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Polynuclear chromium carbonyl complexes of the general formula [Cr4(CO)12^3-OR)4]4-(R = H, C6H5) have been prepared as their Et4N+ salts. Reaction of Cr(CO)6 with excess hydroxide ion in aqueous THE solution yields [Cr4(CO)12(^3-OH)4]4". The X-ray structure of [Et4N]4[Cr4(CO)12(g3-OH)4] has been determined. The structural features of the tetraanion consist of a cubane-like A4B4 (A = chromium, B = oxygen) arrangement with triply bridging M3-hydroxide groups and Cr(CO)3 units. The Cr-Cr distance within the Cr404 core is nonbondiitg (3.33 (5) Á (average)), consistent with a closed-shell metal configuration.The crystals are monoclinic (space group 12/a) with cell constants a = 21.827 (7) Á, b = 12.236 (2) A, c = 22.899 (4) A, ß = 116.49 (2)°, V = 5474 (5) A* 3, and Z = 8. The µ3-1 derivative, [Et4N]4[Cr4(CO)12^3-OC6H5)4], has been prepared by treatment of Cr(CO)3(CH3CN)3 with [Et4N] [OC6H5] in equimolar concentrations and characterized by spectroscopic methods. Polynuclear Chromium Carbonyl Tetraanions Table II. Selected Bond Distances (Á) and Angles (deg) for [Cr4(CO)12(M3-OH)4]4-Cr-0", Cr(l)-0(7) 2.126 (4) Cr(2)-0(7) 2.145 (3) Cr(l)-0(8) 2.122 (4) Cr(2)-Q(7) 2.107 (3) Cr(l)-0(8) 2.111 (3) Cr(2)-0(8) 2.112 (4) Cr--C Cr(l)-C(l) 1.787 ib) Cr(2)-C(4) 1.770 (b) Cr(l)-C(2) 1.787 (7) Cr(2)-C(5) 1.791 (b) Cr(l)-C(3) 1.803 (8) Cr(2)-C(6) 1.798 (7) C-O C(l)-0(1) 1.189 (7) C(4)-0(4) 1.201 (b) C(2)-0(2) 1.174 (7) C(5)-0(5) 1.178 (7) C(3)-0(3) 1.168 (8) C(b)-0(6) 1.174 (7)Cr-OwrCr Cr(l)-0(7)-Cr(2) 100.5 (2) Cr(l)-Q(8)-Cr(l) 106.1 (2) Cr(l)-G(7)-Cr(2) 102.7 (2) Cr(l)-G(8)-Cr(2) 102.7 (2) Cr(2)-0(7)-Cr(2) 106.5 (1) Cr(l)-Q(8)-Cr(2) 102.1 (2)0"3-Cr-0M3 0(7)-Cr(l)-0(8) 74.8 (1) 0(7)-Cr(2)-Q( 7) 72.7 (1) 0(7)-Cr(l)-0(8) 76.1 (1) 0(7)-Cr(2)-0(8) 75.7 (1) 0(8)-Cr(l)-0(8) 72.9 (2) 0(7)-Cr(2)-0(8) 75.4 (1)Cr-C-O Cr(l)-C(l)-0(1) 174.0 (5) Cr(2)-C(4)-Q(4) 174.6 ( 6)Cr(l)-C(2)-0(2)174.3 (6) Cr(2)-C(5)-0(5) 173.5 (6) Cr(l)-C(3)-0(3) 173.7 (6) Cr(2)-C( 6)-Q( 6) 169.4 (6)

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1053protonated carbonyl compounds can relax to their ground states. In this context, the photoreactions of 3 and 4 described here can perhaps be better compared to those of 1,3-dienes rather than conjugated enones.'Secondly, it is of interest to consider the mechanism of the thermal isomerization about the C -0 bonds. Several possibilities can be eliminated from consideration. Thus, the minimal effect of introducing a methyl group a t C3 on the rate of isomerization about the C-0 bond renders a direct rotation process unlikely as this involves a large change in the charge distribution in the system. The absence of any appreciable medium effect eliminates any base addition route, while the substantially different activation energies for isomerism about the C-0 and C2-C3 bonds rules out the intervention of a common 0-methyloxetium ion in these two rearrangements.Rather, it appears that the most likely process occurring is an inversion or "lateral shift" mechanism8 as has also been found with the isoelectronic aldimines and k e t i m i n e~.~ Acknowledgment.

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