Synthesis, Chemical Characterization, and Bonding Analysis of the [Ag{Fe(CO)4}2]3-, [Ag4{.mu.2-Fe(CO)4}4]4-, and [Ag5{.mu.2-Fe(CO)4}2{.mu.3-Fe(CO)4}2]3- Cluster Anions. X-ray Structural Determination of[NMe3CH2Ph]4[Ag4Fe4(CO)16] and [NEt4]3[Ag5Fe4(CO)16]

The new [Ag{Fe(CO)(4))(2)](3-), [Ag-4{mu(2)-Fe(CO)(4)}(4)](4-), and [Ag-5{mu(2)-Fe(CO)(4)}(2){mu(3)-Fe(CO)(4)}(2)](3-) cluster anions have been selectively obtained by reaction of AgBF4 or AgNO3 with Na-2[Fe(CO)(4)](.)xTHF in tetrahydrofuran and/or acetonitrile solutions in 0.5, 1, and 1.25 molar ratios, respectively. [Ag-5{mu(2)-Fe(CO)(4)}(2){mu 3-Fe(CO)(4)}(2)](3-) has been isolated as a tetrasubstituted ammonium or phosphonium salt, by metathesis in methanol with the corresponding halides, and crystallized as either the [NMe(3)CH(2)Ph](+) or [N(PPh(3))(2)](+) salt from acetone-isopropyl alcohol and acetonitrile-isopropyl ether mixtures, respectively. Neither the [Ag{Fe(CO)(4)}(2)](3-) nor the [Ag-4{mu(2)-Fe(CO)(4)}(4)](4-) anion could be isolated as an ammonium salt directly by metathesis, owing to the sensitivity of these ions to protic solvents. Their salts have been obtained indirectly by reaction of metathesized [Ag-5{mu(2)-Fe(CO)(4)}(2){mu(3)-Fe(CO)(4)}(2)](3-) salts with Na-2[Fe(CO)(4)](.)xTHF in THF, and crystallized by layering with toluene. The structures of [NMe(3)CH(2)Ph](4) [Ag-4{mu(2)-Fe(CO)(4)}(4)] and [NEt(4)](3)[Ag-5{mu(2)-Fe(CO)(4)}(2){mu(3)-Fe(CO)(4)}(2)] have been determined by X-ray diffraction studies. [NMe(3)CH(2)Ph](4)[Ag-4{mu(2)-Fe(CO)(4)}(4)] is monoclinic, space group P2(1)/n (No. 14), with a = 11.419(4) Angstrom, b = 29.714(5) Angstrom, c = 19.227(3) Angstrom, and Z = 4 while [NEt(4)](3)[Ag-5{mu(2)-Fe(CO)(4)}(2){mu(3)-Fe(CO)(4)}(2)] is tetragonal, space group P4(2)/mnm (No. 136), with a 14.004(1) Angstrom, c = 14.278(1) Angstrom, and Z = 2. The [Ag-4{mu(2)-Fe(CO)(4)}(4)](4-) and [Ag-5{mu(2)-Fe(CO)(4)}(2){mu(3)-Fe(CO)(4)}(2)](3-) anions both display an almost planar metal atom frame. The molecular structure of the [Ag-4{mu(2)-Fe(CO)(4)}(4)](4-) anion consists of an Ag-4 square, spanned on all the edges by Fe(CO)(4) groups, the iron atoms describing a square of frequency 2 with the central atom missing. The idealized molecular symmetry is D-4h. The structure of the corresponding [Ag-5{mu(2)-Fe(CO)(4)}(2){mu(3)-Fe(CO)(4)}(2)](3-) conforms exactly to a rhombus of frequency 2; the vertices are occupied by the four iron atoms, whereas silver atoms are located in the middle of each edge and in the center of the rhombus. The electron counts of these clusters have been rationalized by extended Huckel molecular orbital calculations and their chemistry has been interpreted on the basis of isolobal analogies.