atom (stereochemical control of valence). The six-coordinate complex, trans-[CoNO(NCS)-(C(6)H(4) [As(CH(3))(2)](2))(2)](+), consists of [Co(III)-(N=O(-))](2+) (angle(Co-N-O) = 135 degrees ), while the pentacoordinate trigonal bipyramidal complex, [CoNO(C(6)H(4)[As(CH(3))(2)](2))(2)] (2+), is best formulated as [Co(I)-(N identical withO(+)](2+) (angle(Co-N-O) = 179 degrees ). Evidence indicates that complexes of (NO)(+) and N(2) are electronically similar. Hence, the principles of stereochemical control of valence may be applied to metal complexes of N(2). In a linearly coordinated M(n) m(N identical withN) complex, valence electrons can be transferred from the metal to the N(2) ligand producing a bent, protonated, and/or metallated M(n+2)-(N=N(2-)) complex. This reduction of N(2) can be effected by the addition of an appropriate ligand to M or by a change in the coordination geometry about M. Stereo-chemical control of valence leads to the rejection of one of the previously proposed mechanisms for reduction by nitrogenase.