Synthesis and Nonlinear Optical Properties of η<SUP>5</SUP>-Monocyclopentadienyliron(II) Acetylide Derivatives. X-ray Crystal Structures of [Fe(η<SUP>5</SUP>-C<INF>5</INF>H<INF>5</INF>)(DPPE)(p-C&tbd1;CC<INF>6</INF>H<INF>4</INF>NO<INF>2</INF>)] and [Fe(η<SUP>5</SUP>-C<INF>5</INF>H<INF>5</INF>)(DPPE)((E)-p-C&tbd1;CC<INF>6</INF>H<INF>4</INF>C(H)&dbd;C(H)C<INF>6</INF>H<INF>4</INF>NO<INF>2</INF>)]

A series of new acetylide complexes of the type [Fe(η⁵-C<INF>5</INF>H<INF>5</INF>)(P&arcraise;P)(p-C&tbd1;CC<INF>6</INF>H<INF>4</INF>R)] (P&arcraise;P = DPPE (=1,2-bis(diphenylphosphino)ethane), (R)-PROPHOS (=(R)-(+)-1,2-bis(diphenylphosphino)propane), R = NO<INF>2</INF>, C<INF>6</INF>H<INF>4</INF>NO<INF>2</INF>, (Z)-C(H)&dbd;C(H)C<INF>6</INF>H<INF>4</INF>NO<INF>2</INF>, (E)-C(H)&dbd;C(H)C<INF>6</INF>H<INF>4</INF>NO<INF>2</INF>) have been synthesized by halide abstraction from the precursors [Fe(η⁵-C<INF>5</INF>H<INF>5</INF>)(P&arcraise;P)(I)] and fully characterized. Quadratic hyperpolarizabilities (β) for the complexes have been determined by hyper-Rayleigh scattering at 1064 nm. The influence on the nonlinear response of acetylide chain length in proceeding from 4-nitrophenylethynyl to 4-nitrobiphenylethynyl and 4-nitro-(E)-stibenylethynyl has been studied, revealing values of the first hyperpolarizabilities among the highest reported for organometallic molecular materials. Comparisons on the nonlinear efficiencies are drawn with the related well-known families of compounds [Ru(η⁵-C<INF>5</INF>H<INF>5</INF>)(PR<INF>3</INF>)<INF>2</INF>(p-C&tbd1;C−(aryl)−NO<INF>2</INF>)] and [Fe(η⁵-C<INF>5</INF>H<INF>5</INF>)(P&arcraise;P)(p-N&tbd1;C−(aryl)−NO<INF>2</INF>)]⁺. Cubic hyperpolarizabilities (γ) determined by Z-scan at 800 nm are consistent with an increase in γ upon replacing Ru by the more easily oxidizable Fe and upon chain-lengthening the delocalizable π-bridging unit (proceeding from 4-C<INF>6</INF>H<INF>4</INF> to (E)-4,4‘-C<INF>6</INF>H<INF>4</INF>CH&dbd;CHC<INF>6</INF>H<INF>4</INF>). X-ray crystallographic structures of complexes [Fe(η⁵-C<INF>5</INF>H<INF>5</INF>)(DPPE)(p-C&tbd1;CC<INF>6</INF>H<INF>4</INF>NO<INF>2</INF>)] and [Fe(η⁵-C<INF>5</INF>H<INF>5</INF>)(DPPE)((E)-p-C&tbd1;CC<INF>6</INF>H<INF>4</INF>C(H)&dbd;C(H)C<INF>6</INF>H<INF>4</INF>NO<INF>2</INF>)] were studied in order to investigate the existence of π-back-donation suggested by spectroscopic and electrochemical data. Crystal packing was analyzed with the aim of assessing the alignment of the molecules in the lattice and hence suggesting the magnitude of NLO properties at the macroscopic level.